MICROWAVE TUBES

IntroductionMicrowave tubes are constructed to overcome the limitations with conventional tubes and UHF tubes.Principle of Operation:-The basic principle of operation of microwave tube involves transfer of power from a source of DC voltage to source of AC voltage by means of a current density modulated electron beam.The same can be achieved by accelerating electrons in a static electric field and retarding them in an AC field.

Contd..The density modulation of the electron beam allows more electrons to be retarded by ac field than accelerated by dc field, which therefore makes possible a net energy be delivered to the ac electric field.Classification of microwave tubes:-The classification is based on different factors.Their mechanism of producing density modulation.The acceleration and retardation of electrons in the ac field.Important types are:-Klystron amplifier (two cavity,multy cavity)Klystron oscillator (reflex klystron)MagnetronTWT

KLYSTRONAklystronis a specializedlinear-beamvacuum tube, invented in 1937 by American electrical engineersRussel and Sigurd Varian, which is used as anamplifierfor highradio frequencies, fromUHFup into themicrowaverange or generator of power at mw frequencies.

Two Cavity Klystron Amplifier

Two cavity klystron amplifier is basically a velocity modulated tube.Construction:-Here a high velocity electron beam is formed , focused and sent down along a glass tube through an input cavity (buncher ), a field free drift space and an output cavity (catcher ) to a collector electrode.The anode is kept positive potential with respect to cathode.The electron beam is passed through the gap A .The input and output is taken from the tube via resonant cavities with the help of coupling loops.

Two cavity klystron amplifier

OPERATIONOperation can be best explained by the help of apple gate diagram

ContdInitial stageAt point B on the input RF cycle, the alternating voltage is zero and going to be positive.At this instant, the electric field across gap A is zero and an electron which passes through the gap A is unaffected by the RF signal.Let this electron is called the reference electron eR and travels with an unaffected velocity v0 = .Where V is the anode to cathode voltage

ContdSecond stageAt point C of the input RF cycle again we are passing electron called late electron el as compared to the reference electron.the late electron is subjected to maximum positive RF voltage and hence travel towards gap B with an increase velocity than V0 (V>V0 ) , and these electron overtakes reference electron.Similarly an early electron ee that passes thorough the gap A slightly before the reference electron is subjected to the maximum negative field.So these electron travels with the reduced velocity than v0.Later these electrons ee are catch up by reference electron.

As a result of these actions the electrons in the bunching limit gradually bunch together as they travel down in the drift space from gap A to B.The drift space converts the velocity modulation into current modulation.

Performance characteristics:-Frequency : 250MHz to 100GHz.Power : 10 kW 500kWPower gain : 15dB 70 dB.Theoretical efficiency : 58%.Applications :-As power output tubesIn UHF TV transmitters.In troposphere scatter applicationRadar transmittersAs power oscillators.

Two cavity Klystron Amplifier

Multicavity KlystronThe problem associated with the two cavity klystron amplifier is that the gain is about 10 to 20 dB.A higher overall gain can be achieved by connecting several two cavity tubes in cascade manner.Instead of these multiple cavities can be used as shown below,

ContdHere each of the intermediate cavities act as a buncher.With four cavities power gain of 50 dB can be easily achieved.The cavities can be tuned to the same frequency.Applications:-These are employed in UHF klystron for TV transmittersIn satellite earth station transmitters.

Two Cavity Klystron OscillatorKlystron amplifier can be converted into oscillator by feeding back a part of catcher output to into the buncher in proper phase.But the feed backing should be satisfy Barkhausen criterion.A = 1The schematic is same as of amplifier expect the feedback need to be added.The feedback must be adjusted to give correct polarity and amplitude which is basically depends on cavity tuning.The criterion for oscillation is given by

Where + /2 is the phase angle difference between buncher and catcher cavity. is the total phase shift between resonator and the feedback cable.

ContdIf the value of is zero means the oscillations are in phase.The maximum power output is obtained at this condition. Also when a small change in the de accelerating voltage it cause change in the frequency since transit angle varies.Tuning of the oscillator is done by adjusting the grid voltage, accelerating voltage and tuning the cavities. High frequency oscillations are obtained by controlling the temperature of the resonators.

BuncherThe cathode controls the number of electrons in the electron beam and focuses the beam. The voltage between the cathode and the cavity resonators (the buncher and the catcher, which serve as reservoirs of electromagnetic oscillations) is the accelerating potential and is commonly referred to as the beam voltage. This voltage accelerates the DC electron beam to a high.