level microwave drive and control come from solid-state circuitry on the face of the antenna that faces the earth. It is also mandatory that the phase of the output of the microwave generators be nearly the same as the phase of the input drive. These two necessary requirements can be achieved simultaneously if the free running frequency of the magnetron, that is, its operating frequency without the injection of any locking signal, is automatically tuned to closely approximate that of the locking signal. If this condition is met, then when the locking signal is injected there will be negligible phase shift between the phase of the drive signal and the phase of the amplified output. It is therefore possible to use a phase comparator that compares the phase of the input signal with that of the amplified output to generate an error signal which is used to tune the magnetron operating frequency to that of the drive signal. The magnetron can be tuned in several different ways. It can be tuned either internally or externally by some kind of mechanical motion that changes the resonant frequency of the tube. It can also be tuned electronically in different ways. The most
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