Klystron Amplifier
The klystron amplifier can be used as an microwave oscillator or amplifier at low and high frequencies.
There are two kinds of klystron amplifier
So lets see 2-cavity klystron amplifier first............
Two Cavity Klystron Amplifier
2-cavity klystron amplifier works on the following principles
Above is an actual picture of Two cavity Klystron Amplifier used for space communication purposes....................Thus klystrons are very helpful at microwave frequencies as amplifiers and oscillators. In the next post we will see the variant of two cavity klystron called Reflex klystron.................
The klystron amplifier can be used as an microwave oscillator or amplifier at low and high frequencies.
There are two kinds of klystron amplifier
- Multicavity Klystron: used as a low power microwave amplifier
- Reflex klystron:used as a low power microwave oscillator
So lets see 2-cavity klystron amplifier first............
Two Cavity Klystron Amplifier
2-cavity klystron amplifier works on the following principles
- Velocity modulation
- Current modulation
As we know in any tube amplifier we need to have electron beam produced in cathode to anode region.So electrons in electron beam are produced in the cathode to anode region and accelerated by the means of an anode voltage V0 .These electrons are allowed to pass through a pair of buncher grid across which an RF(radio frequency) voltage V1sinωt and these electrons are accelerated or de-accelerated depending on the part of cycle during which they enter gap.The accelerated electrons emerge with a velocity higher than the entering velocity v0 and de-accelerated electrons emerge with a velocity lower than v0.While some electrons pass through zero RF field and hence there is no change in their velocity. This phenomenon of the variation of electrons in electron beam is known as Velocity Modulation.
The analysis for velocity modulation is carried out with following assumptions
1.Electrons leave the cathode with velocity=0 and the beam has uniform density in cross section of beam
2.Space charge effets are not considered
3.Magnitude(V1) of input signal<<< De-accelerating voltage(V0)
Current modulation will be seen in the working of 2-cavity klystron amplifier
Operation :
The analysis for velocity modulation is carried out with following assumptions
1.Electrons leave the cathode with velocity=0 and the beam has uniform density in cross section of beam
2.Space charge effets are not considered
3.Magnitude(V1) of input signal<<< De-accelerating voltage(V0)
Current modulation will be seen in the working of 2-cavity klystron amplifier
Operation :
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| Two cavity Klystron Amplifier |
The electron beam is created with the help of the cathode.This electron beam attains a high velocity due to the accelerating anode.This beam passes through the buncher cavity.This beam then passes through the drift space having length 'L' and finally through the catcher cavity(the name is referred as catcher since the output is obtained at this cavity).Finally the beam is collected by the collector(Collector electrode).
The RF input(microwave) is given at the buncher cavity which we want to be amplified.The anode voltage V0 and buncher cavity gap having length 'd' are adjusted such that time taken by beam to pass through d is less than quarter time period of input RF signal.The beam is focussed to travel axially so that it doesn't spread by the means of applied external magnetic field.
Now when the electron beam passes through the buncher cavity during positive half cycle of RF input signal,velocity of beam increses whereas during the negative cycle half cycle of input RF signal velcity of beam decreases . This is the concept of velocity modulation which we have see earlier. In the moving frame of the electron beam, the velocity modulation is equivalent to a plasma oscillations. 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 density modulation, i.e. bunches of electrons.Now lets see this procedure with the help of Applegate diagram..........
Thus the electron beam is velocity modulated to form bunches or undergoes density(Current modulation) with input RF signal.This current modulation of beam produces amplification of RF signal input at the catcher cavity.Thus what we obtain finally is the amplification of RF input signal.
One important observation is that the phase of output signal is opposite to that of input signal.Also many harmonics are generated during amplification.One way to remove this harmonics is to tune the catcher cavity to the fundamental frequency or any other harmonic desired.
Analysis:
Due to potential difference V0 between anode and cathode ,the electrons form a high current density beam with velocity u0
uo=sqrt(2eV0/m)
e= charge of an electron
m= mass of electron
The rime taken by the electron beam to cross cavity gap 'd' = Transit time
Transit time= t2-t1=tg=(d/u0)
Transit angle = Θg=ωtg
RF input signal is given to the buncher cavity
Thus the average RF input in the gap of buncher cavity is
V(av)= ∫V1sinωt.dt (t1<t<t2)
The output comes out to be
V(av)=V1/ωtg[sin(ωt1+Θ(g/2))sin(ωtg/2)]
where ß=[sinΘ(g/2)/Θ(g/2)]=buncher cavity beam coupling coefficient
V(av)=V1ßsin(ωt1+Θ(g/2)
When the electron passes through the buncher cavity their velocity either increases or decreases depending on the state of RF input cycle
Let u(av)=velocity of electron at mid of gap
u(av)/u0 =sqrt[V0+V(av)/V0]
Therefore u(av)/u0 =sqrt[1+{V1ßsin(ωt1+Θ(g/2)/V0}]
where m= V1ß/V0= depth of modulation
u(av)=u0=sqrt[1+msin(ωt1+Θ(g/2)]
Thus we see that electron in beam are velocity modulated by input RF signal with a modulation depth of [V1ß/V0].
