Frequency stabilization of a VCO FM modulator

Facsimile and static presentation processing – Facsimile – Specific signal processing circuitry

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332 19, H04N 562

Patent

active

044867824

ABSTRACT:
A system and method for stabilizing a voltage controlled oscillator (VCO) in an FM modulator is disclosed. The frequencies of a reference carrier and the VCO are sequentially detected in the same frequency discriminator, sampled and stored for later comparison in generating an error signal representing the average frequency difference between these two signals. This error signal is then summed with detected audio information and provided to the VCO as a correction signal for matching the VCO's frequency with the reference frequency of the carrier. VCO signal instabilities due to system drift are thus eliminated for improved signal processing. A time-based switching approach is utilized for sequentially detecting and sampling the reference carrier and the VCO output. One embodiment of the invention utilizes AC coupling in the correction signal feedback loop eliminating DC signal drift and improving system operating stability. The present invention is particularly adapted for operation in a signal decoder such as utilized in a frequency modulated subscription television system wherein the audio carrier is processed and provided to a conventional television receiver.

REFERENCES:
patent: 4006429 (1977-02-01), Talbot
patent: 4145717 (1979-03-01), Guif et al.
patent: 4242649 (1980-12-01), Washburn, Jr.
patent: 4322751 (1982-03-01), Hongu et al.

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