MICROWAVE VACUUM TUBE DEVICES

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Why Ordinary Vacuum Tube Devices can not generate Microwave?



Higher value of inter-electrode capacitance, lead inductance and transit time limits the high frequency operation of Vacuum Tube devices.
.Reducing the area of electrodes or by increasing the distance between them the inter-electrode capacitance can be reduced but at the same time transit time of the device increases, which in turn reduces the operating frequency.


KLYSTRON


The Klystron tube operates on the principle of velocity modulation in stead of current density modulation employed in conventional tubes. Klystron amplifier was introduced by Varian brothers in 1939. Later on Klystron oscillator was developed.


PRINCIPLE OF OPERATION


Electron beam emerges from cathode, interacts with the applied rf wave (signal) within the buncher cavity. By the process velocity of the beam modulates when it comes out the buncher cavity and forms a bunch over the drift space. Finally the bunch moves through the catcher cavity and induces energy into it. Output is taken through catcher cavity.


REFLEX KLYSTRON


Problem of adjustment of feedback path length with the change of frequency does not allow to construct a two cavity klystron amplifier. Hence reflex klystron is a single cavity system operates on the principle of velocity modulation and generates microwave oscillation.



Advantages:
Produces high output power.
Operates up to a very high frequency.


Disadvantages:
Very low efficiency.
Repeller protection is very much essential.