Frequency discrimination and stabilization of square-wave modulated microwave transmissions

The paper describes a method of square-wave (on-off) modulation of the reflector electrode of a reflex klystron which produces a stabilized frequency during the ¿on¿ time. During the ¿off¿ time the voltage applied to the reflector electrode is outside a mode of oscillation and the klystron does not oscillate. The modulation is applied through a special blocking modulator circuit which ensures that the duration of the frequency sweep immediately preceding stabilization is kept to a small value. The stabilization is accomplished by means of a frequency discriminator unit, which feeds back a voltage through the blocking modulator (while the klystron is oscillating) thus stabilizing its frequency. It is therefore essential to obtain a smooth voltage/frequency discriminator characteristic over the swept frequency ranges to eliminate the possibility of stabilizing at more than one frequency. Both theoretical and experimental characteristics of the microwave discriminator are discussed. It is shown that the output-voltage/frequency characteristic of the discriminator is critically dependent on the phasing of sidebands and the (frequency sensitive) phasing of a carrier at a detector crystal. It is also shown that, measured in the plane of the detector crystal, the phase of the carrier should be in quadrature with that of the reference phase of the sidebands, not only at the resonant frequency of the controlling cavity resonator, but also at frequencies well removed from resonance. The theoretical analysis would be useful in other situations where the relative amplitudes and phases of a carrier and two sidebands have to be considered.