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Microwave engg. Model questions
Ques.1. What do you mean by waveguide? Explain the TE and TM modes of rectangular waveguide.
Ans. A waveguide is a special form of transmission line consisting of a hollow, metal tube. The
tube wall provides distributed inductance, while the empty space between the tube walls provide
distributed capacitance.
Waveguides are practical only for signals of extremely high frequency, where the wavelength
approaches the cross-sectional dimensions of the waveguide. Below such frequencies, waveguides are
useless as electrical transmission lines.
When functioning as transmission lines, though, waveguides are considerably simpler than two-
conductor cables -- especially coaxial cables -- in their manufacture and maintenance. With only a single
conductor (the waveguide's shell), there are no concerns with proper conductor-to-conductor spacing,
or of the consistency of the dielectric material, since the only dielectric in a waveguide is air. Moisture is
not as severe a problem in waveguides as it is within coaxial cables, either, and so waveguides are often
spared the necessity of gas filling.
Waveguides may be thought of as conduits for electromagnetic energy, the waveguide itself acting
as nothing more than a director of the energy rather than as a signal conductor in the normal sense of
the word. In a sense, all transmission lines function as conduits of electromagnetic energy when
transporting pulses or high-frequency waves, directing the waves as the banks of a river direct a tidal
wave. However, because waveguides are single-conductor elements, the propagation of electrical
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energy down a waveguide is of a very different nature than the propagation of electrical energy down a
two-conductor transmission line.
All electromagnetic waves consist of electric and magnetic fields propagating in the same direction of
travel, but perpendicular to each other. Along the length of a normal transmission line, both electric and
magnetic fields are perpendicular (transverse) to the direction of wave travel. This is known as the
principal mode, or TEM (Transverse Electric and Magnetic) mode. This mode of wave propagation can
exist only where there are two conductors, and it is the dominant mode of wave propagation where the
cross-sectional dimensions of the transmission line are small compared to the wavelength of the signal.
Ques 2. Explain the propagation of TE waves in rectangular waveguide.
Ans. Propagation of TE Waves in Rectangular Waveguide:-
As we have seen earlier waveguides refer to any structure that can guide electromagnetic (EM) waves
along its axial direction, which include transmission line.
Here we consider waveguide as specifically refers to long metallic structures with only 1 piece of
conductor between the source and load end.
These metallic structures are usually hollow, so that EM fields are confined within the hollow and be
guided along the axial direction.
Applying Maxwells Equations with the proper boundary conditions shows that propagating EM waves
can be supported by the waveguide.
Due to the absence of center conductor, only TE and TM mode exist.
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Ques. 3. What do you mean by phase velocity and group velocity in waveguide?
Ans- Phase Velocity in Waveguide
Since phase velocity vp depends on propagation constant bmn, it too depends on the integer pair (m,n)
hence the property of the TE mode
Fields.
Speed of light in dielectric of (,e)
Group Velocity in Waveguide
The velocity of energy propagation, or the speed that information traveling a waveguide is given by the
Group Velocity vg.
Thus from:
Since vp > c,
The group velocity is thus less than speed of light in vacuum, maintaining the assertion of Relativity
Theory that no mass/energy can travel faster than speed of light.
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Ques.4. Derive the TE modes for rectangular waveguide.
Ans. FOR TE Mode :-
To obtain the TE mode electromagnetic (EM) field pattern, we use the systematic procedure developed
in Chapter 1 Advanced Transmission Line Theory.
We start by solving the pattern function for z-component of the magnetic field and boundary conditions
(1.1)
Problem (1.1) is called Boundary Value Problem (BVP) in mathematics.
Once we know the function of hz(x,y), the EM fields are given by:
(1.2a)
(1.2b)
Expanding the partial differential equation (PDE) of (1.1) in cartesian coordinates:
(1.2)
Using the Separation of Variables Method, we can decompose hz(x,y) into the product of 2 functions
and kc 2 to be the sum of 2 constants:
(1.3a) (1.3b)
Putting these into (1.2), and after some manipulation we obtain 2 ordinary differential equations
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(1.4a) (1.4b)
From elementary calculus, we know that the general solution for (1.4a) and (1.4b) are:
(1.5a)
(1.5b)
Thus hz(x,y) is given by:
(1.6)
A, B, C and D in (1.6) are unknown constants, to be determined by applying the boundary conditions
that the tangential electric field must vanish on the conductive walls of the waveguide. From (1.2b):
Using (1.6) and applying the boundary condition (1.7a):
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Using (1.6) and applying the boundary condition (1.7b):
In the above equations, we can combine the product of AC, lets call it R. Common sense tells us that
R would be different for each pair of integer (m,n), thus we should denote R by: Rmn
From (1.3b), kc and the propagation constant are given by:
Since kc and also depends on the integer pairs (m,n), it is more
appropriate to write these as:
(1.7a)
(1.7b)
With these information, and using (1.2a) and (1.2b), we can write out the complete mathematical
expressions for the EM fields under TE propagation mode for a rectangular waveguide:
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(1.8a)
(1.8b)
(1.8c)
(1.8d)
(1.8e)
Ques.5. Derive TM modes for rectangular waveguide.
Ans. The procedure for obtaining the EM field solution for TM propagation is similar to the TE
procedure.
We start by solving the pattern function for the z-component of the electric field and boundary
conditions (1.1
As in solving TE mode problem, the Separation of Variables Method is used in solving (1.11), and
integer pair (m,n) needs to be introduced in the TM mode solution.
The mathematical expressions for the EM field components thus depends on the integer pair (m,n), and
is denoted by TMmn field.
The derivation details will be omitted here due to space constraint. You can refer to reference [1] for the
procedure.
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The complete expressions for the TMmn field components are show below:
(1.12a)
(1.12b)
(1.12c)
(1.12d)
(1.12e)
Where
(1.13a)
(1.13b)
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Ques.6. Explain circular waveguide.
Ans. For mechanical reasons, a rotating joint must be circular and requires a coaxial line or a section of
circular waveguide.
a.) Transverse electric (TE) and transverse magnetic (TM) waves are propagated in circular waveguides
in almost the same manner as inrectangular waveguides. The field configuration in the circular waveguide
closely follows a sine wave pattern (fig. 65).
b.) The boundary conditions used in the rectangular waveguide also apply to the circular
waveguide. Under these conditions the electric field must be perpendicular to the surface of the
conductor, and the magnetic field parallel to the surface of the conductor. When these boundary
conditions are fulfilled in the circular waveguide, the electric field exists between the center of the
waveguide and the wall, and the magnetic field exists around the inside of the waveguide as shown Figure
82. Field configuration in a circular in figure 82. waveguide.
c.) The dominant mode in the circular waveguide is similar to the dominant mode in the rectangular
waveguide. In the TE mode, the electric field is perpendicular to the direction of propagation. In the TM
mode, the magnetic field is perpendicular to the direction of propagation. The TE mode in figure 83
shows that the electric lines are circular around the center of the waveguide and perpendicular to the
direction of propagation. In the TM mode, the magnetic lines are circular around the center of the
waveguide and perpendicular to the direction of propagation.
The Circular Waveguide Modes;
Figure 7.26 shows a circular waveguide with inner diameter 2 a. We investigate the lossless circular
waveguide with a perfectly conducting wall and free-space inner region. To investigate the TM and TE
modes of the circular cylindric waveguide we derive the fields either from an electric Hertz form ? e or a
magnetic Hertz form ? m exhibiting only a z-component
Circular cylindric waveguide.
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For both cases the Helmholtz equation (3.28) has the following form:
with . With (A.157) we obtain for circular cylindric coordinates
We seek solutions for waves propagating in the positive z-direction and choose the separation
formulation
From this it follows that
We introduce the parameterkc given by
and obtain
Ques.7. What do you understand by striplines. Write advantages and disadvantages of them.
