GB733976A - Improvements in or relating to demodulators for electric carrier current communication systems - Google Patents

Abstract

733,976. Valve circuits for radio reception. STANDARD TELEPHONES & CABLES, Ltd. Sept. 24, 1953, No. 26300/53. Classes 40 (5) and 40 (6). A non-linear resistor is utilized to maintain constant the signal level at a detector. In a reflex circuit, Fig. 1, carrier waves amplitude modulated with speech and also with a pilot tone are applied to the grid of a valve 4 via a resistor 3. They are selected from the anode circuit by a tuned transformer 9 and applied to a bridge rectifier 29, from which the modulation components are returned to the grid of valve 4 via a level control 30 and R.C. network 33, 36, 34. After re-amplification in the valve, the pilot tone is selected from the anode circuit by means of a tuned transformer 11, while the speech signals are obtained from resistor 13 and via a low-pass filter 16. In order to render the output level constant, the secondary of the transformer 9 is loaded by a directly heated thermistor 25 in series with a fixed resistor 26. In another arrangement, Fig. 3, the valve 38 comprises an anode-bend detector having an anode circuit similar to that shown in Fig. 1, but differing in that the secondary of the carrier-frequency transformer 9 is now connected, via a directly-heated thermistor 25 and series resistor 26, to a tuned circuit 39 in the cathode path of the valve 38. In this way variable negative feed-back is provided at carrier frequency, so as to reduce variations in output level. In a modification, Fig. 4 (not shown), the primary of the transformer 9 is coupled to the tuned cathode circuit 39 via a series acceptor circuit, and the secondary of transformer 9 is shunted with the thermistor and series resistor so as to vary the degree of positive feed-back so produced. The valve used in the arrangements of Figs. 3, 4 may alternatively comprise a leaky-grid detector.