MICROWAVE ENGINEERING

EKT 345

CHAPTER 3:

MICROWAVE TUBES

Outline:

3.1 Frequency Limitation of Conventional

Tubes

3.2 Traveling wave tube (TWT) – Electron gun,

RF Interaction circuit, Electron Beam

Focusing, Terms, analysis and applications

3.3 Klystrons - Multi Cavity, Reflex

3.4 Magnetrons – Conventional, Coaxial

3.1 Frequency Limitation of Conventional Tubes

� Conventional tubes :

� Electronic tubes: Triodes, tetrodes, pentodes

� Low output power

� Low microwave frequencies – failed to operate above 1 GHz

� Frequency rise affects:

Inter electrode

capacitance

Lead

inductanceElectron transit

time

Cont’d…

Cont’d…

� Microwave Tubes:

� Linear beam tubes (O-type)

� Dc magnetic field is in parallel with the dc

electric field.

� Crossed-field tubes (M-type)

� Dc electric field and the dc magnetic field

are perpendicular to each other.

Cont’d…

Cont’d…

3.2 Traveling wave tube (TWT)

� Travelling Wave Tube Amplifier:

� High gain > 40 dB

� Low NF < 10 dB

� Wide Band > Octave

� Frequency range:

0.3 – 50 GHz

� Contains electron gun, RF interaction circuit, electron beam focusing magnet, collector

� Amplify a weak RF input signal many thousands of times

Cont’d…

a) Electron gun

� To get as much electron current flowing

into as small a region as possible without

distortion or fuzzy edges

� Sources of electrons for the beam

� 6 elements : gun shells, heater, cathode,

control grid, focus electrode and anode

Cont’d…

b) RF interaction circuit

� Interaction structures : helix, ring bar, ring

loop, coupled cavity

� RF circuit – complex trade off analysis, based

on many interlocking parameters

� Low power level : helix

� Medium power level : ring loop, ring bar

� Power level & frequency increased: RF losses

on the circuit become more appreciate able.

Cont’d…

c) Electron beam focusing� A magnetic field – to hold the electron beam together as it travels through the interaction structure of the tube

� The beam tends to disperse or spread out as a result of the natural repulsive forces between electrons.

� Methods of magnetic focusing

� Solenoid magnetic structure

� Permanent magnet

� Periodic permanent magnet (PPM)

� Radial magnet PPM

Cont’d…

d) The collector

� To dissipate the electrons in the form of

heat as they emerge from the slow wave

structure

� Accomplished by thermal conduction to a

colder outside surface – the heat is

absorbed by circulated air or a liquid

Terms, Analysis and applicationsTerms

1. Gain compression� the amount of gain decrease from the small signal

condition (normally 6dB)

2. Beam Voltage� the voltage between the cathode and the RF structure

3. Synchronous Voltage� the beam voltage necessary to obtain the greatest

interaction between the electrons in the electron beam and the RF wave on the circuit

4. Gain� the ratio of RF output power to RF input power (dB)

5. Phase Characteristic� Phase shift – the phase of output signal relative to the

input signal

� Phase sensitivity – the rate of phase change with a specific operating parameter

Cont’d…

Analysis : TWTA

Cont’d…

Gain Characteristic

Cont’d…

� Applications

� in medium power and higher power

satellite transponder output

� electronic counter measure system (ECM)

� military applications – phased array radar

3.3 Klystrons

� In microwave region, performs the

functions of generates, receives and

amplifies signals

� Configurations:

� Reflex – low power microwave oscillator

� Multicavity – low power microwave

amplifier

Cont’d…

a) Reflex Klystron

- has a reflector and one cavity as

a resonator

- reflex action of electron beam

- performance

- Frequency range: 2-200 GHz

- BW: ± 30 MHz for ∆VR: ±10 V

- Power o/p: 10mW – 2.5W

- used as microwave source in lab,

microwave transmitter

- frequency modulation and

amplitude modulation

Cont’d…

� Mechanism of oscillation

� The electron passing

through the cavity gap -

experience the RF field

� velocity modulated

Cont’d…

� a: Electrons which

encountered the positive half

cycle of the RF field in the

cavity gap will be

accelerated

� b: Electrons which

encountered zero RF field

will pass with unchanged

original velocity

� c: Electrons which

encountered the negative

half cycle will be retarded

and entering the repeller

space.

Cont’d…

b) Multicavity Klystron

� Two cavity Klystron Amplifier

� Assumption for RF amplification

� Transit time in the cavity gap is very small compared to the period of the input RF signal cycle

� Input RF signal amplitude is very small compared to the dc beam voltage

� The cathode, anode, cavity grids and collectors are all parallel

� No space charge or debunching take place at the bunch point

� The RF fields are totally confined in the cavity gaps, zero in the drift space

� Electrons leave the cathode with zero initial velocity

Cont’d…

Cont’d…

Analysis : Reflex Klystrons

Cont’d…

Analysis : Multicavity Klystrons

Cont’d…

� Three cavity klystron

amplifier

� Addition of

intermediate cavities

between the input and

output cavities :

improve amplification,

power output,

frequency BW and

efficiency.

Cont’d…

Comparison between TWT Amplifier

and Klystron Amplifier

3.4 Magnetrons

� Magnetrons oscillators:

� to generate high microwave power required in radar and communication systems.

� Crossed-field tube (M type)

� High power oscillator uses a TW cylindrical magnetron tubes

� Can deliver a peak power o/p up to 40MW, dc voltage 50kV at 10GHz

� Efficiency : 40 to 70%

� Applications: radar transmitters, industrial heating, microwave oven (standard power=600W, frequency= 915MHz or 2450 MHz)

Cont’d…

Cont’d…

a) Conventional magnetrons:

� a type of diode in which magnetic field is set up perpendicular to the electric field existing between the cathode and the plate

� Basic types

� Cyclotron-frequency� Operates by virtue of resonance between the period of RF

oscillation and the rotational motion of the electron

� Negative resistance� Operates on the principle of a static negative resistance

characteristic between the anode sections

� Travelling wave � Depends upon the mean velocity of the electrons being

synchronized with the velocity of the travelling wave component of the RF interaction field between the cathode and anode

Cont’d…

b) Coaxial magnetrons:� An extension of conventional magnetron structure by the addition of a third element

� A coaxial cavity that surrounds the anode forming its inner wall

� The operation frequency is determined by the resonant frequency of the surrounding cavity.

� Physically larger and heavier than conventional magnetrons.

� Pushing and pulling factors

Cont’d…

Advantages of Coaxial over Conventional

� Positive control of operating mode

� Very high Q, giving low pulling, low pushing, good spectrum and high efficiency

� Absence of straps in the structure

� Large internal volume giving good storage life

� Ease of tuning with no degradation of magnetron performance

Cont’d…

Analysis : Magnetrons