Automatic frequency deviation detection and correction apparatus

Automatic frequency deviation correction apparatus for a frequency modulated system, comprising:

• a source of RF carrier signal of center frequency Fc;
• a source of modulating signal of frequency Fm, having positive and negative half cycles and coupled to modulate the frequency of said carrier signal about said center frequency, as a function of the amplitude of said modulating signal, to produce a frequency modulated output signal;
• a gate circuit;
• a feedback arrangement connecting said frequency modulated output signal to said gate circuit as a feedback signal;
• first and second counters;
• a gate select means for sensing said positive and negative half cycles of said modulating signal and operable to cause said gate circuit to provide said feedback signal to said first counter, which counts the number of cycles in said feedback signal during said positive half cycle of said modulating signal;
• said gate select means being additionally operable to cause said gate circuit to provide said feedback signal to said second counter, which counts the number of cycles in said feedback signal during said negative half cycle of said modulating signal;
• a microprocessor connected to said first and second counters and operable to obtain the count difference between said first and second counters, and from said count difference, determine the deviation of said frequency modulated output signal from said center frequency;
• said microprocessor being further operable to calculate and generate a correction signal if the difference between a predetermined desired deviation and said deviation determined by said microprocessor is greater than a predetermined value;
• said microprocessor applying said correction signal to vary the amplitude of said modulating signal until said difference does not exceed said predetermined value.