Abstract An AFC defeat circuit in a television receiver for disabling the receiver''s automatic frequency control (AFC) system during manual fine-tuning of the input tuner stage. The circuitry utilizes the potential developed across a storage capacitor in the output circuit of a ''''normally OFF'''' transistor to provide a d-c voltage to the high-potential end of a voltage divider biasing network in the AFC system. There is further provided a conventional, finetuning control knob coated with an electron-conductive material to derive an input signal during manual fine-tuning from lowfrequency radiation picked up by the user''s body. The input signal, approximating a sine wave, alternately switches the transistor between non-conductive (OFF) and conductive (ON) states, charging and discharging the storage capacitor, respectively. When the transistor is OFF, the capacitor is charged through a high impedance; but when the transistor is ON, the capacitor discharges through the low impedance of the transistor. Due to the difference in RC time constants between the charge and discharge cycles, the collector output will be very nearly ground whenever the fine-tuning knob is actuated. Accordingly, when the fine-tuning knob is touched, the voltage divider bias is reduced to ground potential disabling the AFC system.
In a television receiver having an automatic frequency control (AFC) system and means for defeating said AFC system during receiver fine-tuning adjustments by coupling userintercepted low frequency radiaTions in the vicinity of the receiver to a conductive fine-tuning knob, comprising in combination: a capacitor; switching means including a transistor switch having an input coupled to said control knob and an output including said capacitor for establishing a short discharge time for said capacitor during conduction of said transistor switch; coupling means, including an impedance for establishing a long charge time for said capacitor, coupling said capacitor to said AFC system; said transistor switch being driven conductive and non-conductive in accordance with the frequency of said userintercepted radiation coupled thereto, whereby the difference between said charge time and said discharge time for said capacitor results in a change in voltage across said capacitor for disabling said AFC system.
Claim 2
Dependent Claim
147 words
In a television receiver having an automatic frequency control (AFC) system and means for defeating said AFC system during receiver fine-tuning adjustments by coupling userintercepted low frequency radiaTions in the vicinity of the receiver to a conductive fine-tuning knob, comprising in combination: a capacitor; switching means including a transistor switch having an input coupled to said control knob and an output including said capacitor for establishing a short discharge time for said capacitor during conduction of said transistor switch; coupling means, including an impedance for establishing a long charge time for said capacitor, coupling said capacitor to said AFC system; said transistor switch being driven conductive and non-conductive in accordance with the frequency of said userintercepted radiation coupled thereto, whereby the difference between said charge time and said discharge time for said capacitor results in a change in voltage across said capacitor for disabling said AFC system.
Claim 3
Dependent Claim
47 words
In a television receiver as set forth in claim 1, further including a source of B+ potential and a ground reference; said storage capacitor being connected at one terminal to said source of B+, through said impedance element, and to said ground reference at its other terminal.
Claim 4
Dependent Claim
43 words
In a television receiver as set forth in claim 2, wherein said coupling means further include a second transistor switch having an input coupled to said capacitor and an output coupled to said AFC system for supplying operating bias to said AFC system.