Wideband FM modulator and AFC control loop therefor

DETAILED DESCRIPTION The present invention pertains to an improved digital frequency discriminator for use in a wideband FM modulator, which discriminator utilizes a stable reference frequency source and a counter synchronized to the input signal to produce a pulse signal having an average duty factor that varies linearly with the frequency of the input signal, a second counter continuously counting the cycles of the reference frequency to provide a pulse signal having a constant duty factor approximately equal to the average duty factor of the pulse signal from the counter at a desired mean frequency of the input signal, and circuitry for receiving the two pulse signals and providing a DC control voltage equivalent to the difference between the average voltages thereof, which control voltage is utilized to control the VCO that supplies the input signals to the discriminator toward the desired mean frequency.
The AFC loop including the VCO also contains a high speed voltage translator for converting the analog output of the VCO to digital pulses and a divider for reducing the frequency of the pulses.
Other features including common semiconductor substrates and constant frequency adjusting voltages are incorporated to reduce errors due to temperature differentials and the like.
It is an object of the present invention to provide a new and improved digital frequency discriminator.
It is a further object of the present invention to provide a new and improved digital frequency discriminator in conjunction with an improved AFC loop for use in a wideband FM modulator.
It is a further object of the present invention to provide a method of increasing the center frequency stability of a wideband FM modulator utilizing a VCO.
These and other objects of this invention will become apparent to those skilled in the art upon consideration of the accompanying specification, claims and drawings.