Frequency demodulator for small modulation indices - with F.M. oscillation to be demodulated applied to a phase comparator

摘要

Modulation indices are smaller than or equal to 1, and the phase comparator can be in the form of a mixer, to which the signal from a freely oscillating VCO oscillator is also applied. The comparator output voltage is applied through a phase control loop with a loop filter to the oscillator whose frequency is synchronised with and phase locked into the frequency of the oscillation to be demodulated. The control voltage represents the demodulation product. Oscillation to be demodulated is at first applied to a Schmitt trigger with hysteresis, and then to the phase demodulator, whose phase characteristic is triangular. The loop filter and the rest of the control loop is such that the bandwidth in it is much narrower than the highest modulation frequency. An equaliser is connected to the output for the demodulated signal, which corrects distortions caused by the loop bandwidth narrowing.