Ans. Stripline is a conductor sandwiched by dielectric between a pair of groundplanes, much like a
coax cable would look after you ran it over with your small-manhood indicating SUV (let's not go
there...) In practice, "classic" stripline is usually made by etching circuitry on a substrate that has a
groundplane on the opposite face, then adhesively attaching a second substrate (which is metalized
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on only one surface) on top to achieve the second ground plane. Stripline is most often a "soft-board"
technology, but using low-temperature co-fired ceramics (LTCC), ceramic stripline circuits are also
possible
Advantages and disadvantages of strip line
y Strip line is a TEM (transverse electromagnetic) transmission line media, like coax. The filling
factor for coax is unity, and "Keff" is equal to ER. This means that it is non-dispersive. Whatever
circuits you can make on micro strip (which is quasi-TEM), you can make better using strip line,
unless you run into fabrication or size constraints. Stripline filters and couplers always offer better
bandwidth than their counterparts in micro strip, and the rolloff of strip line BPFs can be quite
symmetric (unlike micro strip). Strip line has no lower cutoff frequency (like waveguide does).
y Another advantage of stripline is that fantastic isolation between adjacent traces can be achieved
(as opposed to microstrip). The best isolation results when a picket-fence of vias surrounds each
transmission line, spaced at less than 1/4 wavelength. Stripline can be used to route RF signals
across each other quite easily when offset stripline is used.
y Disadvantages of stripline are two: first, it is much harder (and more expensive) to fabricate than
microstrip, some old guys would even say it's a lost art. Lumped-element and active components
either have to be buried between the groundplanes (generally a tricky proposition), or transitions
to microstrip must be employed as needed to get the components onto the top of the board.
y The second disadvantage of stripline is that because of the second groundplane, the strip widths
are much narrower for a given impedance (such as 50 ohms) and board thickness than for
microstrip. A common reaction to problems with microstrip circuits is to attempt to convert them
to stripline. Chances are you'll end up with a board thickness that is four times that of yourmicrostrip board to get equivalent transmission line loss. That means you'll need forty mils thick
stripline to replace ten mil thick microstrip! This is one of the reasons that soft-board
manufacturers offer so many thicknesses.
Ques.8. Define microstriplines. Explian its effective dielectric constant.
Ans. Microstrip transmission lines consist of a conductive strip of width "W" and thickness "t"
and a wider ground plane, separated by a dielectric layer (a.k.a. the "substrate") of thickness "H"as shown in the figure below. Microstrip is by far the most popular microwave transmission line,
especially for microwave integrated circuits and MMICs. The major advantage of microstrip
over stripline is that all active components can be mounted on top of the board. Thedisadvantages are that when high isolation is required such as in a filter or switch, some externalshielding may have to be considered. Given the chance, microstrip circuits can radiate, causing
unintended circuit response. A minor issue with microstrip is that it is dispersive, meaning that
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signals of different frequencies travel at slightly different speeds. Microstrip does not support a
TEM mode, because of its filling factor. For coupled lines, the even and odd modes will not havethe same phase velocity. This property is what causes the asymmetric frequency of microstrip
bandpass filters, for example.Variants of microstrip include embedded microstrip and coatedmicrostrip, both of which add some dielectric above the microstrip conductor.
Effective dielectric constant
Because part of the fields from the microstrip conductor exist in air, the effective dielectricconstant "Keff" is somewhat less than the substrate's dielectric constant (also known as the
relative permittivity). Thanks to Brian KC2PIT for reminding us the term "relative dielectricconstant" is an oxymoron only used my microwave morons!) According to Bahl and Trivedi[1],
the effective dielectric constant eff (a.k.a. Keff) of microstrip is calculated by:
All microstrip equations are approximate. The above equations ignore strip thickness, so we
wouldn't recommend relying on them for critical designs on thick copper boards.
The effective dielectric constant is a seen to be a function of the ratio of the width to the height
of the microstrip line (W/H), as well as the dielectric constant of the substrate material. Becareful, the way it is expressed here it is also a function of H/W! We have a table of "hard"
substrate material properties here, and "soft" substrate material properties here, in case you wantto look up the dielectric constant of a specific material.
Note that there are separate solutions for cases where W/H is less than 1, and when W/H is
greater than or equal to 1. These equations provide a reasonable approximation for eff(effective dielectric constant). This calculation ignores strip thickness and frequency dispersion,
but their effects are usually small.
Ques.9. Give all the characteristics of microstrip lines.
Ans. The different characteristics of microstrip lines are as follows:
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1. Wavelength
Wavelength for any transmission line can be calculated by dividing free space wavelength by the
squareroot of the effective dielectric constant, which is explained above.
2. Characteristic impedance
The characteristic impedance Z0 is also a function of the ratio of the height to the width W/H
(and ratio of width to height H/W) of the transmission line, and also has separate solutionsdepending on the value of W/H. According to Bahl and Trivedi[1], the characteristic impedance
Z0 of microstrip is calculated by:
3. Effect of metal thickness on calculations
Having a finite thickness of metal for the conductor strips tends to increase the capacitance of the
lines, which effects the effand Z0 calculations. We'll add this correction factor at a later date.
4. Effect of cover height on calculations Having a lid in close proximity raises the capacitanceper length, and therefore lowers the impedance. We suggest that if your impedance calculation is
important, to use EDA software to make the final calculation on line widths!
Ques.10. Derive the modes for circular waveguide.
Ans. The Circular Waveguide Modes;
Figure shows a circular waveguide with inner diameter 2 a. We investigate the lossless circular
waveguide with a perfectly conducting wall and free-space inner region. To investigate the TMand TE modes of the circular cylindric waveguide we derive the fields either from an electric
Hertz form ?e or a magnetic Hertz form ?m exhibiting only az-component
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Figure 7.26: Circular cylindric waveguide.
For both cases the Helmholtz equation (3.28) has the following form:
with . With (A.157) we obtain for circular cylindric coordinates
We seek solutions for waves propagating in the positive z-direction and choose the separationformulation
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From this it follows that
We introduce the parameterkc given by
and obtain
Ques.11. Explain the scattering matrix. Write its properties.
Ans.Scattering(S) Parameter-
Scattering parameters orS-parameters (the elements of a scattering matrix orS-matrix) describe the
electrical behavior of linear electrical networks when undergoing various steady state stimuli by electrical
signals.
The S-parameters are members of a family of similar parameters, other examples being: Y-parameters,[1]
Z-parameters,[2] H-parameters, T-parameters or ABCD-parameters.[3][4]They differ from these, in the
sense that S-parameters do not use open or short circuit conditions to characterize a linear electrical
network; instead matched loads are used. These terminations are much easier to use at high signal
frequencies than open-circuit and short-circuit terminations. Moreover, the quantities are measured in
terms of power.
Many electrical properties of networks of components (inductors, capacitors, resistors) may be expressed
using S-parameters, such as gain, return loss, voltage standing wave ratio (VSWR), reflection coefficient
and amplifier stability.
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The term 'scattering' is more common to optical engineering than RF engineering, referring to the effect
observed when a plane electromagnetic wave is incident on an obstruction or passes across dissimilar
dielectric media. In the context of S-parameters, scattering refers to the way in which the traveling
currents and voltages in a transmission line are affected when they meet a discontinuity caused by the
insertion of a network into the transmission line. This is equivalent to the wave meeting an impedance
differing from the line's characteristic impedance.
S-parameters are readily represented in matrix form and obey the rules of matrix algebra.
The S-parameter matrix describing an N-port network will be square of dimension 'N' and will therefore
contain elements. At the test frequency each element or S-parameter is represented by a unitless
complex number that represents magnitude and angle, i.e. amplitude and phase. The complex number
may either be expressed in rectangular form or, more commonly, in polar form. The S-parameter
magnitude may be expressed in linear form or logarithmic form. When expressed in logarithmic form,
magnitude has the "dimensionless unit" of decibels. The S-parameter angle is most frequently expressed
in degrees but occasionally in radians. Any S-parameter may be displayed graphically on a polar diagram
by a dot for one frequency or a locus for a range of frequencies...he following information must be
defined when specifying any S-parameter:
(1)The characteristic impedance (often 50 ).
(2) The allocation of port numbers.(3) Conditions which may affect the network, such as frequency, temperature, control voltage, and bias
current, where applicable.
Ques.12. Explain properties scattering matrix by virtue of different types of networks.
