Patent GB-1373362-A (GB)
Title: Circuit arrangment for automatic signal seeking and tuning
IPC: H03J7/26
Filed: 1971-11-05
Abstract: 1373362 Radio receiving circuits PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 5 Nov 1971 [10 Nov 1970] 51537/71 Addition to 1310180 Heading H3Q A circuit for automatic signal seeking and tuning in a receiver, particularly a radio or T.V. receiver whose tuning elements, particularly capacity diodes are electronically variable, while the tuning voltage is derived from a capacitor 1, Fig. 1, optionally after previous amplification i.e. in amplifier 9 which is charged or discharged through separate current sources, i.e. transistors 2, 3, which are controllably a frequency discriminator 24, the tuning voltage being applied to a hysteresis circuit (e.g. Schmitt trigger) 10 which controls the constant current sources, is characterized in that two similar gates 11, 12 each having a first and a second input are arranged between the hysteresis circuit 10 and the constant current sources 2, 3, each first input of the gates 11, 12 being connected to a separate one of two phase opposed outputs of the hysteresis circuit 10, the two second inputs of the gates 11, 12 being connected to the output of a bi-stable flip-flop circuit 13 controlled by a voltage (e.g. AGC) which is dependent on the amplitude of the received signal. In the embodiment, the gates 11, 12 are NAND gates with their outputs respectively coupled to the base of transistor 3 via a diode 14 and to the base of transistor 2 via a diode 15 and a further transistor 6. Frequency discriminator 24 has its output coupled to the base of transistor 17 which forms with transistor 18 a long-tailed pair, the collectors of transistors 17, 18 being respectively coupled to the bases of transistors 3, 2. The base circuit of transistor 17 is shunted by the collector to emitter path of an auxiliary transistor 22, the base of which is coupled to the trigger circuit 13. The amplifier 9 may comprise an IGFET transistor. In the absence of a signal, if switch 16R is closed, the output of bi-stable flip-flop 13 becomes high, and also the output 10a of hysteresis circuit 10, output 10b being low. Transistor 22 is then conductive, short-circuiting the output from discriminator 24 and earthing the base of transistor 17. The output llc of gate 11 is low, diode 14 consequently conductive, so rendering transistor 3 conductive to charge capacitor 1: the output 12c of gate 12 is high rendering transistor 6 and thus transistor 2 non-conductive. When a signal is intercepted the AGC voltage biases bi-stable flip-flop 13 so that its output is low: the output from gate 11 is then high, cutting off diode 14, and the transistor 22 is rendered non-conductive, so that the output from discriminator 24 controls both charging and discharging transistors 3, 2 respectively, via the amplifier comprising transistors 17, 18. If, on the other hand, switch 16L is closed the output 10a of hysteresis circuit 10 is low and output 10b high. The 11c of gate 11 is high, rendering diode 14 non- conductive, so that transistor 3 is cut off: output 12c of gate 12 is however low, rendering diode 15 conductive so that transistor 6 and thereby the discharging transistor 2 is rendered conductive. Upon interception of a signal the output of gate 12 becomes high, transistor 22 is cut off and the tuning again controlled by discriminator 24. Hysteresis circuit 10.-The two controlling inputs of a bistable flip-flop 35, Fig. 2 are connected to the collectors of transistors 27, 28 respectively, an overriding manual control being provided in the form of switches 16L, 16R respectively. When zero control voltage, derived from amplifier 9 (Fig. 1) appears on line 25 transistor 28 is biased to cut-off, rendering its collector voltage high, and transistor 26 is also biased to cut off so that transistor 27 is biased conductive and its collector potential is low. When the control voltage in line 25 rises, transistor 26 is biased conductive so that transistor 27 is non-conductive and its collector potential becomes low. Transistor 28, however remains non-conductive, since the voltage applied to its base via potentiometer 33, 34 is insufficient to cause conduction: when, however the control voltage on line 25 reaches its maximum value, the voltage on the base of transistor 28 is sufficient to cause conduction, the collector potential falls, and so the bi-stable flip-flop 35 is caused to reverse its state.