Excitation of cavity:
It is desired that that grid spacing be as small as possible to ensure maximum coupling.To achieve this transit angle (Θg) should be kept small......which means that transit time(tg) should be kept small. Also the the Q factor of the cavity must be high.
For excitation two techniques are used
The RF input(microwave) is given at the buncher cavity which we want to be amplified.The anode voltage V0 and buncher cavity gap having length 'd' are adjusted such that time taken by beam to pass through d is less than quarter time period of input RF signal.The beam is focussed to travel axially so that it doesn't spread by the means of applied external magnetic field.
Now when the electron beam passes through the buncher cavity during positive half cycle of RF input signal,velocity of beam increses whereas during the negative cycle half cycle of input RF signal velcity of beam decreases . This is the concept of velocity modulation which we have see earlier. In the moving frame of the electron beam, the velocity modulation is equivalent to a plasma oscillations. 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 density modulation, i.e. bunches of electrons.Now lets see this procedure with the help of Applegate diagram..........
 |
| Applegate Diagram |
Thus the electron beam is velocity modulated to form bunches or undergoes density(Current modulation) with input RF signal.This current modulation of beam produces amplification of RF signal input at the catcher cavity.Thus what we obtain finally is the amplification of RF input signal.
One important observation is that the phase of output signal is opposite to that of input signal.Also many harmonics are generated during amplification.One way to remove this harmonics is to tune the catcher cavity to the fundamental frequency or any other harmonic desired.
Analysis:
Due to potential difference V0 between anode and cathode ,the electrons form a high current density beam with velocity u0
uo=sqrt(2eV0/m)
e= charge of an electron
m= mass of electron
The rime taken by the electron beam to cross cavity gap 'd' = Transit time
Transit time= t2-t1=tg=(d/u0)
Transit angle = Θg=ωtg
RF input signal is given to the buncher cavity
Thus the average RF input in the gap of buncher cavity is
V(av)= ∫V1sinωt.dt (t1<t<t2)
The output comes out to be
V(av)=V1/ωtg[sin(ωt1+Θ(g/2))sin(ωtg/2)]
where ß=[sinΘ(g/2)/Θ(g/2)]=buncher cavity beam coupling coefficient
V(av)=V1ßsin(ωt1+Θ(g/2)
When the electron passes through the buncher cavity their velocity either increases or decreases depending on the state of RF input cycle
Let u(av)=velocity of electron at mid of gap
u(av)/u0 =sqrt[V0+V(av)/V0]
Therefore u(av)/u0 =sqrt[1+{V1ßsin(ωt1+Θ(g/2)/V0}]
where m= V1ß/V0= depth of modulation
u(av)=u0=sqrt[1+msin(ωt1+Θ(g/2)]
Thus we see that electron in beam are velocity modulated by input RF signal with a modulation depth of [V1ß/V0].
Excitation of cavity:
It is desired that that grid spacing be as small as possible to ensure maximum coupling.To achieve this transit angle (Θg) should be kept small......which means that transit time(tg) should be kept small. Also the the Q factor of the cavity must be high.
For excitation two techniques are used
- Cylindrical cavity excitation by axial current
- Cylindrical cavity excitation by velocity modulated beam
- Axial current flows through cavity to excite it
- For coupling efficiency we have to modify the dimensions
- Axial symmetry guarantees TM(0m0) modes
- Velocity modulated beam excites cavity
- For coupling efficiency DC potentail and dimensions of the cavity should be modified
- Axial symmetry guarantees TM(0mn) modes
- Frequency - 250 MHz to 100 GHz
- Gain- 16 to 70 dB
- Bandwidth-10-50 Mhz
- Power- 10kW- 500kW(continuous mode) and 30MW (pulsed mode)
- Noise Figure- 15 to 20 dB
- Efficiency-60%
- UHF TV Transmitter
- Troposphere scatter transmitter
- Communication Satellites
- Radar Transmitter
- Also as a power oscillator
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| High-power klystron used for spacecraft communication at the Canberra Deep Space Communications Complex. |
Above is an actual picture of Two cavity Klystron Amplifier used for space communication purposes....................Thus klystrons are very helpful at microwave frequencies as amplifiers and oscillators. In the next post we will see the variant of two cavity klystron called Reflex klystron.................
nice explanation
ReplyDeletethanx bro... helped a lot :)
ReplyDeleteNice Explanation...thanks
ReplyDeletethank you..nice explanation
ReplyDeleteGood explanation about the topic
ReplyDelete