microwave engineering ekt 345 - unimap portalportal.unimap.edu.my/portal/page/portal30/lecturer...
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![Page 1: MICROWAVE ENGINEERING EKT 345 - UniMAP Portalportal.unimap.edu.my/portal/page/portal30/Lecturer Notes... · CHAPTER 3: MICROWAVE TUBES Outline: 3.1 Frequency Limitation of Conventional](https://reader030.vdocuments.site/reader030/viewer/2022012322/5a9df6647f8b9ada718bce38/html5/thumbnails/1.jpg)
MICROWAVE ENGINEERING
EKT 345
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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
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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
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Cont’d…
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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.
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Cont’d…
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Cont’d…
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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
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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
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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.
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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
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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
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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
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Cont’d…
Analysis : TWTA
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Cont’d…
Gain Characteristic
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Cont’d…
� Applications
� in medium power and higher power
satellite transponder output
� electronic counter measure system (ECM)
� military applications – phased array radar
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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
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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
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Cont’d…
� Mechanism of oscillation
� The electron passing
through the cavity gap -
experience the RF field
� velocity modulated
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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.
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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
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Cont’d…
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Cont’d…
Analysis : Reflex Klystrons
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Cont’d…
Analysis : Multicavity Klystrons
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Cont’d…
� Three cavity klystron
amplifier
� Addition of
intermediate cavities
between the input and
output cavities :
improve amplification,
power output,
frequency BW and
efficiency.
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Cont’d…
Comparison between TWT Amplifier
and Klystron Amplifier
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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)
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Cont’d…
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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
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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
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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
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Cont’d…
Analysis : Magnetrons
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