Insight into Reflex Klystron

Reflex Klystron:

So basically what was the need for reflex klystron when we had multicavity klystron?

Lets consider 2-cavity klystron oscillator.For proper working as an oscillator the output voltage needs to be fed back to the input and if positive feedback is taken then only the klystron will oscillate.But 2-cavity klystron oscillator is not used because the slight change in frequency is used to tune both the cavities(buncher and catcher) to get positive feedback.Also the multicavity klystron suffer from the noise caused beacuse bunching process is never complete and electrons never arrive at random at the catcher cavity.Hence multicavity klystrons are not used in receivers
                                                                                       This limitations of 2-cavity klystron are overcome by REFLEX klystron oscillator.




Operation:

Reflex Klystron

                                
 The cathode generates the electron beam.This beam is accelerated by the acclearing grid(G).
When the beam passes through the grid cap of length(d), the electrons suffer either acceleration or de-acceleration depending on positive or negative half cycles of RF input applied.Thus velocity modulation takes place just like 2-cavity klystron.
                                                                               Lets consider any reference electron.when this particular electron passes through the grid gap 'd' and if it experiences positive half cycle of input RF signal,its velocity increases,acceleration takes place.When this reference electron passes through cavity gap 'd' and if it experiences zero RF input signal ,no velocity change occurs.When the electron passes through grid gap 'd' and it experiences negative potential of input RF signal,its velocity decreases .All these velocity modulated electrons will be repelled back to  cavity by repeller due to its negative potential.The repeller distance'L' and the repeller voltage can be adjusted to receive all velocity modulated electrons at same time on positive peak of the cavity RF input.
                                                                                                                                                    Thus all these velocity modulated electrons are bunched together.When the bunching process occurs the electrons lose kinetic energy.The loss of kinetic energy is thus transferred to the cavity to conserve total power.If the power delivered by bunching process by the bunched electrons to  cavity is greater than the power loss in cavity,the electromagnetic (EM) field amplitude at resonant frequency of cavity will increase to produce microwave oscillations.This RF power is given to load.When the power delivered by the bunching process by the electrons becomes equal to the total power loss in cavity system ,a steady state microwave oscillations are generated at resonant frequency of cavity.
Lets see the Applegate diagram( which is a graph of the electron paths in a two-cavity klystron tube, showing how electron bunching occurs) for the velocity modulation.......

Applegate Diagram

                                                 

The bunched electrons can deliver maximum coupling and maximum power to cavity when power is collected at the peak of RF input signal.The Applegate diagram clearly demonstrates that the amount of power collected and mode of oscillations depends on distance between cavity and repeller as well as  repeller voltage.

The analysis of Reflex klystron for calculating RF power is carried out under following assumptions:

1. Cavity grids and repeller are in parallel and very large in extent
2. No RF field is excited in repeller space
3. Electrons are not intercepted by cavity anode grid
4. No debunching takes place in repeller space
5. Cavity gap RF voltage <<< DC Beam voltage (V0)

Let T= time period at the resonant frequency
      to=time taken by refrence electron to travel in repeller space between entering the repeller space at 'B' and returning to the cavity at positive peak value on formation of bunch,then
                                      
                                                                t0=[n+(3/4)]T
                                                                t0=NT
                                                          where N=n+3/4                 {n=0,1,2........}

Thus by adjusting repeller voltage for a given dimension of Reflex Klystron, bunches can be formed at N=n+3/4 positive half cycle.


Thus accordingly......Modes of oscillations are given as N=3/4,7/4(i.e 1 3/4),11/4 (i.e2 3/4)
for modes n=0,1,2...........respectively.It is obvious that lowest order mode 3/4 occurs for maximum value of repeller voltage when transit time of electrons 't0' in repeller space is minimum.Actually higher modes occurs at lower repeller voltages.At lowest order mode we have maximum repeller voltage .....hence acceleration of bunched electrons on return is maximum....and therefore the power output of lowest mode is the Maximum.

The above demonstration of the modes is shown in the below graph: 

Mode curves for Reflex Klystron

Above is a roughly drawn graph for mode curves..U will get an idea by looking at the graph about the modes of operation.



Output Power:

                                 P(rf)= [{V0I0*J(X)*(V0+VR)}/2*pi*fL]*sqrt(e/2mv0)

where
V0=DC Beam voltage
I0=DC Beam current
VR=Repeller voltage
X=bunching parameter= pi*N* β*V1/V0
N= n+3/4
 β= Beam coupling coefficient (explained in two cavity klystron)
 V1=Instantaneous amplitude of input RF voltage
J(X)=Bessel function of X 

Output Frequency: 

Output frequency can be controlled electronically by adjusting the repeller voltage(VR)...GIVEN BY

                                                           f=[(V0+VR)N/L*sqrt(V0)*0.0674]
where
V0=DC beam voltage
VR=Repeller voltage
L=Repeller length in cms


Efficiency:

                                                                η=P(RF out)/P(DC in)


Modulation:

Basically a Reflex klystron is used as a microwave source for laboratory experiments.
To avoid use of costly microwave receiver or power meter,the RF signals are low frequency modulated and this modulated signal is probed and detected by a crystal detector.The detected signal usually carries original amplitude and phase variation information of signal are measured using low frequency receiver to obtain the desired parameters.

The two modulations used are

1.Amplitude modulation by square wave
2.Frequency modulation by saw tooth wave  


Characterisctics:

• Frequency Range- 2-200 GHz
• Bandwidth- 30MHz
• Power output- 10mW-2.5W
• Efficiency- 20 to 30%

 

Applications:

•  Signal source in microwave generators
•  Local oscillators in receivers
•  Pump oscillators in parametric amplifiers
• FM oscillator in low power microwave links

Experimental Setup for Reflex Klystron

Above is an practical experimental setup of Reflex Klystron.Experimental Set-up uses low power reflex klystron along with its klystron power supply and klystron mount, as a microwave signal source along with other standard high quality components and VSWR meter, enables to perform under mentioned experiments in the laboratory. The waveguide components are based on rectangular waveguide equipped with rectangular plain flanges .The set up is complete in all respect and requires only CRO( the kind of used one in colleges to teach about Reflex Klystron).