Ans. Properties of S matrix:
1) Reciprocal and non-reciprocal networks:A reciprocal network is one in which the power
losses are the same between any two ports regardless of direction of propagation (scattering parameter S21=S12, S13=S31, etc.) A network is known to be reciprocal if it is passive and
contains only isotropic materials. Examples of reciprocal networks include cables, attenuators,and all passive power splitters and couplers. Anisotropic materials have different electrical
properties (such as relative dielectric constant) depending on which direction a signal propagatesthrough them. One example of an anisotropic material is the class of materials known as ferrites,
from which circulators and isolators are made. Two classic examples of non-reciprocal networksare RF amplifiers and isolators. In both cases, scattering parameter S21 is much different from
S12.A reciprocal network always has a symmetric S-parameter matrix. That means thatS21=S12, S13=S31, etc. All values along the lower-left to upper right diagonal must be equal. A
two-port S-parameter matrix (at a single frequency) is represented by:
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If you are measuring a network that is known to be reciprocal, checking for symmetry across thediagonal of the S-parameter matrix is one simple check to see if the data is valid. Here is an
example of S-parameters of a network that is either a non-reciprocal network, or your technicianhas a drinking problem Although the data shows the part is well matched (S11 and S22
magnitudes are low), and low loss (S21 and S12 magnitudes are high). The magnitudes of S12and S21 are equal, so what is the problem? The phase angles of S12 and S21 are significantly
different. That can't be right.
2) Lossless networks:
For a network to be lossless, all of the power (or energy) that is incident at any one port has to beaccounted for by summing the power output at the other ports with the power reflected at the
incident port. None of the power is converted to heat or radiated within a lossless network. Notethat an active device is not in the same category as a lossless part, since power is added to the
network through its bias connections. Within the S-parameter matrix of a lossless network, thesum of the squares of the magnitudes of any row must total unity (unity is a fancy way of saying"one"). If any of the rows' sum-of-the-squares is less than one, there is a lossy element within the
network, or something is radiating.
3) Passive devices versus active devices:
A passive device contains no source that could add energy to your signal, with one exception.The first law thermodynamics, conservation of energy, implies that a passive device can't
oscillate. An active device is one in which an external energy source is somehow contributing tothe magnitude of one or more responses
Ques.13. Derive S parameters of scattering matrix.
Ans. Scattering Parameters
A scattering matrix (S-parameter matrix) is one way to describe the operation of a linear, time-invariant two-port circuit. A two-port network is defined as any linear device where a signal goes
in one side and comes out the other. The S-parameter matrix is rapidly becoming very popular asa way to characterize connectors and cables for high-speed applications above 1 Gb/s.
The measurement setup associated with S-parameters is as follows (Figure 1).
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From the test equipment, two cables having characteristic impedance Z0 lead to the left and rightsides, respectively, of the device under test (DUT).
Using the first (left-side) cable, inject a sinusoidal signal (in1) of unit amplitude into the DUT.
The test equipment records the amplitude and phase of the signal (out1) reflected back onto thefirst cable from the DUT, and also the amplitude and phase of the signal (out 2) conveyed through
the DUT to the second cable on the other side.
In a separate experiement, using the second (right-side) cable, inject a sinusoidal signal (in 2) of
unit amplitude into the DUT. The test equipment records the amplitude and phase of the signal(out2) reflected from the right side of the DUT, and the amplitude and phase of the signal (out1)
conveyed through the DUT to the other (left) side. The complete S-parameter matrix is acombination of these four basic measurements.
The four elements of an S-parameter matrix may be reported as complex numbers (with real and
imaginary parts) or in logarithmic units (as dB magnitude and phase). An n-port microwavenetwork has n arms into which power can be fed and from which power can be taken. In general,
power can get from any arm (as input) to any other arm (as output).
There are thus n incoming waves and n outgoing waves. We also observe that power can be
reflected by a port, so the input power to a single port can partition between all the ports of thenetwork to form outgoing waves. Associated with each port is the notion of a "reference plane"
at which the wave amplitude and phase is defined. Usually the reference plane associated with acertain port is at the same place with respect to incoming and outgoing waves. The n incoming
wave complex amplitudes are usually designated by the n complex quantities an, and the noutgoing wave complex quantities are designated by the n complex quantities bn. The incoming
wave quantities are assembled into an n-vector A and the outgoing wave quantities into an n-
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vector B. The outgoing waves are expressed in terms of the incoming waves by the matrixequation B = SA where S is an n by n square matrix of complex numbers called the "scattering
matrix". It completely determines the behaviour of the network. In general, the elements of thismatrix, which are termed "s-parameters", are all frequency-dependent.
For example, the matrix equations for a 2-port are
b1 = s11 a1 + s12 a2
b2 = s21 a1 + s22 a2
And the matrix equations for a 3-port are
b1 = s11 a1 + s12 a2 + s13 a3
b2 = s21 a1 + s22 a2 + s23 a3
b3 = s31 a1 + s32 a2 + s33 a3
The wave amplitudes an and bn are obtained from the port current and voltages by the relations a
= (V + ZoI)/(2 sqrt(2Zo)) and b = (V - ZoI)/(2 sqrt(2Zo)). Here, a refers to an if V is Vn and I Infor the nth port. Note the sqrt(2) reduces the peak value to an rms value, and the sqrt(Zo) makes
the amplitude normalised with respect to power, so that the incoming power = aa* and theoutgoing power is bb*. A one-port scattering parameter s is merely the reflection coefficient
gamma, and as we have seen we can relate gamma to the load impedance zL = ZL/Zo by theformula gamma = (zL-1)/(zL+1).
Similarly, given an n by n "Z-matrix" for an n-port network, we obtain the S matrix from theformula S = (Z-I)(Z+I)^-1, by post-multiplying the matrix (Z-I) by the inverse of the matrix
(Z+I). Here, I is the n by n unit matrix. The matrix of z parameters (which has n squaredelements) is the inverse of the matrix of y parameters.
Ques.14. Write a short note on reciprocity in s- matrix.
Ans. Reciprocity
A network will be reciprocal if it is passive and it contains only reciprocal materials that influence the
transmitted signal. For example, attenuators, cables, splitters and combiners are all reciprocal networks
and in each case, or the S-parameter matrix will be equal to its transpose. Networks which
include non-reciprocal materials in the transmission medium such as those containing magnetically biased
ferrite components will be non-reciprocal. An amplifier is another example of a non-reciprocal network.
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An interesting property of 3-port networks, however, is that they cannot be simultaneously reciprocal,
loss-free, and perfectly matched.
A reciprocal network is one in which the power losses are the same between any two ports regardless of
direction of propagation (scattering parameter S21=S12, S13=S31, etc.) A network is known to be
reciprocal if it is passive and contains only isotropic materials. Examples ofreciprocal networks include
cables, attenuators, and all passive power splitters and couplers.
Anisotropic materials have different electrical properties (such as relative dielectric constant) depending
on which direction a signal propagates through them. One example of an anisotropic material is the class
of materials known as ferrites, from which circulators and isolators are made. Two classic examples of
non-reciprocalnetworks are RF amplifiers and isolators. In both cases, scattering parameter S21 is much
different from S12.
A reciprocal network always has a symmetric S-parameter matrix. That means that S21=S12, S13=S31,
etc. All values along the lower-left to upper right diagonal must be equal. A two-port S-parameter matrix
(at a single frequency) is represented by:
Ques.15. Explain briefly about lossless network.
Ans. Lossless networks
A lossless network is one which does not dissipate any power, or : . The sum of the
incident powers at all ports is equal to the sum of the reflected powers at all ports. This implies that the S-
parameter matrix is unitary, that is , where is the conjugate transpose of
and is the identity matrix.
For a network to be lossless, all of the power (or energy) that is incident at any one port has to be
accounted for by summing the power output at the other ports with the power reflected at the incident
port. None of the power is converted to heat or radiated within a lossless network. Note that an active
device is not in the same category as a lossless part, since power is added to the network through its bias
connections.
Within the S-parameter matrix of a lossless network, the sum of the squares of the magnitudes of any row
must total unity (unity is a fancy way of saying "one"). If any of the rows' sum-of-the-squares is less than
one, there is a lossy element within the network, or something is radiating.
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Ques.16. What do you mean by waveguide junction. List various types of junction.
Ans.Waveguide Junctions:
Figure 1: H-type T- junction
Different types of junctions affect the energy in different ways. The T- Junction is the most
simple of the commonly used waveguide junctions. T-junctions are divided into two basic types,the E-TYPE and the H-TYPE.
H-type T-junction
An H-type T-junction is illustrated in the beside figure. It is called an H-type T-junction becausethe long axis of the B arm is parallel to the plane of the magnetic lines of force in the
waveguide. The E-field is fed into arm A and in-phase outputs are obtained from the B and Carms. The reverse is also true.
Figure 2: E-type T- junction
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E-type T-junction
This junction is called an E- type T junction because the junction arm extends from the main
waveguide in the same direction as the E-field in the waveguide. The outputs will be 180 out ofphase with each other.
