Patent GB-1493839-A (GB)
Title: Broadcast receiver tuning circuit
IPC: H03J5/02
Filed: 1975-02-24
Abstract: 1493839 Tuning radio receivers; transistor gating circuits PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 24 Feb 1975 [27 Feb 1974] 7618/75 Headings H3Q and H3T A broadcast receiver tuning circuit 1 is tuned by a voltage obtained from a capacitor 23 whose charge can be changed by a current source 25 controlled by an automatic tuning correction signal from 15 and by a desired tuning voltage temporarily applied from a tuning selector 65 to the capacitor via a switch 59 which is operated by a time constant circuit 79 for a time so as to immediately tune to a desired frequency. As shown by pressing a button of the tuning selector 65 a signal generated by a switch 73 operates the time constant circuit to close switch 59 for a time so as to connect the preset tuning voltage of the associated potentiometer 67 to the capacitor 23. During the time the preset tuning voltage is applied via switch 59 a switch 33 disconnects the AFC voltage. A mechanical or electrical memory 81 ensures that it can be seen which potentiometer 67, 69 or 71 is connected to output 63. The selector 65 may have touch controls. A switch 85 enables switch 59 to be maintained closed as long as required for adjustment of the potentiometers to a desired tuning setting. When the receiver is switched on a supply circuit 95 energizes a time constant circuit 89 to maintain the switch 59 closed for a time so that the station last selected by the selector 65 is tuned to even if a station finder switch 41 is on. If the signal received by the receiver becomes too weak as determined by a signal amplitude detector 19 the switch 59 is closed under control of the detector 19 so that the tuning selected by the selector 65 is maintained until the received signal becomes stronger again. An alternative such as for car radios is when the field strength is sufficient the voltage on the capacitor can be written into a memory via a switch which then couples the output of the memory to the capacitor when the field strength becomes too weak. This memory may be a potentiometer adjustable by a motor. Such an arrangement is disclosed in Specification 1,493,312. The station finder 41 can select discharge or charge of the capacitor so that the tuning is swept until a received signal produces a stop signal from 15 via a delay circuit 53 to stop input 55. The switch 59 and time constant circuits 79, 89 can be formed of a transistor circuit (Fig. 2, not shown). A capacitor (231) provides a delay in a transistor (223) conducting when the receiver is switched on so that a transistor (205) turns on the transistor switch 59 for this delay time. An input 77 from the selector 65 causes the capacitor (231) to discharge and turn on the transistor (205) and switch 59 for the delay time until the capacitor has charged sufficiently to turn the transistor (223) on and transistor (205) and switch 59 off. In integrated form when the transistor (205) is on the output 57 is the same potential as the input 61. The circuit can be made independent of temperature. A diode (203) prevents the V be  of a transistor (201) of the switch 59 from becoming too large. The switch 59 can act as an amplifier so that the rate at which the charge on the capacitor 23 charges via a transistor (211) or discharges via diodes (215, 217) can be made independent of the settings of the tuning potentiometers.