A ferrite microwave modulator employing feedback

The amplitude modulation of microwaves produced by the magnetization of ferrites is non-linear and suffers from hysteresis. Hence, square-wave modulation is the only function in which the distortions are not objectionable. The paper describes a feedback method of applying the modulation signal, providing a linearity substantially that of the feedback crystal used to detect the modulated microwave signal, and reducing the effect of hysteresis by an amount approximating the feedback loop gain. Pure sine-wave modulation is achieved at low frequencies, in which the second-harmonic sidebands are more than 45 dB below the fundamental. Linear modulation, by sawtooth and square waveforms, is also achieved, in which the modulation envelope faithfully reproduces the applied signal. The employment of ferrite microwave modulators in engineering applications will involve techniques which are already standard for lower frequencies with conventional components, and the paper establishes the feasibility of using the powerful method of envelope feedback to control them.