Magic-T-Hybrid Junction
A simplified version of the magic-T-hybrid junction is shown in the figure. The magic-Tjunction can be described as a dual electromagnetic plane type of T-junction. It is a combination
of the H-type and E-type T.junction therefore. The most common applications of this type of junction are for example as the mixer section for microwave radar receivers or as a part of a
measurement system.
If a signal is fed into the E-plane arm of the magic-T, it will divide into two out-of-phasecomponents (arm B and C). The signal entering the E-arm will not enter the H-plane arm
because of the zero potential existing at the entrance of the H-plane arm. The potential must bezero at this point to satisfy the boundary conditions of the E-plane arm.
Normally a magic-T needs an impedance matching (shown in the figure as matching screws).
Figure 3: Magic-T Hybrid
Ques.17. Explain Magic tee.
Ans. MAGIC-T HYBRID JUNCTION. A simplified version of the magic-T hybrid junction is
shown in figure 3-64. The magic-T is a combination of the H-type and E-type T junctions. The
most common application of this type of junction is as the mixer section for microwave radar
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receiversMagic-T hybrid junction. If a signal is fed into the b arm of the magic-T, it will divide
into two out-of-phase components.
As shown in figure 3-65, view A, these two components will move into the a and c arms. The signalentering the b arm will not enter the d arm because of the zero potential existing at the entrance of the
d arm. The potential must be zero at this point to satisfy the boundary conditions of the b arm.
This absence of potential is illustrated in views B and C where the magnitude of the E field in the
b arm is indicated by the length of the arrows. Since the E lines are at maximum in the center of
the b arm and minimum at the edge where the d arm entrance is located, no potential difference
exists across the mouth of the d arm. Figure 3-65.Magic-T with input to arm b. In summary, when
an input is applied to arm b of the magic-T hybrid junction, the output signals from arms a and c are
180 degrees out of phase with each other, and no output occurs at the d arm. The action that occurs
when a signal is fed into the d arm of the magic-T is illustrated in figure 3-66. As with the H-type T
junction, the signal entering the d arm divides and moves down the a and c arms as outputs that are
in phase with each other and with the input. The shape of the E
fields in motion is shown by the numbered curved slices. As the E field moves down the d arm,
points 2 and 3 are at an equal potential. The energy divides equally into arms a and c, and the E
fields in both arms become identical in shape. Since thepotentials on both sides of the b arm are
equal, no potential difference exists at the entrance to the b arm, resulting in no output.
Ques.18. Describe directional coupler.
Ans.DIRECTION COUPLER:-
Directional couplers are four-port circuits where one port is isolated from the
input port. Directional couplers are passive reciprocal networks All four ports are (ideally) matched, andthe circuit is (ideally) lossless. Directional couplers can be realized in microstrip, stripline, coax and
waveguide. They are used for sampling a signal, sometimes both the incident and reflected waves (this
application is called a reflectometer, which is an important part of a network analyzer). Directional
couplers generally use distributed properties of microwave circuits, the coupling feature is generally a
quarter (or multiple) quarter-wavelengths.
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A directional coupler has four ports, where one is regarded as the input, one is regarded as the "through"
port (where most of the incident signal exits), one is regarded as the coupled port (where a fixed fraction
of the input signal appears, usually expressed in dB), and an isolated port, which is usually terminated. If
the signal is reversed so that it enter the "though" port, most of it exits the "input" port, but the coupled
port is now the port that was previously regarded as the "isolated port". The coupled port is a function of
which port is the incident port.
Ques.20.Write short note on waveguide attenuators.
Ans. Waveguide Attenuators:-Wave guide attenuators are Low Power Fixed Attenuators, Low PowerVariable Attenuators and a range of Precision Variable Attenuators. Fixed Low Power attenuators. All of
the standard fixed attenuators are manufactured from selected waveguide tube.The attenuating element is
manufactured from a metallised glass fibre reinforced PTFE, resistive card vane or an absorptive
composite material. The vane version is supported in the waveguide using two metal rods and is
accurately positioned to give a desired value between 0 and 40dB as required. The composite absorber is
positioned and glued into the tube (the attenuation is based on thelength of the absorber
Variable Attenuators
Based upon the same construction as the Low Power Fixed Attenuators, the metalIised glass fibre
reinforced PTFE resistive card vane is positioned in the Waveguide using a backlash free, spring
controlled piston, precisely fitted in a machined housing to give a high degree of mechanical stability. The
Attenuation is varied by means of a knurled finger-control knob, and a locking screw is provided for
repetitive measurements, or, in the case of the variable precision devices, the attenuation is varied by
means of a standard micrometer drive.
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Ques .21. Explain circulator.
Ans. A waveguide circulator used as an isolator by placing a matched load on port 3. The label
on the permanent magnet indicates the direction of circulation.
A circulator is a passive non-reciprocal three- or four-port device, in which microwave or radio
frequency power entering any port is transmitted to the next port in rotation (only). Thus, to
within a phase-factor, the scattering matrix for an ideal three-port circulator is
When one port of a three-port circulator is terminated in a matched load, it can be used as an
isolator, since a signal can travel in only one direction between the remaining ports. There are
circulators for LF, VHF, UHF, microwave frequencies and for light, the latter being used in
optical fiber networks. Circulators fall into two main classes: 4-port waveguide circulators based
on Faraday rotation of waves propagating in a magnetized material, and 3-port "Y-junction"
circulators based on cancellation of waves propagating over two different paths near a
magnetized material. Waveguide circulators may be of either type, while more compact devicesbased on striplines are of the 3-port type. Sometimes two or more Y-junctions are combined in a
single component to give four or more ports, but these differ in behavior from a true 4-port
circulator.
In radar, circulators are used to route outgoing and incoming signals between the antenna, the
transmitter and the receiver. In a simple system, this function could be performed by a switch
that alternates between connecting the antenna to the transmitter and to the receiver. The use of
chirped pulses and a high dynamic range may lead to temporal overlap of the sent and received
pulses, however, requiring a circulator for this function.
Ques.12. Write down different types of directional couplers.
Ans. Different types of couplers are as follows:-
Forward versus backward wave couplers:-
Waveguide couplers couple in the forward direction (forward-wave couplers); a signal incident on port 1
will couple to port 3 (port 4 is isolated). Microstrip or stripline coupler are "backward wave" couplers. In
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the schematic above, that means for a signal incident on port 1, port 4 is the coupled port (port 3 is
isolated).
COUPLER RULE OF THUMB:-
The coupled port on a microstrip or stripline directional coupler is closest to the input port because it is a
backward wave coupler. On a waveguide broadwall directional coupler, the coupled port is closest to the
output port because it is aforward wave coupler.
The Narda coupler below is made in stripline (you have to cut it apart to know that, but just trust us),
which means it is a backward wave coupler. The input port is on the right, and the port facing up is the
coupled port(the opposite port is terminated with that weird cone-shaped thingy which voids thewarrantee if you remove it. Luckily Narda usually prints an arrow on the coupler to show how to use it,
but the arrow is on the side that is hidden in the photo.
On the waveguide coupler below, the input is on the left, while the coupled port is on the right, pointing
toward your left ear. There is a termination built into the guide opposite the coupled port, although you
can't see it.
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Bethe-hole coupler:-
This is a waveguide directional coupler, using a single hole, and is works over a
narrow band. In waveguide, a two-hole coupler, two waveguides share a broad wall. Holes are 1/4 wave
apart. In the foreword case the coupled signals add, in the reverse they subtract (180 apart) and disappear.
Coupling factor is controlled by hole size. The "holes" are often x-shaped, and...
Bi-directional coupler:-
A directional coupler where the isolated port is not internally terminated. You can use such a
coupler to form a reflectometer, but it is recommended (use the dual-directional coupler you cheapskate!)
Dual-directional coupler:-
Here we have two couplers in series, in opposing directions, with the isolated ports internally terminated.
This component is the basis for the reflectometer.
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Hybrid couplers
A hybrid coupler is a special case, where a 3 dB split is desired between the through path and the coupled
path. There are two types of hybrid couplers, 90 degree couplers (such as Langes or branchlines) and 180
degree hybrids (such as rat-races and magic tees).
Ques.22. What do you mean by Non reciprocal devices?
