Monograph BBC Monograph 76 Download
The monograph describes an electronic method of varying the effective value of an inductance and derives the theoretical value of an inductance and derives the theoretical value of this inductance. This method is exploited in an oscillator whose frequency is determined by a control voltage. An expression is obtained for the voltage required to cause a certain deviation in frequency, and in the course of this analysis it is shown that the centre frequency should be reasonably stable over a period of time. An evaluation is made of the distortion which may be introduced by this circuit when driven by a sinusoidal waveform, and of the conditions to be fulfilled for minimum distortion. It is further shown that is possible to produce a pre-emphasised frequency response without the use of an external network.
A practical modulator design is described and the performance analysed theoretically. The results are compared with those obtained in practice. It is thereby shown that the apparatus would be of adequate performance to be used in broadcasting transmitters for a pilot tone stereophonic transmission system.