LIMITATIONS AT MICROWAVE

FREQUENCIES
 INTRODUCTION
• Generation of MW’s done using
– Vacuum tube technology
– Semiconductor technology
• At MW freq size becomes smaller – less power
handling capacity
• Conventional vacuum tubes such as triodes,
tetrodes and pentodes are only useful till low MW
freq
 VACUUM TUBES : TRIODES
• In the triode, electrons are released
into the tube from the
metal cathode by heating it, a process
called thermionic emission.
• The cathode is heated red hot by a
separate current flowing through a thin
metal filament.
• Virtually all the air is removed from the
tube, so the electrons can move freely.
• The negative electrons are attracted to
the positively-charged plate (anode),
and flow through the spaces between
the grid wires to it, creating a current
through the tube from cathode to
plate.
 VACUUM TUBES : TRIODES
• The magnitude of this current can be controlled by a voltage
applied between the cathode and the grid.
• A more negative voltage on the grid will repel some of the
electrons, so fewer get through to the plate, reducing the plate
current.
• A positive voltage on the grid will attract more electrons from the
cathode, so more reach the plate, increasing the plate current.
• Therefore a low power varying (AC) signal applied to the grid can
control a much more powerful plate current; resulting
in amplification.
• Variation in the grid voltage will cause identical proportional
variations in the plate current. By placing a suitable load resistance
in the plate circuit, the varying current will cause a varying voltage
across the resistance which is much larger than the input voltage
variations, resulting in voltage gain.
 VACUUM TUBES : TRIODES
• The triode is a normally "on" device; and current flows to the plate with
zero voltage on the grid.
• The plate current is progressively reduced as the grid is made more
negative with respect to the cathode.
• Usually a constant DC voltage ("bias") is applied to the grid to set the DC
current through the tube, and the varying signal voltage is superimposed
on it.
• A sufficiently negative voltage on the grid, usually around 3-5 volts, will
prevent any electrons from getting through to the plate, turning off the
plate current. This is called the "cutoff voltage".
• Since below cutoff the plate current ceases to be proportional to the grid
voltage, the voltage on the grid must remain above the cutoff voltage for
faithful (linear) amplification.
 ENERGY TRANSFER MECHANISM
• Amplifiers & oscillators transfer power from a DC
supply to AC signal.
• MW Signal placed in cavity gap & electrons are
forced to cross the gap at times when they face max
opposition.
• Max tfr of energy from electrons to signal across
cavity gap when the field due to signal opposes
electron motion.
 LIMITATIONS AT HIGH FREQ
The efficiency of vacuum tubes is independent of
freq up to a certain limit. When frequency increase
beyond that limit, several combine to rapidly
decrease tube efficiency.
High freq limitations (Above 1GHz)
– Inter electrode capacitance effect
– Lead inductance effect
– Transit time effect
– Gain BW limitation
– Effect due to RF losses(skin effect)
– Effect due to radiation losses
 LIMITATIONS
• Inter electrode capacitance
– With increase in i/p freq, effective grid to
cathode impedance or reactance (Xc=1/2πfc)
of the tube decreases due to inter electrode
capacitance and output voltage decrease due
to shunting effect. because at higher
frequencies Xc becomes almost short.
– These can be reduced by decrease the area of
the electrode (C=A/d) ie by using smaller
electrodes or by increasing the distance
between electrodes.
– Beyond 100MHz it becomes so small that the
signal gets short circuited within the tube.
 LIMITATIONS
• As freq increase, the reactance XL=2πfL increase
and hence the voltage appearing at the active
electrodes are less than the voltage at the bias
pins. this results in reduced gain for the tube
amplifier.
• L is proportional to reactance( L=l/A), L can be
decreased by using large sized short leads
without base pins ie by increasing A and
decreasing l. reduces the power handling
capability.
• Input admittance becomes large enough to
cause overload of the input circuit resulting in
decrease in circuit efficiency
 LIMITATIONS
• Transit time
– Time to travel from cathode to plate is insignificant at low
freq
– Time taken by an electron to travel fm cathode to anode
plate of an electron tube is called transit time
– Transit time is given as t=d/v
– Transit angle is given as θ=ωt=ωd/v
– At low freqs transit time is small, electron travel fm
cathode to anode as the voltage changes fm +ve to –ve
– At high freqs the voltage changes fm +ve to –ve fm 1 to
100 times in one transit time
– However at high freq transit times in excess of 0.1 cycle
causes a phase shift between plate current and grid
voltage resulting in decreasing in tube efficiency
• Gain bw limitation
Large gain over a low bw
• Effect due to RF losses(skin effect)
Losses will increase at higher freqs therefore losses can
reduced by increasing the size of the conductors
 LIMITATIONS
• Inter electrode capacitance can be reduced by
moving the electrodes further apart or by
reducing size of tube and electrodes
• However moving electrodes apart increases
transit time and reducing size of the tube
limits power handling capacity
• Upper freq limit 1GHz
KLYSTRON
 KLYSTRON
• Types
– Two cavity Klystron
– Reflex klystron
 ENERGY TRANSFER MECHANISM
• Amp & Osc are circuit arrng which tfr pwr fm DC
supply to AC signal
• During interaction between particle and field,
energy tfr takes place
• When field favors the particle motion, tfr is from
fd to particle
• If field opposes particle motion, tfr of energy is in
opp dirn resulting in loss of energy by the particle
and gain of an equal amount of energy by the
field
 ENERGY TRANSFER MECHANISM
• If the field happens due to current carrying
signal, energy tfr to the field strengthens the
signal resulting in gain
• During interaction gain/loss of energy by
particle is equal to the loss/gain by the field
 ENERGY TRANSFER MECHANISM
• In most microwave tubes, the signal is placed in cavity
gap and electrons are forced to cross the cavity gap at
times when they face max opposition
• Crossing the gap under opposition leads to tfr of
energy to cavity gap signal
• When the gap voltage is sinusoidal and time varying
and the charge crossing it is continuous and uniform,
which is usually the case, no net tfr of energy takes
place between cavity and charge crossing the gap
• It is because the energy tfr is equal and opp in dirn
during a half cycle when compared to previous half
cycle resulting in no net tfr of energy in a cycle
 ENERGY TRANSFER MECHANISM
• To have a net energy tfr, preferably max, from
electron beam to gap signal voltage, the
distributed charge is compressed into a thin
sheet or bunch,
• This bunch now requires less time to cross the
gap and it is arranged such that the bunch
crossing is at peak gap voltage so that bunch
faces max opposition/retardation from the
signal voltage
 TYPES
• Magnetron (Cross field or”M”type device Focussing field
is perpendicular to accelerating field)