Ans. Non-reciprocal devices
A non-reciprocal circuit device comprising a first inductance element L1 disposed between a first
input/output port P1 and a second input/output port P2, a first capacitance element Ci parallel-connected
to the first inductance element L1 to constitute a first resonance circuit, a resistance element R parallel-connected to the first parallel resonance circuit, a second inductance element L2 disposed between a
second input/output port P2 of the first resonance circuit and a ground, a second capacitance element Cfa
parallel-connected to the second inductance element L2 to constitute a second resonance circuit, a third
inductance element Lg disposed between the second resonance circuit and the ground, and a third
capacitance element Cfb disposed between a second input/output port P2 of the first resonance circuit and
the ground.
Discription-
This invention relates to a non-reciprocal circuit device having non-reciprocal transmission characteristics
to high-frequency signals, particularly to a non-reciprocal circuit device suitable for mobilecommunications systems such as cellphones, etc.
Non-reciprocal circuit devices such as isolators are used in mobile communications equipments utilizing
frequencies from several hundreds MHz to several tens GHz, such as base stations and terminals of cell
phones, etc. In transmission systems ofmobile communications equipments, for instance, isolators are
disposed between power amplifiers and antennas to prevent unnecessary signals from returning to the
power amplifiers, thereby stabilizing the impedance of the power amplifiers on the loadside. Accordingly,
the isolators are required to have excellent insertion loss characteristics, reflection loss characteristics and
isolationcharacteristics.
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Cell phones have become handling wider frequency bands (wideband), and pluralities of
transmission/receiving systems such as WCDMA, PDC, PHS, GSM, etc. (multi-band, multi-system, etc.)
to adapt to increasing numbers of users. Accordingly,non-reciprocal circuit devices have been getting
required to be operable in wider frequency bands. One of data transmission technologies, which uses a
cell phone network for GSM and TDMA systems, is an enhanced data GSM environment (EDGE). When
twobands of GSM850/900 are used, a frequency passband required for the non-reciprocal circuit device is824-915MHz.
To obtain a wideband, non-reciprocal circuit device, various factors of causing unevenness, such as
inductance generated in lines connecting reactance elements, floating capacitance generated by
interference between electrode patterns, etc.,should be taken into consideration. In the two-port isolator,
however, unnecessary reactance components are connected to the first and second parallel resonance
circuits, resulting in the deviation of the input impedance of the two-port isolator fromthe desired level.
As a result, there appears impedance mismatching between the two-port isolator and the other circuits
connected thereto, leading to deteriorated insertion loss and isolation characteristics.
Accordingly, the first object of the present invention is to provide a non-reciprocal circuit device having a
wide operation frequency band.
The second object of the present invention is to provide a non-reciprocal circuit device with easy input
impedance matching, which has excellent insertion loss characteristics, reflection characteristics and
harmonicssuppression.
Ques.23. Give the characteristics of non reciprocal devices.
Ans. The characteristics of non reciprocal devices are as follows:-
1.A non-reciprocal circuit device comprising a first inductance element L1 disposed between a first
input/output port P1 and a second input/output port P2, a first capacitance element Ci parallel-connected
to said first inductance element L1 to constitute a first resonance circuit, a resistance element R parallel-
connected to said first parallel resonance circuit, a second inductance element L2 disposed between
asecond input/output port P2 of said first resonance circuit and a ground, and a second capacitance
element Cfa parallel-connected to said second inductance element L2 to constitute a second resonance
circuit, and a third inductance element Lg disposedbetween said second resonance circuit and the ground,
and a third capacitance element Cfb disposed between a second input/output port P2 of said first
resonance circuit and the ground.
2. The non-reciprocal circuit device according to claim 1, wherein said first inductance element L1 has
smaller inductance than that of said second inductance element L2.
3. The non-reciprocal circuit device according to claim 1, wherein at least one of the first capacitance
element Ci, the second capacitance element Cfa and the third capacitance element Cfb is constituted by
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Ans. A highly sensitive low-noise amplifier for ultrahigh-frequency and microwave radiosignals, utilizing as the active element an inductor or capacitor whose reactance is varied
periodically at another microwave or ultra-high frequency. A varactor diode is most commonlyused as the variable reactor. Amplification of weak signal waves occurs through a nonlinear
modulation or signal-mixing process which produces additional signal waves at otherfrequencies. This process may provide negative-resistance amplification for the applied signal
wave and increased power in one or more of the new frequencies which are generated. See alsoVaractor.
There are several possible circuit arrangements for obtaining useful parametric amplification.
The two most common are the up-converter and the negative-resistance amplifier. In both types,the pump frequency is normally much higher than the input-signal frequency. In the up-
converter, a new signal wave is generated at a higher power than the input wave. In the negative-resistance device, negative resistance is obtained for the input signal frequency, causing an
enhancement of signal power at the same frequency. See also Negative-resistance circuits.
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Ques.26. Explain the property of parametric amplifier due to which it is commonly used in receivers with
MASER.
Ans. The most important advantage of the parametric amplifier is its low level of noisegeneration. The parametric amplifier finds its greatest use as the first stage at the input of
microwave receivers where the utmost sensitivity is required. Its noise performance has beenexceeded only by the maser. Maser amplifiers are normally operated under extreme refrigeration
using liquid helium at about 4 K above absolute zero (452F). The parametric amplifier doesnot require such refrigeration but in some cases cooling to very low temperatures has been used
to give improved noise performance that is only slightly poorer than the maser.
We know that :-
Q[charge in a capacitor] = C x V
therefore
V [voltage across a capacitor] = Q/C
Figure 2-45A. - parametric amplifier. CIRCUIT
Figure 2-45B. - parametric amplifier.
The pump signal causes the capacitor in view (A) to vary at a 12-gigahertz rate. The 3-gigahertz
input signal enters via a four-port ferrite circulator, is developed in the signal cavity, and appliedacross the varactor. The nonlinear action of the varactor produces a 9-gigahertz differencefrequency (fp-fs) with an energy-level higher than the original input signal.
The difference (idler) frequency is reapplied to the varactor to increase the gain and to produce
an output signal of the correct frequency. The 9-gigahertz idler frequency recombines with the12-gigahertz pump signal and produces a 3-gigahertz difference signal that has a much larger
amplitude than the original 3-gigahertz input signal. The amplified signal is sent to the ferritecirculator for transfer to the next stage. As with tunnel-diode amplifiers, the circulator improves
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stability by preventing reflection of the signal back into the amplifier. Reflections would beamplified and cause uncontrollable oscillations. The ferrite circulator also serves as an isolator to
prevent source and load impedance changes from affecting gain. Typically, the gain of aparametric amplifier is about 20 dB. The gain can be controlled with a variable attenuator that
changes the amount of pump power applied to the varactor.
Parametric amplifiers are relatively simple in construction. The only component is a varactordiode placed in an arrangement of cavities and waveguides. The most elaborate feature of the
amplifier is the mechanical tuning mechanism. Figure 2-46 illustrates an actual parametricamplifier.
Ques.26. Explain Manley rowe power relation.
Ans. MANLEY ROWE RELATION:-
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Ques.27. Describe devise structure of IMPATT diode.
Ans. An IMPATT diode (IMPact ionization Avalanche Transit-Time) is a form of high
power diode used in high-frequency electronics and microwave devices. They are typically madewith silicon carbide owing to their high breakdown fields.
They operate at frequencies between about 3 and 100 GHz or more. A main advantage is theirhigh power capability. These diodes are used in a variety of applications from low
power radar systems to alarms. A major drawback of using IMPATT diodes is the high levelofphase noise they generate. This results from the statistical nature of the avalanche process
Nevertheless these diodes make excellent microwave generators for many applications
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Device structure:-
The IMPATT diode family includes many different junction and metal semiconductor devicesThe first IMPATT oscillation was obtained from a simple silicon p-n junction diode biased into a
reverse avalanche break down and mounted in a microwave cavity. Because of the strongdependence of the ionization coefficient on the electric field, most of the electronhole pairs are
generated in the high field region. The generated electron immediately moves into the N region,while the generated holes drift across the P region. The time required for the hole to reach the
contact constitutes the transit time delay.
The original proposal for a microwave device of the IMPATT type was made by Read andinvolved a structure. The Read diode consists of two regions (i) The Avalanche region (a region
with relatively high doping and high field) in which avalanche multiplication occurs and (ii) thedrift region (a region with essentially intrinsic doping and constant field) in which the generated
holes drift towards the contact. A similar device can be built with the configuration in whichelectrons generated from the avalanche multiplication drift through the intrinsic region.
