Multi-speed ramp for a varactor tuning system

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325453, 325468, H04B 116

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active

039434494

ABSTRACT:
An electronic all channel television tuning system has a presettable counter for deriving the number of the receivable television channel by counting, on a continuing sample basis, the varactor diode tuner local oscillator frequency. The derived channel number is supplied as an input of a comparator which includes separate tens and units channel number comparison sections. A channel number selector supplies an encoded desired channel number to another input of the comparator and to a frequency band decoder which establishes appropriate preset information for the counter. The system local oscillator frequency is varied by a voltage ramp circuit having two pairs of matched current sources and sinks of different capacity for supplying current, through respective transmission gates, to charge or discharge a tuning capacitor at different rates. A difference in the tens section of the comparator activates the transmission gates for both the larger and smaller capacity source-sink pairs, producing the high ramp speed. After equality in the tens digits only one of the small capacity source-sink pair is activated, under control of the units section of the comparator, to produce the low ramp speed.

REFERENCES:
patent: 3560858 (1971-02-01), Sakai et al.
patent: 3736513 (1973-05-01), Wilson
patent: 3748645 (1973-07-01), Kawashima
patent: 3766483 (1973-10-01), Wolfram
patent: 3818353 (1974-06-01), Sakamoto
patent: 3822405 (1974-07-01), Sakamoto
IEEE Spectrum on "TV Receiver Tuning Systems of the Future," by Doyle et al., Vol. 15, No. 2, 7/1969.

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