• Travelling wave tubes

• Two cavity Klystron Linear or “O” type tube
Electron beam parallel to electric

• Reflex Klystron field

TWO CAVITY KLYSTRON
 TWO CAVITY KLYSTRON

•Electrons emitted from cathode accelerated towards neck of reentrant cavity by

High DC voltage V01 with uniform acceleration
• RF input signal Vs=V1sinωt1 is fed into first cavity called buncher cavity
• At zero signals electrons continue the onward passage
•During positive/negative half cycle electrons are accelerated retarded with
reduction in speed
•Variation in velocity of electrons at different angles of wave, called velocity
modulation causes electrons to arrive at different times in drift space and bunch
 TWO CAVITY KLYSTRON
• The bunched electrons arrive at the second cavity called catcher cavity. The
arr of the bunched electrons is so arranged midway between the grid of the
neck of the catcher cavity at the time of retarding phase of the field so as to
deliver most of its energy.

• To produce the retarding field in catcher cavity we need not use an external
excitation. The bunch of electrons passing through the cather cavity induces
currents and associated fields in the resonator gap. Out of the several
components of the field, only the component which opposes the motion of the
electron bunch can accept energy from the bunch growing further, thus
providing more and more retardation to the subsequently arr bunches.

• Other components accelerate bunches, loosing in the process their energy

and ultimately dying down.