An IMPATT diode generally is mounted in a microwave package. The diode is mounted with its
highfield region close to a copper heatsink so that the heat generated at the diode junction canbe readily dissipated. Similar microwave packages are used to house other microwave devices.
Ques.28. Explain the principle of working of IMPATT diode.
Ans. Impact ionization
If a free electron with sufficient energy strikes a silicon atom, it can break the covalent bond of
silicon and liberate an electron from the covalent bond. If the electron liberated gains energy bybeing in an electric field and liberates other electrons from other covalent bonds then this process
can cascade very quickly into a chain reaction producing a large number of electrons and a large
current flow. This phenomenon is called impact avalanche.
At breakdown, the n region is punched through and forms the avalanche region of the diode.
The high resistivity region is the drift zone through which the avalanche generated electronsmove toward the anode.
Consider a dc bias VB, just short of that required to cause breakdown, applied to the diode. Let
an AC voltage of sufficiently large magnitude be superimposed on the dc bias, such that duringthe positive cycle of the AC voltage, the diode is driven deep into the avalanche breakdown. At
t=0, the AC voltage is zero, and only a small pre-breakdown current flows through the diode.
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As t increases, the voltage goes above the breakdown voltage and secondary electron-hole pairsare produced by impact ionization. As long as the field in the avalanche region is maintain above
the breakdown field, the electron-hole concentration grows exponentially with t. Similarly thisconcentration decays exponentially with time when the field is reduced below breakdown
voltage during the negative swing of the AC voltage. The holes generated in the avalancheregion disappear in the p+ region and are collected by the cathode. The electrons are injected into
the i
zone where they drift toward the n+ region. Then, the field in the avalanche region reaches itsmaximum value and the population of the electron-hole pairs starts building up. At this time, the
ionization coefficients have their maximum values. The generated electron concentration doesnot follow the electric field instantaneously because it also depends on the number of electron-
hole pairs already present in the avalanche region. Hence, the electron concentration at this pointwill have a small value. Even after the field has passed its maximum value, the electron-hole
concentration continues to grow because the secondary carrier generation rate still remains aboveits average value. For this reason, the electron concentration in the avalanche region attains its
maximum value at, when the field has dropped to its average value. Thus, it is clear that theavalanche region introduces a 90o phase shift between the AC signal and the electron
concentration in this region.
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With a further increase in t, the AC voltage becomes negative, and the field in the avalancheregion drops below its critical value. The electrons in the avalanche region are then injected into
the drift zone which induces a current in the external circuit which has a phase opposite to that ofthe AC voltage. The AC field, therefore, absorbs energy from the drifting electrons as they are
decelerated by the decreasing field. It is clear that an ideal phase shift between the diode currentAnd the AC signal is achieved if the thickness of the drift zone is such that the bunch of electron
is collected at the n+ - anode at the moment the AC voltage goes to zero. This condition isachieved by making the length of the drift region equal to the wavelength of the signal. This
situation produces an additional phase shift of 90o between the AC voltage and the diode current.
Ques.29. Draw the V-I characteristics of IMPATT diode. Also write down its applications.
Ans.
IMPATT V-I CHARACTERISTICS
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Application:-
These IMPATT diodes makes excellent microwave generator for many applications:
y Parametric amplifier
y Parametric up converter
y Parametric down converter
y Negative resistance parametric amplifier
Ques.30. Explain working principle of TRAPATT diode.
Ans. It is derived from the IMPATT diode and is closely related to it.It is a high efficiencymicrowave generator capable of operating from several hundred MHz to several GHz. The basic
operation of the oscillator is a semiconductor pn junction diode reversw biased to current
densities well in excess of those encountered in normal avalanche operation
OPERATION
The basic scheme used to generate these picoseconds-kilovolt signals. A pulse generator withan amplitude larger than the breakdown voltage of the diode is applied to the diode in
the reverse direction. When the pulse is applied to the circuit the diode will first breakdown, i.e., the diode will look like a zener diode, and then if the amplitude of the driving
signal is large enough, so that a large current flows in the circuit, the diode will go intosecond breakdown Second breakdown can be thought of as a change in diode voltage from
the primary breakdown voltage to some much lower value. Since KVL must be maintained
in the circuit this is usually accompanied by an increase in the current flowing through thedevice. Destruction of the device is usually associated with second breakdown. However, if theamount of energy passed through the diode is limited, destruction is avoided. This usually means
narrow pulses,
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The apparent velocity of the plasma should be much larger than the saturation velocity ofelectrons and holes in the semiconductor for proper operation. The plasma is formed by exciting
electrons below the valence band into states above the conduction band. The result is generationof a gaseous conductor (plasma) in picoseconds. Externally this appears as a switch closing in
a time dependent on the plasma formation
Ques.31. Mention advantages and disadvantages of IMPATT diodes.
Ans. Advantages:
1. Its efficiency high as compared to IMPATT diode.
2. It can be used at high frequency from MHz to GHz.
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Disadvantages:
1. It used at high noise figure.
2. It used at upper microwave frequencies is high.
3. It generates strong harmonics due to short duration of current pulse.
Ques.32. Write down the applications of TRAPATT diode.
APPLICATION:
In pulse radar as local oscillator
In radio altimeter.
Air borne and marine radar.
In microwave beacons and landing system.
In low power Doppler radar.
Ques.33. Explain BARITT DIODE. Also write its applications.
Ans. BARITT DIODE (barrier injected transit time device):-
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When no bias voltage is applied, the electric field profile of this structure is determine by built-infield region of pn and np junction.
When the bias voltage is applied the condition is changed and one junction become forward
biased and one become reversed biased.The possibility offered by BARITT diode is restricted as8GHz as compared to IMPATT diode which is offered 100GHz.
But as in compared of TED performance BARITT diode is good as its high power efficiency.
One more advantage of BARITT diode is that it is simple to fabricate using a sophisticatedMaterial. They also have low noise.
APPLICATION:
They are primarily used for amplifier rather than oscillator because of lower
efficiencies.
Ques.34. Write the basic principle of klystron amplifier.
Ans. A klystron is a specialized linear-beam vacuum tube (evacuated electron tube). Klystrons are used as
amplifiers at microwave and radio frequencies to produce both low-power reference signals for
superheterodyne radar receivers and to produce high-power carrier waves for communications and the
driving force for modern particle accelerators.
Klystron amplifiers have the advantage (over the magnetron) of coherently amplifying a reference signal
so its output may be precisely controlled in amplitude, frequency and phase. Many klystrons have a
waveguide for coupling microwave energy into and out of the device, although it is also quite common
for lower power and lower frequency klystrons to use coaxial couplings instead. In some cases a coupling
probe is used to couple the microwave energy from a klystron into a separate external waveguide.
All modern klystrons are amplifiers, since reflex klystrons, which were used as oscillators in the past,
have been surpassed by alternative technologies.
The name klystron comes from the stem form - (klys) of a Greek verb referring to the action of
waves breaking against a shore, and the end of the word electron.
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During the second World War, the Axis powers relied mostly on (then low-powered) klystron technology
for their radar system microwave generation, while the Allies used the far more powerful but frequency-
drifting technology of the cavity magnetron for microwave generation. Klystron tube technologies for
very high-power applications, such as synchrotrons and radar systems, have since been developed.
Klystrons amplify RF signals by converting the kinetic energy in a DC electron beam into radio frequency
power. A beam of electrons is produced by a thermionic cathode (a heated pellet of low work functionmaterial), and accelerated by high-voltage electrodes (typically in the tens of kilovolts). This beam is then
passed through an input cavity. RF energy is fed into the input cavity at, or near, its natural frequency to
produce a voltage which acts on the electron beam. The electric field causes the electrons to bunch:
electrons that pass through during an opposing electric field are accelerated and later electrons are slowed,
causing the previously continuous electron beam to form bunches at the input frequency. To reinforce the
bunching, a klystron may contain additional "buncher" cavities. The RF current carried by the beam will
produce an RF magnetic field, and this will in turn excite a voltage across the gap of subsequent resonant
cavities. In the output cavity, the developed RF energy is coupled out. The spent electron beam, with
reduced energy, is captured in a collector.
Ques.35. Describe the working of two cavity Klystron amplifier.
