Patent GB-1330988-A (GB)
Title: Receiver for pulse modulated signals
IPC: H03G3/30
Filed: 1972-07-20
Abstract: 1330988 Automatic gain control; peak detectors; transistor amplifying circuits TULL AVIATION CORP 20 July 1972 [29 July 1971] 33881/72 Headings H3T and H4R In a radio receiver for pulse modulated carrier signals, comprising at least an R.F. stage, a detector and an audio stage there is provided a peak detector circuit receiving signals from the R.F. stage and operable to detect signal peaks and connected in a control loop to automatically control the gain of the R.F. stage, and a normalizing circuit connected to derive from the carrier signal a direct current voltage level corresponding to the carrier signal level together with the audio output voltages, the normalizing circuit being operable to control the gain of the receiver so as to maintain the D.C. voltage level substantially constant. An embodiment comprises R.F. amplifier 10, Fig. 1, mixers 12, I.F. amplifier 14 and detector 16; a preselection tuner 42 and local oscillators 44 are controlled from a channel selection matrix 38. The output of detector 16 is fed to one contact of switches 28B, 28A, and also to controlled attenuator 30 whose output passes via low-pass filter 32 and amplifier 34 to the other contact of switch 28B. The output of amplifier 34 is also applied to control amplifier 36 which controls attenuator 30. An output from detector 16 is also fed to peak detector 26 the output of which is fed to the second contact of switch 28A. With switches 28A, 28B in the position shown, the output from detector 16 is fed directly to instrumentation unit 20 and also to an automatic gain control amplifier 22 controlling the stages 10, 14. When switch 29 is closed, relay 28 is energized, so that instrumentation unit 20 is now connected to the output of amplifier 34, and A.G.C. amplifier 22 is fed from the peak detector 26. Peak detector.-The output of signal detector 16 (Fig. 1) is fed to a diode 46, Fig. 2, to charge capacitor 48. The voltage across capacitor 48 is applied to output line 60 via emitter-follower 50 and transistor 52 which are connected in a feedback loop so that the input impedance of transistor 50 is high and the discharge rate of capacitor 48 high. Normalizing circuit.-The controlled attenuator 30, Figs. 1, 3, comprises a photo-resistor 70 in series with the signal path, whose impedance is varied by means of lamp element 72. Its output passes via low-pass filter 32 and resistor 84 to the base of transistor 80, which forms, with transistor 78 a long-tailed pair, a controllable D.C. comparison voltage being applied to the base of transistor 78 from potentiometer 86. An output from the collector of transistor 78 is fed via transistor 82 to output line 34A; overall negative feedback is applied to the arrangement by means of resistor 110. The output on line 34A goes to the instrumentation unit (20, Fig. 1) and also to a further amplifier comprising transistors 130, 132 connected as a long-tailed pair; a controllable D.C. comparison voltage is applied to the base of transistor 132 from Zener diode 150 via potentiometer 148. The output from the collector of transistor 132 is fed via a further amplifier comprising transistor 136 to the lamp element 72; a capacitor 156 provides a short-circuit for any A.C. signal.