MICROWAVE THEORY AND LABORATORY

QUESTION BANK 1 What are the types of microwave transmission lines used in microwave circuits? 2 Why Scattering Matrix is required? 3 Explain the principle of operation of H Plane Tee

4 Explain the principle of operation of E plane Tee 5 Explain the principle of operation of Magic Tee 6 Explain the principle of operation of Directional Coupler. 7 Derive scattering matrix for H plane Tee 8 Derive Scattering Matrix for E plane Tee 9 Derive Scattering Matrix for Magic Tee

10 Derive Scattering Matrix for Directional Coupler

11 Show that a 3-Port network cannot be matched in all the three ports 12 Write a note on Windows 13 Write a note on Posts 14 Write a note on Tuning Screws 15 Write a note on Coupling Probes 16 Write a note on Bends

17 Write a note on Phase Shifters 18 With neat diagrams explain the operation of a short circuit plunger 19 Explain how waveguide irises can be used to realize resonant effects. 20 Explain how waveguide Irises can be used to realize capacitance. 21 Explain the working of a dielectric phase shifter. 22 Mention the applications of Magic Tee 23 Mention the applications of Directional Coupler

24 Mention applications of Cavity resonator. 25 Two identical 30 dB directional couplers are used in a waveguide to sample incident

and reflected powers. The outputs of two couplers are 120 and 25 watts respectively. Find the VSWR.

26 The distance between two minima points in a slotted waveguide is 1 cm. The measured VSWR at these points is 2.56. Find the true VSWR.

27 Calculate the SWR of a transmission system operating at 9 GHz. The waveguide

transmission system has dimensions = (4 * 3 cm). The distance measured between twice minimum points is 1.2 mm on a slotted line. Assume TE 1.0 mode. Also find its reflection coefficient.

28 With a neat diagram explain the construction of a ridge wave guide . What are the advantages and disadvantages of ridge waveguide over ordinary rectangular waveguide.

29 What is a wave guide impedance transformer ?

30 With neat diagrams explain the operation of a short circuit plunger 31 What are the properties of S matrix ? Derive the S matrix representation of a three

port circulator

32 Give the constructional features and working principle of Isolator and Magic Tee 33 Explain the working of Bethe Hole Coupler with its S Matrix representation 34 What is scattering matrix ? State and explain the properties of a scattering matrix.

35 With the help of scattering matrix, explain the properties of a Magic Tee. 36 Explain how magic tee is used as a duplexer in radar system. 37 What are the problems faced by the active devices while operating at Microwave

Frequencies.

38 Discrete passive components cannot be used at microwave frequencies. Justify the statement.

39 Explain the transit time effect at high frequencies of conventional tubes. How is this

eliminated in microwave tubes .

40 Explain the effect of circuit Reactance on the performance of the Conventional Electron Tubes at HUF.

41 If the useful range of a triode is limited to a point where the Transit Time becomes 1/10th of the time period, calculate upto what frequency one can operate a tube with cathode anode spacing of 5 mm when the anode voltage is 250 volts.

42 How Applegate Diagram helps in explaining the working of Reflex Klystron.

43 With the help of a neat diagram explain the operating principles of a Reflex Klystron Oscillator. Compare its performance with BWO.

44 What devices depend on the slow wave for their operation? What devices depend on magnetic field interaction for their operation.

45 What are the advantages of more than two cavities in a Klystron? 46 Describe the two cavity Klystron amplifier, with the aid of schematic diagrams. 47 With the help of a neat diagram explain the operating principles of Travelling Wave

Tube.

48 How does velocity modulation take place in Klystron? Explain with Applegate diagrams 49 How does a traveling wave tube Amplifier work? Explain with neat sketches

50 A TWT has the following characteristics .Beam voltage Vo=2KV Beam current IO = 4 mA frequency = 8 GHz length N= 50 and characteristic impedance ZO = 20 ohms Determine the gain parameter and the power gain in decibels

51 What are the differences between Klystron Amplifier and TWTA 52 Explain the working of TWTA 53 Explain how varactor diode can be used as degenerate amplifier 54 Describe the construction, working and application of Gunn Diode. 55 An n type GaAs Gunn Diode has the following parameters.