Ans. Two-cavity klystron amplifier:-
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In the two-chamber klystron, the electron beam is injected into a resonant cavity. The electron beam,
accelerated by a positive potential, is constrained to travel through a cylindrical drift tube in a straight
path by an axial magnetic field. While passing through the first cavity, the electron beam is velocity
modulated by the weak RF signal. In the moving frame of the electron beam, the velocity modulation is
equivalent to a plasma oscillation. Plasma oscillations are rapid oscillations of the electron density in
conducting media such as plasmas or metals.(The frequency only depends weakly on the wavelength). So
in a quarter of one period of the plasma frequency, the velocity modulation is converted to densitymodulation, i.e. bunches of electrons. As the bunched electrons enter the second chamber they induce
standing waves at the same frequency as the input signal. The signal induced in the second chamber is
much stronger than that in the first.
Ques.36. How could you convert a two cavity klystron as an oscillator.
Ans. Two-cavity klystron oscillator:-
The two-cavity amplifier klystron is readily turned into an oscillator klystron by providing a feedback
loop between the input and output cavities. Two-cavity oscillator klystrons have the advantage of being
among the lowest-noise microwave sources available, and for that reason have often been used in theilluminator systems of missile targeting radars. The two-cavity oscillator klystron normally generates
more power than the reflex klystrontypically watts of output rather than milliwatts. Since there is no
reflector, only one high-voltage supply is necessary to cause the tube to oscillate, the voltage must be
adjusted to a particular value. This is because the electron beam must produce the bunched electrons in
the second cavity in order to generate output power. Voltage must be adjusted to vary the velocity of the
electron beam (and thus the frequency) to a suitable level due to the fixed physical separation between the
two cavities. Often several "modes" of oscillation can be observed in a given klystron.
Ques.37. Describe the working principle of reflex klystron.
Ans. In the reflex klystron (also known as a 'Sutton' klystron after its inventor), the electron beam passes
through a single resonant cavity. The electrons are fired into one end of the tube by an electron gun. After
passing through the resonant cavity they are reflected by a negatively charged reflector electrode for
another pass through the cavity, where they are then collected. The electron beam is velocity modulated
when it first passes through the cavity. The formation of electron bunches takes place in the drift space
between the reflector and the cavity. The voltage on the reflector must be adjusted so that the bunching is
at a maximum as the electron beam re-enters the resonant cavity, thus ensuring a maximum of energy is
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transferred from the electron beam to the RF oscillations in the cavity. The voltage should always be
switched on before providing the input to the reflex klystron as the whole function of the reflex klystron
would be destroyed if the supply is provided after the input. The reflector voltage may be varied slightly
from the optimum value, which results in some loss of output power, but also in a variation in frequency.
This effect is used to good advantage for automatic frequency control in receivers, and in frequency
modulation for transmitters. The level of modulation applied for transmission is small enough that the
power output essentially remains constant. At regions far from the optimum voltage, no oscillations areobtained at all. This tube is called a reflex klystron because it repels the input supply or performs the
opposite function of a klystron.
The reflex klystron contains a REFLECTOR PLATE, referred to as the REPELLER, instead of the output
cavity used in other types of klystrons. The electron beam is modulated as it was in the other types of
klystrons by passing it through an oscillating resonant cavity, but here the similarity ends. The feedback
required to maintain oscillations within the cavity is obtained by reversing the beam and sending it back
through the cavity. The electrons in the beam are velocity-modulated before the beam passes through the
cavity the second time and will give up the energy required to maintain oscillations. The electron beam is
turned around by a negatively charged electrode that repels the beam. This negative element is the
repeller mentioned earlier. This type of klystron oscillator is called a reflex klystron because of the reflex
action of the electron beam.
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Ques.38. Explain the effect of modes of operation of reflex klystron at its output .
Ans. The reflex klystron operates in a different mode for each additional cycle that the electrons remain in
the repeller field. Mode 1 is obtained when the repeller voltage produces an electron transit time of 3/4
cycle. Additional modes follow in sequence. Mode 2 has an electron transit time of 1 3/4 cycles; mode 3
has an electron transit time of 2 3/4 cycles; etc. The physical design of the tube limits the number of
modes possible in practical applications. A range of four modes of operation are normally available. Theactual mode used (1 3/4 cycles through 4 3/4 cycles, 2 3/4 cycles through 6 3/4 cycles, etc.) depends upon
the application. The choice of mode is determined by the difference in power available from each mode
and the band of frequencies over which the circuit can be tuned.
OUTPUT POWER. - The variation in output power for different modes of operation can be explained by
examining the factors which limit the amplitude of oscillations. Power and amplitude limitations are
caused by the DEBUNCHING process of the electrons in the repeller field space. Debunching is simply
the spreading out of the electron bunches before they reach electrostatic fields across the cavity grid . The
lower concentration of electrons in the returning bunches provides less power for delivery to the
oscillating cavity. This reduced power from the bunches, in turn, reduces the amplitude of the cavity
oscillations and causes a decrease in output power. In higher modes of operation the electron bunches areformed more slowly. They are more likely to be affected by debunching because of mutual repulsion
between the negatively charged electrons. The long drift time in the higher modes allows more time for
this electron interaction and, as a result, the effects of debunching are more severe. The mutual repulsion
changes the relative velocity between the electrons in the bunches and causes the bunches to spread out.
Figure 2-12. - Electronic tuning and output power of a reflex klystron.
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Electronic tuning does not change the center frequency of the cavity, but does vary the frequency within
the mode of operation. The amount the frequency can be varied above or below the center frequency is
limited by the half-power points of the mode, as shown in figure 2-12. The center frequency can be
changed by one of two methods One method, GRID-GAP TUNING, varies the cavity frequency by
altering the distance between the grids to change the physical size of the cavity. This method varies the
capacitance of the cavity by using a tuning screw to change the distance between the grids mechanically.
The cavity can also be tuned by PADDLES or SLUGS that change the inductance of the cavity.
Ques.39. Write the applications of klystron amplifier.
Ans. Klystrons produce microwave power far in excess of that developed by solid state. In modern
systems, they are used from UHF (hundreds of MHz) up through hundreds of gigahertz (as in the
Extended Interaction Klystrons in the CloudSat satellite). Klystrons can be found at work in radar,
satellite and wideband high-power communication (very common in television broadcasting and EHF
satellite terminals), medicine (radiation oncology), and high-energy physics (particle accelerators and
experimental reactors). At SLAC, for example, klystrons are routinely employed which have outputs in
the range of 50 megawatts (pulse) and 50 kilowatts (time-averaged) at frequencies nearing 3 GHz.
Ques.40. Write the working principle of TWT amplifiers.
Ans. Operation
Power supply arrangements for a typical travelling wave tube
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The power supply arrangements for a typical TWT are shown in above. The voltages andcurrents given are for a 10 W output tube, but the alignment details apply to almost all tubes.
However, manufacturers' data regarding electrode voltages and tube operating conditions should
always be referred to before running any part icular tube.
It is very important that a suitable matched load be connected to the output of the amplifier, asthe power reflected from any mismatch at the output is dissipated in the helix and can burn it out.
For the same reason the antenna must be properly matched.
The beam current is controlled by the grid-cathode voltage. In modern TWTS, the beam
focussing is preset and no adjustment is necessary, but if the focussing is adjustable the tubeshould be run initially at a low beam (collector) current, and the beam focussing magnets
adjusted for minimum helix current. The helix voltage should also be set for minimum helixcurrent. With the tube running at its specified collector current, RF drive can be applied. The
collector current will hardly change, but the helix voltage should be set for maximum outputconsistent with not exceeding the tube voltage or helix current ratings. If the focussing is
adjustable this should be readjusted for minimum helix current, since the RF drive will defocus
the beam slightly. As the helix is fragile and will not dissipate more than a certain power withoutdamage, the helix current should be metered, and a current trip incorporated to cut the powersupplies to the tube if the helix current becomes excessive. The EHT supplies to the tube should
be well smoothed, since ripple will phase-modulate the output and give a rough note.
If the collector dissipates more than about 100 W it may be necessary to use a blower to cool thecollector end of the tube. Typical efficiency of the TWTA is about 10 per cent, though some
modern tubts may reach 40 per cent.
