Patent GB-1310180-A (GB)
Title: Cirucit arrangement for automatic signal seeking and tuning
IPC: H03J7/24
Filed: 1970-11-26
Abstract: 1310180 Radio receiving circuits; transistor bistable and sawtooth generator circuits PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 26 Nov 1970 [29 Nov 1969] 56235/70 Headings H3Q and H3T In an arrangement for automatic tuning in a receiver having electronically controllable tuning elements, a tuning voltage is developed across a capacitor which is charged and discharged by separate current sources: it is characterized by means of applying the tuning voltage to a hysteresis circuit having first and second conditions and which controls the current sources, and by means for disabling the control of the current sources by the hysteresis circuit and for allowing them to be controlled by a discriminator circuit for automatic frequency control, when a control signal (i.e. A.G.C.) which depends on the strength of the received signal exceeds a predetermined level. Capacitor 1, Fig. 1 can be charged from two sources 2, 3 of constant current; only one can be switched on at one time and dependent on which the capacitor is charged or discharged and voltage varying linearly with time produced: this is amplified by amplifier 4 of high input resistance and applied to varactor diodes (not shown). This voltage also controls via line 25 a Schmitt trigger 5, having hysteresis properties (Fig. 2, not shown) which is switched over every time when the highest and also the lowest tuning voltage is reached: its D.C. output voltage controls a three level switch 6, the output of which controls constant current sources 2, 3 via line 7. In addition, there is applied over line 8 to the sources 2, 3 a direct voltage which lies midway between the upper and lower levels of switch 6; if the voltage defference between lines 7 and 8 is positive source 3 is switched in, while if negative, source 2 is switched in. When during tuning a station of sufficient strength is intercepted a control A.G.C. voltage is developed which reverses bistable flip-flop 9; thus, via line 36, controls switch 6, switching it to the mid position in which the D.C. voltages on lines 7, 8 are equal and the control of sources 2, 3 by the Schmitt trigger 5 is disabled. An A.F.C. voltage from source 38 is then applied via amplifier 10 to the sources 2, 3 and takes charge of the timing. Transmitter seeking towards higher or lower frequencies is effected by means of circuit 11 which includes two push buttons determining the seeking direction: dependent on which is actuated Schmitt trigger 5 receives a pulse whose polarity controls it in such a manner that seeking is in the desired direction. At the same time bistable flip-flop 9 receives a pulse whereby it reverts to its original state so that switch 6 is no longer retained at its intermediate voltage value: finally the A.F.C. control voltage at the output of amplifier 11 is short-circuited. Schmitt trigger circuit and three level switch.- The Schmitt trigger 5, Fig. 3 comprises transistors 12, 13, cross-coupled by resistors 19, 20 and 21, 22 and capacitors 25, 34: the trigger circuit can be placed in either of its stable states by applying a positive or negative pulse, as the case may be, to the common emitter circuit by means of push-buttons 31, 33. The three level switch comprises transistors 26, 28, coupled by common emitter resistor 27 from which the output is taken over line 7: the base of transistor 26 receives the collector potential of transistor 13, and the arrangement is driven to its intermediate state by biasing the base of transistor 28 over line 36.