Electron Drift Velocity –2.5 * 1005 m/s Negative Electron Mobility - 0.014 m2 /vs

Relative dielectric constant = 13.8 Determine the criterion for classifying modes of operation

56 Explain Gunn Effect. Why Gunn Devices are called Diodes. 57 With a neat sketch explain the construction of a Gunn diode.

58 Explain how Gunn diode is used as an oscillator. 59 The drift velocity of electrons is 2x 10e 7 cm/sec through the active region of length

.001 cm. Calculate the natural frequency of the Gunn diode and critical voltage.

60 How are Gunn domains formed? How does the domain formation lead to microwave generation?

61 A specimen of n type GaAs used to fabricate a Gunn Diode has a length = 200 m and

cross-sectional are =3.5 * 103 cm2. What should be the voltage applied to get a Threshold field of 2800 V/cm.

62 What are the applications of Gunn Oscillators and Amplifiers? 63 With the aid of a suitable sketch describe the construction and operation of PIN Diode. 64 Discuss the performance and applications of Schottky Barrier Diode

65 Draw the schematic diagram of an IMPATT Diode and fully explain the two effects that combine to produce 180 degree phase difference between the applied voltage and

resulting current pulse.

66 Briefly describe the basic operation mechanism of TRAPATT Diode using suitable diagram.

67 Compare IMPATT, TRAPATT & GUNN Oscillators. 68 Explain the different modes of operation of a Gunn diode

69 Describe the construction of a PIN diode switch 70 Discuss the principle of operation of IMPATT oscillator 71 What are transit time devices? Explain at least two of them 72 Explain the following

(i) Radiometry (ii) Satellite communication (iii) Hazards of microwave communication

73 Write short notes on a) TRAPATT (b) Microwave radiation hazards (c) Microwave heating

Updated as on 16/07/2012

Q.1 Choose the correct or the best alternative in the following: (2 × 10) a. Impedance inversion may be obtained with : (A) a short-circuited stub (B) an open circuited stub

(C) a quarter wave line (D) a wave line b. In order to couple two generators to a waveguide system without coupling them to each other , one could not use a : (A) Rat race (B) E-plane T (C) Hybrid ring (D) Magic T c. For low attenuation, the best transmission medium is : (A) flexible waveguide (B) ridged wave-guide

(C) rectangular waveguide (D) coaxial line

d. A PIN diode is : (A) A metal semiconductor point-contact diode. (B) A microwave mixer diode. (C) Often used as a microwave detector (D) Suitable for use as a microwave switch. e. The gain bandwidth frequency of a microwave transistor fT , is the frequency

at which : (A) Alpha of transistor falls by 3dB (B) Beta of transistor falls by 3dB (C) Power gain of transistor falls to unity (D) Beta of transistor falls to unity

f. The main disadvantage of the two-hole directional coupler is : (A) low directional coupling (B) poor directivity

(C) narrow bandwidth (D) high SWR g. Calculate the output power of given circuit, if input power is 1mW

(A) 0 dBm. (B) 66 dBm.

(C) 10 dB (D) None of these h. A wave guide can be considered as : (A) Band pass filter (B) Band stop filter (C) Low pass filter (D) High pass filter i. If in a rectangular waveguide for which a = 2b, the cutoff frequency for TE02 :

mode is 12 GHz, the cutoff frequency for TM11 mode is (A) 3_5 GHz (B) 3 GHz (C) 12 GHz (D) 6_5 GHz j. ___________is directly measured in sweep reflectometer (A) SWR (B) Impedance (C) Return loss (D) reflection coefficient

Answer any FIVE Questions out of EIGHT Questions. Each question carries 16 marks. Q.2 a. Starting from the two time harmonic Maxwell’s curl equation in cylindrical co-ordinates. Express the transverse field components Er, EØ, Hr, and HØ in

terms of the longitudinal components Ez and Hz. What equation must Ez and Hz satisfy. (8) b. Explain Rectangular Cavity Resonators and derive an expression for its resonant frequency. (8) Q.3 a. A 200 volt (rms) generator having an internal resistance of 200W is feeding a lossless transmission line. The characteristic impedance and the length of the line are 200W and 10 m respectively. The line is terminated

in a resistive load of 100W . If the operate frequency is 37.5 MHz. Calculate the (i) Current drawn from the generator.

