Patent GB-544696-A (GB)
Title: Improvements in detectors for frequency modulation
IPC: H03G3/28
Filed: 1941-01-31
Abstract: 544,696. Frequency modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Jan. 31, 1941, No. 1327. Convention date, Jan. 31, 1940. [Class 40 (v)] In a frequency-modulation receiver, the difference between the output voltages of the two rectifiers of the frequency discriminator acts on a bias resistor to cut off the audio-frequency amplifier when the circuit is substantially mistuned. As shown in Fig. 2, an I.F. amplifier 10 feeds a known type of discriminator, e.g. that described in Specification 489,094, comprising coupled tuned circuits 12, 13, rectifiers 14, 15 and resistances 17, 18, the detected signal being applied through condenser 22 to the input of a pentode audio-amplifier 23. Any D.C. voltage component present across 17, 18, due to mistuning of the receiver, is applied through a resistance 31 to the suppresser grid of a pentode valve 29. When this voltage is positive, the anode current through resistance 35 rises, the anode voltage falls and a diode valve 39 becomes conductive and permits current to pass through a resistance 25, thus biasing one of the grids of the valve 23 to cut-off. When the voltage applied to the suppresser grid of the valve 29 is negative, the screen current of that valve rises and the same effect is produced by rendering a diode 38 conductive. To suppress noise in the absence of signal, the voltage across resistance 18 is applied through a smoothing circuit 41, 45 to the grid of a valve 42. When there is no signal the valve 42 passes current and its anode is sufficiently negative to render a third diode 44 conductive. Current passes through the biasing resistor 25 and the valve 23 is cut off. In a modification (Fig. 3, not shown), the voltage across resistance 18 is applied to the first grid of the pentode valve 29 and the valve 42 is dispensed with. A diode limiter is interposed between this grid and earth, to prevent the grid from becoming too negative. To ensure sudden and complete cut-off of the amplifier 23, a trigger circuit of the kind described in Specification 452,927 and comprising two reciprocably-coupled pentode valves 48, 50, Fig. 4, may be used. The voltage across the resistances 17, 18 is applied jointly to the control grids of these valves, the reciprocal coupling being afforded by direct connections of their anodes and screen grids. When a signal is correctly tuned in, there is no D.C. voltage drop across 17, 18 and both control grids of the valves 48, 50 are at earth potential. Owing to the bias resistance 55 in the cathode lead of the valve 50, this valve is cut off and the valve 48 is conductive. The voltage across the resistance 25 in the anode circuit of the valve 50 is small, and the audio-amplifier 23 is operative. If the circuit is mistuned, either a positive voltage appears on the grid of valve 50, or a negative voltage on the grid of valve 48, voltages of the opposite signs being prevented by the limiting diodes 52, 53. In either case, the valve 50 tends to pass current and the valve 48 to reduce its current flow, and the valves react cumulatively on each other until valve 48 is cut oft, and valve 50 passes a large current which causes the resistance 25 to bias the audio-amplifier to cut-off. If there is no appreciable received signal, the absence of voltage across the resistance 18 permits valve 51 to pass current through its cathode resistance 54. Diode 53 becomes operative, and the control grid of the valve 50 more positive, thus initiating the reversing action of the valves 48, 50 and cutting off the audio amplifier 23.