• Net result is only the field that provides retardation to bunch of electrons
exists and grows in the gap.
 TWO CAVITY KLYSTRON

•The electron then leaves the catcher cavity with reduced velocity and get
terminated at the cathode
• The catcher cavity resonator is tuned to the same freq as buncher cavity.
 TWO CAVITY KLYSTRON

• Velocity modulation process

• Bunching process
• Output power
BUNCHING PROCESS
APPLEGATE DIAGRAM
CATCHER CAVITY CURRENT
 OUTPUT POWER
• Since current induced by electron beam in the
walls of the catcher cavity is directly
proportional to the amplitude of the
microwave input voltage V1,
– fundamental component of the induced
microwave current in the catcher is given by
 TWO CAVITY KLYSTRON
Performance characteristics
Sno Feature Typical values
1. Power ouput 10KW-
500KW(CW),30MW(Pulsed)
2. Frequency 250MHz-100GHz
4. Efficiency 30-40%(Theoretical 58%)
5. Bandwidth 10-60MHz
6. Power gain 15-70dB

Applications
1. As power output tubes in
• UHF TV transmitters
• Troposphere scatter transmitters
• Satellite comn ground station
• Radar transmitters
2. As oscillators and amplifiers in radar & comn eqpt
REFLEX KLYSTRON
 REFLEX KLYSTRON
• If fraction of output power is fed back to the input cavity
and if the loop gain has a magnitude of unity with a phase
shift of multiple 2π, the Klystron will oscillate
• However two cavity Klystron not used because, when the
osc freq is varied, the resonant freq of each cavity and the
feedback path phase shift must be readjusted for a positive
feedback.
• Therefore, Reflex Klystron is single cavity that overcomes
disadvantages of two cavity Klystron
• Used in applications where variable freq is reqd
• Low power output = 10-500mW, freq range = 1-25GHz
• Efficiency = 20-30%
 REFLEX KLYSTRON
• Electron beam injected fm
cathode
• Velocity modulated by cavity-
gap voltage
• Some electrons accelerated by
accelerating field enter the
repeller space with greater
velocity than those with
unchanged velocity
• Some electrons decelerated by
the retarding field enter the
repeller region with less
velocity
• All electrons turned around by
the repeller voltage then pass
through the cavity gap in
bunches that occur once per
cycle
 REFLEX KLYSTRON
• On their return journey
the bunched electrons
pass through the gap
during the retarding
phase of the alternating
field and give up their
kinetic energy to the
electromagnetic energy
of the field in the cavity.
• Oscillator output energy
is then taken out from
the cavity.
REFLEX KLYSTRON
APPLEGATE DIAGRAM
POWER O/P & FREQ CHARACTERISTICS
 REFLEX KLYSTRON
Performance characteristics
Sno Feature Typical values
1. Power ouput 1mW-2.5W
2. Frequency 4GHz-200GHz
4. Efficiency 10-20%,Theoretical 22.7%
5. Bandwidth 10-60MHz
6. Tuning freq range 2W at 5GHz to 10mW at 30GHz

Applications
Where variable freq is reqd
• In radar receivers
• Local osc in microwave receiver
• Portable microwave links
• Signal source in microwave generator of variable freq
• Pump osc in parametric amp
Introduction: - A TWT is a vacuum tube that is used in wireless communication
or satellite system to amplified radio frequency signal in MW range
Structure: -
 The basic structure of a TWT consists of a cathode and filament heater plus an
anode that is biased positively to accelerate the electron beam forward and to
focus it into a narrow beam.
 The electrons are attracted by a positive plate called the collector, which has given
a high dc voltage.
 The length of the tube is usually many wavelengths at the operating frequency.
 Surrounding the tube are either permanent magnets or electromagnets that keep
the electrons tightly focused into a narrow beam.
 TRAVELLING WAVE TUBES
Function: -
• When sw on the cct the cathode starts the emission of
electrode.
• Focussing electrode to focus electron in narrow band.
• When RF signal is not applied, the emitted electron will reach
to collector without any distortion.
• When the RF signal is applied at the helix tube, then
acceleration +ve half cycle and deceleration during the –ve half
cycle.
• The volume of bunch will beam stronger and stronger as the
electron appears towards to anode end.
• Therefore at the o/p of helix stronger electric fd created by
buncher cavity. which will result to produce the amplified o/p
signal
• In helix structure, the speed of electron moving in the helix is
synchronization with the speed of the electron emitted by
cathode therefore the system is knows as slow wave structure
 Performance characteristics
1. Frequency of operation : 0.5 GHz – 95 GHz
2. Power outputs:
5 mW (10 – 40 GHz – low power TWT)
250 kW (CW) at 3 GHz (high power TWT)
10 MW (pulsed) at 3 GHz
3. Efficiency : 5 – 20 % ( 30 % with depressed
collector)

