Frequency modulation by inductance variation: a magnetically-stable ferrite modulator

The factors affecting the production of frequency modulation are considered when the modulator is either a reactance valve or a variable inductance employing magnetic variation of the core permeability. It is shown that whilst the effectiveness of the reactance valve is reduced at high oscillator frequencies or with large fractional frequency deviations, the inductance modulator is not affected to the same extent. The losses inherent in the magnetic core of the inductance modulator are considerably reduced by using magnetic ferrites; such a ferrite modulator is described. The effects of magnetic hysteresis, in producing oscillator frequency instability, are minimized by applying a specific level of polarization from a permanent magnet. A figure of merit is developed by which the frequency instability can be calculated from a knowledge of the prevailing magnetic conditions. The ferrite frequency modulator is particularly suitable for use with variable frequency oscillators where the equipment is to be portable and free from spurious sub-harmonic outputs. Results are given which serve to indicate the performance of this modulator.