GB489636A - Carrier wave transmission systems - Google Patents

Abstract

489,636. Wireless receiving systems; automatic control systems. STANDARD TELEPHONES & CABLES, Ltd. Jan. 29, 1937, No. 2728. Convention date, March 26, 1936. [Class 40 (v)] [Also in Group XXXV] in a receiving system for frequency modulated waves, a portion of the low-frequency output is used to correct for frequency drift and to reduce noise and distortion. A push-pull highfrequency detector 5, with the local oscillator 11, shown of the Barkhausen type, produces a frequency-modulated intermediate frequency which is amplified at 15, and converted at 18 to amplitude modulation by reason of circuits 23, 24, 25, 26 symmetrically off tune. Detectors 28, 29 supply low-frequency to the output transformers 30, 31. If the intermediate frequency drifts slowly, the voltages across resistances 34, 35 become unbalanced, the resulting potential being passed over conductors 43 and a filter 44 to an amplifier 47 and used to vary the anode bias of the B-K oscillator 11 causing its frequency to drift also, and so maintaining the intermediate frequency constant. Alternating potentials corresponding to signals, distortion, and noise, appear across potentiometers 41, 42 and are partly fed back over the same circuit as the frequency-correcting potentials and operate in the same manner, in effect reducing the modulation depth of the intermediate frequency signals. If the modulation depth at the transmitter is now increased to make up for the loss in signal strength, an increased signal-to-noise ratio will result. To reduce distortion, however, it is more advantageous not to increase the depth of modulation but to use an A.F. amplifier 32. In a modification, Fig. 2 (not shown), the frequency - correcting and audio - frequency potentials are fed back over separate paths enabling separate control of the two effects.