(ii) Magnitude and phase of the current flowing in the load and (iii) Power delivered to the load. (8) b. Briefly explain the stub matching technique. (8)

Q.4 a. What are directional couplers? Define directivity, coupling factor, isolation and insertion loss of directional couplers. Also, discuss in brief Bethe hole coupler and bi-directional couplers. (8) b. A symmetrical directional coupler with infinite directivity and a forward attenuation of 20 dB is used to monitor the power delivered to a load ZL . Bolometer 1 introduces a VSWR of 2 on arm 4; Bolometer 2 is matched to arm 3. If Bolometer 1 reads 8mW and bolometer2 reads 2mW, find the

amount of power dissipated in the load ZL and VSWR on arm 2. (8)

Q.5 a. What is scattering matrix? Discuss the following properties of ‘S’ matrix

(i) Symmetry property (ii) Unitary property (iii) Phase shift property (8) b. Prove with the help of Maxwell’s equations that TEM mode cannot exist in rectangular waveguide. (8)

Q.6 a. With the help of two-valley model along with the emphasis of drift

velocity, explain the negative resistance property of a gunn diode. (10) b. Write a short note on MASERS. (6) Q.7 a. Describe how can the power of a microwave generator be measured using Bolometer. (8) b. (i) A varactor diode has a cut off frequency of 2450 MHz. What is the operating Q at a frequency of 54 MHz? (ii) A silicon varactor has a zero potential capacitance of 100 pF. If the m

factor is ½, what is the capacitance when the applied voltage is –4 V d.c.? Assume a barrier potential of 0.65 V and K=1. Q.8 a. Briefly explain the concept of Duct propagation. (8) b. A rectangular air filled copper wave guide with dimension 0.9 inch × 0.4 inch cross section and 12¢¢ length is operated at 9.2 GHz with dominant

mode. Find

(i) Cut-off frequency (ii) Guide wavelength (iii) Phase velocity (iv) Characteristic Impedance. (8)

Q.9 Write short notes on any TWO: (i) Critical frequency (ii) Skip distance (iii) Maximum usable frequency (2×8)

Q.1 Choose the correct or the best alternative in the following: (2×10) a. A transmission line has following parameters R = 2 _/m, G = 0.5 m mho/m, L = 8 nH/m, C = 0.23 pF, f = 1GHz, Its characteristic impedance is given by ______. (A) 50 _ (B) 75 _ (C) 100 _ (D) 179.44+j26.50

b. A certain transmission line has a characteristic impedance of 75 + j0.01 _ and is terminated in a load impedance of 70 + j50 _. The reflection coefficient is _____ (A) 0.8+j40 (B) 0.6+j50

(C) 0.9+j60 (D) 0.08+j0.32 c. A micro wave circulator is a multiport junction where the power can flow from _______

(A) port(1) to port(2) and port(2) to port(3) etc (B) port(3) to port(2) and port(2)to port(1) (C) port 1 to all other ports (D) microwave circulators cannot be used for power carrying purposes d. A two cavity Klystron is a widely used microwave amplifier operated by ______ and ________ . (A) Velocity and current modulation

(B) Electron motion (C) On same principals as low frequency tubes (D) Slow wave structure e. Crossed field tubes derive their names from the fact that

(A) D.C electric field and D.C magnetic field are perpendicular to each other (B) A.C magnetic field and A.C electric field are horizontal to each other (C) A.C magnetic field and A.C electric field are perpendicular to each other