The transfer characteristic of a travelling wave tube amplifier
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The transfer characteristic is essentially linear, which permits the tube to be used to amplify ssb-one of its great advantages in an amateur context. As the input is increased, however, the
amplifier saturates. There is no harm to the tube in operating at saturated output power, exceptthat amplification is no longer linear, although if appreciable harmonic power is generated this
may be reflected at the output transition and damage the helix through over dissipation. Theoutput from the amplifier can also be amplitude-modulated by a signal on the grid, but the
attendant phase modulation is quite high; this method is not normally used to produce a greatdepth of modulation, other than to operate the TWT in the pulsed mode. This is because at some
voltages between maximum and minimum output, beam interception by the helix occurs, whichcauses excessive helix dissipation unless the transitions are rapid. Phase modulation is obtained
by varying the helix voltage over a small range. Typically, plus or minus 100V from 2 kVnominal helix voltage will give 2 rad phase shift, with 1-2 dB reduction in output, which occurs
because the gain is very sensitive to cathode-helix voltage.
Ques.41. What do you mean by helix structure. Explain its utility to be used in TWT.
Ans. The essential principle of operation of a TWT lies in the interaction between an electronbeam and an RF signal. The velocity, v, of an electron beam is given by:
An anode voltage of 5 kV gives an electron velocity of 4.2 x 10*7 mso*-1. The signal wouldnormally travel at c, the velocity of light (3x10*8 ms*-1), which is much faster than any
'reasonable' electron beam (relativistic effects mean that the electron mass actually increases asits velocity approaches c, so that achieving electron velocities approaching c is a complicated
business), If, however, the signal can be slowed down to the same velocity as the electron beam,it is possible to obtain amplification of the signal by virtue of its interaction with the beam. This
is usually achieved using the helix electrode, which is simply a spiral of wire around the electronbeam,
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The geometery of the helix
Without the helix, the signal would travel at a velocity c. With the helix, the axial signal velocity
is approximately c x (p /2a) where a, p are shown above, so the signal is slowed by the factorp/2a. Note that this is independent of signal frequency. The signal travelling along the helix is
known as a slow wave, and the helix is referred to as a slow-wave structure, The condition forequal slow-wave and electron-beam velocities is therefore approximately
The interaction between the beam and the slow wave takes the form of 'velocity modulation' of
the beam (ie some electrons are accelerated and some retarded) forming electron bunches withinthe beam. The beam current therefore becomes modulated by the RF signal, and the bunches
react with the RF fields associated with the slow wave travelling down the helix, resulting in anet transfer of energy from the beam to the signal, and consequent amplification. Since there areno resonant structures involved in this interaction, amplification is obtained over a wide
bandwidth. In fact the principal factors which limit bandwidth are the input/output couplingarrangements.
It should also be mentioned that it is possible to construct an oscillator, utilizing the so-called
backward wave, whose energy travels in the reverse direction to the electron beam. These tubesare known as backward wave oscillators (BWOs) and have the advantage of a very wide tunable
range (an octave or more). They have been used extensively in swept frequency sources(sweepers), but are rapidly being displaced by Gunn diodes and, more recently, transistor
sources.
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Ques.42. Explain construction and operation of magnetron.
Ans. All cavity magnetrons consist of a hot cathode with a high (continuous or pulsed) negative
potential by a high-voltage, direct-current power supply. The cathode is built into the center ofan evacuated, lobed, circular chamber. A magnetic field parallel to the filament is imposed by a
permanent magnet. The magnetic field causes the electrons, attracted to the (relatively) positiveouter part of the chamber, to spiral outward in a circular path rather than moving directly to this
anode. Spaced around the rim of the chamber are cylindrical cavities. The cavities are open alongtheir length and connect the common cavity space. As electrons sweep past these openings, they
induce a resonant, high-frequency radio field in the cavity, which in turn causes the electrons tobunch into groups. A portion of this field is extracted with a short antenna that is connected to a
waveguide (a metal tube usually of rectangular cross section). The waveguide directs theextracted RF energy to the load, which may be a cooking chamber in a microwave oven or a
high-gain antenna in the case of radar.
The sizes of the cavities determine the resonant frequency, and thereby the frequency of emitted
microwaves. However, the frequency is not precisely controllable. The operating frequency varies with
changes in load impedance, with changes in the supply current, and with the temperature of the tube.[5]
This is not a problem in uses such as heating, or in some forms of radar where the receiver can be
synchronized with an imprecise magnetron frequency. Where precise frequencies are needed, other
devices such as the klystron are used.
The magnetron is a self-oscillating device requiring no external elements other than a power supply. A
well-defined threshold anode voltage must be applied before oscillation will build up; this voltage is a
function of the dimensions of the resonant cavity, and the applied magnetic field. In pulsed applications
there is a delay of several cycles before the oscillator achieves full peak power, and the build-up of anode
voltage must be coordinated with the build-up of oscillator output.[5]
The magnetron is a fairly efficient device. In a microwave oven, for instance, a 1.1 kilowatt input will
generally create about 700 watt of microwave power, an efficiency of around 65%. (The high-voltage and
the properties of the cathode determine the power of a magnetron.) Large S-band magnetrons can produce
up to 2.5 megawatts peak power with an average power of 3.75 kW.[5] Large magnetrons can be water
cooled. The magnetron remains in widespread use in roles which require high power, but where precise
frequency control is unimportant.
Ques.43. Write the application of magnetron.
Ans. In radar devices the waveguide is connected to an antenna. The magnetron is operated with very
short pulses of applied voltage, resulting in a short pulse of high power microwave energy being radiated.
As in all radar systems, the radiation reflected off a target is analyzed to produce a radar map on a screen.
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Several characteristics of the magnetron's power output conspire to make radar use of the device
somewhat problematic. The first of these factors is the magnetron's inherent instability in its transmitter
frequency. This instability is noted not only as a frequency shift from one pulse to the next, but also a
frequency shift within an individual transmitter pulse. The second factor is that the energy of thetransmitted pulse is spread over a wide frequency spectrum, which makes necessary its receiver to have a
corresponding wide selectivity. This wide selectivity permits ambient electrical noise to be accepted into
the receiver, thus obscuring somewhat the received radar echoes, thereby reducing overall radar
performance. The third factor, depending on application, is the radiation hazard caused by the use of high
power electromagnetic radiation. In some applications, for example marine radar mounted on a
recreational vessel, a radar with a magnetron output of 2 to 4 kilowatts is often found mounted very near
an area occupied by crew or passengers. In practical use, these factors have been overcome, or merely
accepted, and there are today thousands of magnetron aviation and marine radar units in service. Recent
advances in aviation weather avoidance radar and in marine radar have successfully implemented
semiconductor transmitters that eliminate the magnetron entirely.
Heating:-In microwave ovens the waveguide leads to a radio frequency-transparent port into the cooking
chamber.
Lighting:-In microwave-excited lighting systems, such as a sulfur lamp, a magnetron provides the
microwave field that is passed through a waveguide to the lighting cavity containing the light-emitting
substance (e.g., sulfur, metal halides, etc.)
Ques.44. Write the advantages of TWT over its disadvantages.
Ans. The problem unfortunately still remains that TWTS are very difficult to acquire.
Nevertheless, their high power output, high gain, and ease of operation make them the ideal wayto run power at frequencies above about 4 GHz, where conventional tubes like the 2C39A run
out of steam, and they represent practically the only way to run high-power SSB. It is relativelyeasy to generate the 1 mW or so of SSB at microwave frequencies required to drive most TWTS
[3], which will produce over 1 W of power. This, in conjunction with high-gain antennas,permits advantage to be taken of tropospheric scatter as a reliable propagation mode.
Ques.45. Explain transfer characteristics of TWT.
Ans. If the collector dissipates more than about 100 W it may be necessary to use a blower tocool the collector end of the tube. Typical efficiency of the TWTA is about 10 per cent, though
some modern tubts may reach 40 per cent.
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The transfer characteristic of a travelling wave tube amplifier
The transfer characteristic is essentially linear, which permits the tube to be used to amplify ssb-
one of its great advantages in an amateur context. As the input is increased, however, theamplifier saturates. There is no harm to the tube in operating at saturated output power, except
that amplification is no longer linear, although if appreciable harmonic power is generated thismay be reflected at the output transition and damage the helix through overdissipation.
The output from the amplifier can also be amplitude-modulated by a signal on the grid, but the
attendant phase modulation is quite high; this method is not normally used to produce a greatdepth of modulation, other than to operate the TWT in the pulsed mode. This is because at some
voltages between maximum and minimum output, beam interception by the helix occurs, whichcauses excessive helix dissipation unless the transitions are rapid. Phase modulation is obtained
by varying the helix voltage over a small range. Typically, plus or minus 100V from 2 kVnominal helix voltage will give 2 rad phase shift, with 1-2 dB reduction in output, which occurs
because the gain is very sensitive to cathode-helix voltage.
It