PH0101 Unit 2 Lecture 6 39

 Applications of TWT
1. Low noise RF amplifier in broad band microwave receivers.
2. Repeater amplifier in wide band communication links and
long distance telephony.
3. Due to long tube life (50,000 hours against ¼th for other
types), TWT is power output tube in communication
satellite.
4. Continuous wave high power TWT’s are used in
troposcatter links (due to larger power and larger
bandwidths).
5. Used in Air borne and ship borne pulsed high power radars.

PH0101 Unit 2 Lecture 6 40

 TRAVELLING WAVE TUBES
• In Klystrons and Magnetrons, due
to resonant structure BW is
limited
• TWT is a
– broadband device
– Non resonant structure
• Two main constituents of TWT
– Electron beam
– Structure supporting a slow
EM wave
• Types
– Helix TWT
– Coupled cavity TWT
– Gridded-control TWT
 TRAVELLING WAVE TUBES

• Differences with Klystron

– Interaction of electron beam and RF field in TWT is
continuous over the entire length of the circuit
– Wave in TWT is propagating wave
– In coupled cavity TWT there is coupling between the
cavities
• Two requirements of TWT
– Velocity of wave propagating along structure should be
smaller than light so that the beam & wave can travel
along the structure with equal vel
– Wave should have an E-field component parallel to the
beam axis so that interaction can take place between
electrons & slow wave longitudinal E-field
 TRAVELLING WAVE TUBES :WORKING PRINCIPLE

•Electron beam is accelerated to a velocity, slightly more

than the phase velocity of RF wave
• Axial wave accelerates electron during one half cycle and
decelerates during the second half cycle
• At any point of time there exists more electrons in the
decelerating half cycle resulting in net transfer of energy
from electrons to the RF wave
 TRAVELLING WAVE TUBES :
WORKING PRINCIPLE
• The strengthened wave
offers more deceleration
to the incoming electrons,
increasing electron
concentration in its
region thereby increasing
energy transfer to the
beam manifold
• Resulting in exponential
growth of the signal along
the length
 Unit 2 Lecture 6
Klystron Oscillator
Reflex Klystron
Traveling Wave Tube
Biological effect of microwaves

PH0101 Unit 2 Lecture 6 45

 Klystron Oscillator
A klystron is a vacuum tube that can be used
either as a generator or as an amplifier of power,
at microwave frequencies.
Two cavity Klystron Amplifier

PH0101 Unit 2 Lecture 6 47

 Applications
 As power output tubes
1. in UHF TV transmitters
2. in troposphere scatter transmitters
3. satellite communication ground station
4. radar transmitters
 As power oscillator (5 – 50 GHz), if used as a
klystron oscillator

PH0101 Unit 2 Lecture 6 48

 Reflex Klystrons

The reflex klystron has been the most used

source of microwave power in laboratory
applications.

PH0101 Unit 2 Lecture 6 49

 Construction

 A reflex klystron consists of an electron gun, a cavity with a pair

of grids and a repeller plate as shown in the above diagram.
 In this klystron, a single pair of grids does the functions of both
the buncher and the catcher grids.
 The main difference between two cavity reflex klystron amplifier
and reflex klystron is that the output cavity is omitted in reflex
klystron and the repeller or reflector electrode, placed a very
short distance from the single cavity, replaces the collector
electrode.

PH0101 Unit 2 Lecture 6 50

 Working
 The cathode emits electrons which are accelerated forward by
an accelerating grid with a positive voltage on it and focused
into a narrow beam.
 The electrons pass through the cavity and undergo velocity
modulation, which produces electron bunching and the beam is
repelled back by a repeller plate kept at a negative potential
with respect to the cathode.
 On return, the electron beam once again enters the same grids
which act as a buncher, therby the same pair of grids acts
simultaneously as a buncher for the forward moving electron
and as a catcher for the returning beam.