(D) There is no relation between A.C magnetic field and A.C electric field f. X band pulsed cylindrical magnetron has magnetic flux density BO = 0.336 wb/mt2, its cyclotron angular frequency is ________. (A) 5.91×1010 rad (B) 11×1010 radians (C) 5×105 rad (D) 6×105 radians g. A certain Si JFET has the following parameters channel height a = 0.1μm, Electron

concentration Nd = 8×1017cm-3 Relative dielectric constant _r = 11.8 Then the pinch off voltage is given by ______. (A) 60 volts (B) 106.6 volts (C) 88 volts (D) 6.66 volts

h. The phase velocity of a TEM wave can be expressed by the relation which is the velocity of light in an unbounded dielectric is given by __________. (A) Vp = w/_g (B) Vp = w/_o

(C) Vp = w/wo (D) Vp = _g/ _o i. The tunnel diode is a ____________ resistance semiconductor p-n junction diode (A) positive (B) negative (C) high (D) low j. In a directional coupler all four ports are completely matched, and then diagonal elements of the S matrix are given by ________. (A) S11 = S22 = S33 = S44 = 0 (B) S11= S22 = S33 = S44 =1/_2

(C) S11= S22 = S33 = S44 =1 (D) S11= S22 = S33 = S44 =3/2 Answer any FIVE Questions out of EIGHT Questions. Each question carries 16 marks. Q.2 a. With the help of transmission line equation derive the expression for

characteristic impedance Z0 and phase velocity Vp. (6) b. A transmission line has a characteristic impedance of 50+j0.01_ and is

terminated in a load impedance of 73-j42.5_. Calculate the (i) Reflection coefficient (ii) The standing wave ratio (6)

c. Write a note on single stub matching (4) Q.3 a. Explain power is losses in rectangular waveguides. (8) b. An air filled rectangular wave guide of inside dimensions 7×3.5 cms operates in the dominant TE10 mode as shown in Fig.1

(i) Find cut off frequency (ii) Phase velocity of wave at 3.5GHz (iii) _g, the guide wave length (8)

Q.4 a. What are directional couplers, explain with a neat diagram. Derive an expression for an S matrix of directional coupler (8) b. Writ short notes on: (i) Microwave Hybrids (ii) Microwave Isolator (8)

Q.5 a. Explain the principles of operation of microwave tunnel diode. (6)

b. Draw a neat diagram of TRAPATT diode; explain the principle of operation with neat figures (4) c. Avalanche zone velocity of a TRAPATT diode has following parameters. Doping concentration NA = 2×1015 cm-3, current density J = 20 KA/cm2. Calculate the avalanche-zone velocity. (6) Q.6 a. Write the schematic diagram of two cavity klystron amplifier and explain the

velocity modulation process (8) b. A two cavity Klystron has following parameters Vo = 1000 volts, Ro = 40 K_, Io = 25 mA, f = 3GHz. The gap spacing in either cavity is d = 1mm spacing between two cavities L = 4cms, Effective shunt impedance, excluding beam loading Rsh = 30K_. Calculate the efficiency of the amplifier neglecting beam loading. (8) Q.7 a. Draw the schematic diagram of a cylindrical magnetron oscillator and explain its

action (6)

b. Obtain Hull cut off magnetic equation and cut off voltage for cylindrical magnetron. (5) c. An X band pulsed cylindrical magnetron has the following operating parameters. Anode voltage Vo = 26 K volts, Beam current Io = 27 A, Magnetic flux density Bo = 0.336 wb/m2. Radius of cathode cylinder a = 5 cms, Radius of

vane edge to center = b = 10 cms, compute the (i) cyclotron angular fr (ii) Cut off voltage for a fixed Bo (iii) The cut off magnetic flux density (5) Q.8 a. Explain the microstrip lines and the associated losses. (8) b. A lossless parallel strip line has strip width W, _rd = 6 and thickness d = 4 mm. Calculate (i) Required width W of the conducting strip in order to have a characteristic impedance of 50 _ (ii) Strip line capacitance (iii) Strip line impedance (iv) The phase velocity in parallel strip line (8)

Q.9 a. Write short note on (i) Monolithic microwave integrated circuit

(ii) DC Sputtering (8) b. Briefly explain MMIC fabrication techniques. (8)