Facsimile and static presentation processing – Facsimile – Specific signal processing circuitry
Patent
1987-08-03
1989-01-03
Chin, Tommy P.
Facsimile and static presentation processing
Facsimile
Specific signal processing circuitry
455258, 455260, H04N 550, H04B 118
Patent
active
047961023
ABSTRACT:
An automatic frequency control circuit (AFC) is provided for use in a television receiver having a local oscillator (LO), an intermediate frequency (IF) demodulator section, and a phase locked loop (PLL) for acquiring and phase locking onto the IF signal. The AFC circuit comprises an acquistion circuit for sensing and detecting a no signal condition, an out of lock signal condition and phase locked condition. During a no signal condition the acquisition circuit enables offset and clamp circuits for respectively setting the nominal operating frequencies of the voltage controlled oscillator (VCO) of the PLL and the LO to the center frequency of the IF passband. Upon appearance of an IF signal the acquisition circuit enables a sweep circuit as the receiver is in an out of lock condition in order to sweep the VCO downward from the passband center frequency to just above the sound carrier frequency and then sweeps the VCO frequency upwards toward the video carrier frequency. During the downward sweep of the VCO the acquisition circuit disables the phase detector of the PLL to inhibit spurious lock up. Upon the VCO phase locking to the IF signal the clamp, sweep and offset circuits are disabled while the phase detector is enabled.
REFERENCES:
patent: 4461035 (1984-07-01), Sakamoto
patent: 4490743 (1984-12-01), Mycynek
patent: 4575761 (1986-03-01), Carlson
patent: 4596046 (1986-06-01), Richardson
patent: 4625331 (1986-11-01), Richardson
Bingham Michael D.
Chin Tommy P.
Motorola Inc.
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