PH0101 Unit 2 Lecture 6 51

Reflex Klystron oscillator

PH0101 Unit 2 Lecture 6 52

 Working
 The feedback necessary for electrical oscillations is developed
by reflecting the electron beam, the velocity modulated
electron beam does not actually reach the repeller plate, but is
repelled back by the negative voltage.
 The point at which the electron beam is turned back can be
varied by adjusting the repeller voltage.
 Thus the repeller voltage is so adjusted that complete bunching
of the electrons takes place at the catcher grids, the distance
between the repeller and the cavity is chosen such that the
repeller electron bunches will reach the cavity at proper time to
be in synchronization.
 Due to this, they deliver energy to the cavity, the result is the
oscillation at the cavity producing RF frequency.

PH0101 Unit 2 Lecture 6 53

 Performance Characteristics
1. Frequency: 4 – 200 GHz
2. Power: 1 mW – 2.5 W
3. Theoretical efficiency : 22.78 %
4. Practical efficiency : 10 % - 20 %
5. Tuning range : 5 GHz at 2 W – 30 GHz at 10
mW

PH0101 Unit 2 Lecture 6 54

 Applications

 The reflex klystrons are used in

1. Radar receivers
2. Local oscillator in microwave receivers
3. Signal source in microwave generator of
variable frequency
4. Portable microwave links
5. Pump oscillator in parametric amplifier

PH0101 Unit 2 Lecture 6 55

 Traveling Wave Tube

Traveling Wave Tube (TWT) is the most versatile

microwave RF power amplifiers.

The main virtue of the TWT is its extremely wide band

width of operation.

PH0101 Unit 2 Lecture 6 56

Basic structure of a Traveling
Wave Tube (TWT)

PH0101 Unit 2 Lecture 6 57

 Basic structure
 The basic structure of a TWT consists of a cathode and
filament heater plus an anode that is biased positively to
accelerate the electron beam forward and to focus it into a
narrow beam.
 The electrons are attracted by a positive plate called the
collector, which has given a high dc voltage.
 The length of the tube is usually many wavelengths at the
operating frequency.
 Surrounding the tube are either permanent magnets or
electromagnets that keep the electrons tightly focused into a
narrow beam.

PH0101 Unit 2 Lecture 6 58

 Features
 The unique feature of the TWT is a helix or coil that
surrounds the length of the tube and the electron beam
passes through the centre or axis of the helix.
 The microwave signal to be amplified is applied to the end of
the helix near the cathode and the output is taken from the
end of the helix near the collector.
 The purpose of the helix is to provide path for RF signal.
 The propagation of the RF signal along the helix is made
approximately equal to the velocity of the electron beam
from the cathode to the collector

PH0101 Unit 2 Lecture 6 59

 Functioning
 The passage of the microwave signal down the helix
produces electric and magnetic fields that will interact
with the electron beam.
 The electromagnetic field produced by the helix
causes the electrons to be speeded up and slowed
down, this produces velocity modulation of the beam
which produces density modulation.
 Density modulation causes bunches of electrons to
group together one wavelength apart and. these
bunch of electrons travel down the length of the tube
toward the collector.
PH0101 Unit 2 Lecture 6 60
 Functioning

 The electron bunches induce voltages into the helix

which reinforce the voltage already present there. Due
to that the strength of the electromagnetic field on the
helix increases as the wave travels down the tube
towards the collector.
 At the end of the helix, the signal is considerably
amplified. Coaxial cable or waveguide structures are
used to extract the energy from the helix.

61
 Advantages

1. TWT has extremely wide bandwidth. Hence, it can

be made to amplify signals from UHF to hundreds of
gigahertz.
2. Most of the TWT’s have a frequency range of
approximately 2:1 in the desired segment of the
microwave region to be amplified.
3. The TWT’s can be used in both continuous and
pulsed modes of operation with power levels up to
several thousands watts.

