GB1429965A - Broadcast receiver including an automatic frequency control circuit - Google Patents

Abstract

1429965 Tuning radio receivers; disabling AFC; transistor level variation circuits PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 12 Nov 1973 [15 Nov 1972] 52377/73 Headings H3Q, H3A and H3T A broadcast receiver such as a radio or T.V. has a tuning member which is coupled to an electrical level adjusting circuit 31, Fig. 1, for adjusting the electrical level at its output simultaneously with adjusting the receiver tuning and a level variation detector 43 which has its input 41 coupled to the output 29 of the level adjusting circuit and its output 45 coupled to an operational signal input 47 of a circuit 19 for suppressing automatic correction of the receiver tuning during variation of the tuning. Changes in tuning voltage at 29 during tuning selection by switches 36, 38, 40 and/or adjustment of potentiometers 35, 37, 39 and when the receiver is switched on producing a varying supply 33, are detected by level variation detector 43 which turns on one of transistors 53 or 55 to short-circuit the output 11, 13 from the frequency or phase deviation detection circuit 9 so that during tuning the tuning voltage at 29 is applied directly via 21 to the oscillator 3. After completion of tuning a storage capacitor 115, Fig. 2, at the output of the level variation detector 43 maintains the transistor 53 or 55 on for a time to prevent immediate return of the automatic tuning correction to the oscillator 3. As a smoothing capacitor 56 is discharged by conduction of transistor 53 or 55 during tuning there is no residual voltage to cause unwanted detuning when the automatic tuning correction is switched in again and this is useful in remote control receivers which employ stepwise tuning. In a modification when variable capacitors or inductors are used for coarse tuning the oscillator, the connection between 29 and 25 is removed, 25 is then connected to a fixed potential and the level adjusting circuit 31 consists of a single potentiometer the wiper of which is mechanically coupled to means for varying the capacitors or inductors. The tuning correction suppression circuit 19 may be used with digital tuning circuits. The receiver may be of the kind having an automatic station finder in which a tuning voltage varies until a station is found as determined by a further output from the detection circuit 9. The level variation detector 43 is shown in detail in Fig. 2. Any variation in level of the input signal at 41 is fed via a differentiating circuit 57, 61, 63, 65, phase splitter transistors 59, 67, transistors 83, 85 which operate as rectifiers, transistor 87 to a threshold circuit 107, 109, the transistor 107 of which is normally cut off so as to cause an output across the storage capacitor on occurrence of the signal variation at the input 41.