PH0101 Unit 2 Lecture 6 62

 Performance characteristics
1. Frequency of operation : 0.5 GHz – 95 GHz
2. Power outputs:
5 mW (10 – 40 GHz – low power TWT)
250 kW (CW) at 3 GHz (high power TWT)
10 MW (pulsed) at 3 GHz
3. Efficiency : 5 – 20 % ( 30 % with depressed
collector)

PH0101 Unit 2 Lecture 6 63

 Applications of TWT
1. Low noise RF amplifier in broad band microwave receivers.
2. Repeater amplifier in wide band communication links and
long distance telephony.
3. Due to long tube life (50,000 hours against ¼th for other
types), TWT is power output tube in communication
satellite.
4. Continuous wave high power TWT’s are used in
troposcatter links (due to larger power and larger
bandwidths).
5. Used in Air borne and ship borne pulsed high power radars.

PH0101 Unit 2 Lecture 6 64

 MAGNETRONS
• klystron, reflex klystron and TWT are linear beam tubes generally called O tubes or
original type, The other type of microwave tubes are cross field tubes in which the
electric and magnetic field are perpendicular to each other. The principal tube in
this type, called the M type is the magnetron.
• It may be noted in klystrons that the electrons carrying energy are in contact with
the RF field in the resonant cavity only for a short duration. However, if the
electrons can be made to interact with RF field for a longer duration higher
efficiency can be obtained. This has been done in TWT and in magnetron also the
same technique is utilised.
There are three types of magnetrons.
1. Negative Resistance type
2.Cyclotron frequency type
3. Travelling wave or Cavity type.
• Negative resistance Magnetrons make use of negative resistance between two
anode segments but have low efficiency and are useful only at low frequencies
(<500 MHz).
Cyclotron frequency magnetrons depend upon synchronism between an
alternating component of electric and periodic oscillation of electrons in a direction
parallel to this field. These are useful only for frequencies greater than 100 MHz.
Cavity magnetrons depend upon the interaction of electrons with a rotating
electro-magnetic field of constant angular velocity. These provide oscillations of
very high peak power and hence are Very useful in radar applications. This being
the most useful one
 MAGNETRON
Structure
• It is a diode usually of cylindrical configuration with a thick
cylindrical cathode at the center and coaxial cylindrical
block of copper as anode.
• In the anode block are cut a number of holes and slots
which act as resonant anode cavities.
• The space between the anode and cathode is the
interaction space and to one of the cavities is connected a
coaxial line or WG for extracting o/p.
• it is cross fd devices as the electric fd between anode and
cathode is radial whereas the magnetic fd produced by a
permanent magnet is axial.
• The permanent magnet is placed such that the magnetic
lines are parallel to the vertical cathode and perpendicular
to the electric fd between cathode and anode.
 RF STRUCTURE

Cathode
MAGNETRON : HULL CUTOFF VOLTAGE
AND HULL CUTOFF MAGNETIC DENSITY
 OSCILLATION MECHANISM
• Electron motion perpendicular to both E & B fields.
• E’s when come across RF field in same direction then gain
velocity – More curling
• F=evB
• Retarded e’s = less curling = drift towards anode
• Accelerated e’s = more curling= curl back away from anode
 MAGNETRON
Function: -
• When a magnetron is first turned “ON” only noise
is present due to electron cloud circling around
the anode.
• Noise consists of all freq’s incl the one at which
the cavity will resonate.
• Oscillations build up rapidly by amp of noise
signal at correct freq.
ROTATING BUNCHED
 MAGNETRON
Performance characteristics
Sno Feature Typical values
1. Power ouput Up to 40 MW at 10
ghz
2. Frequency 500MHz-12GHz
4. Efficiency 80%

Applications
1. Pulsed Radar with large pulse powers
2. Voltage tunable Magnetrons are used in oscillators on telemetry and missile
applications
3. Fixed freq, CW Magnetrons are used for industrial heating and microwave ovens
(500MHz-2.5GHz range, 300W-10KW power outputs, efficiency of 50%
4. Used in mw cookers
Note :
• Very noise
• Can not maintain phase coherency
THANK YOU