Patent GB-533138-A (GB)
Title: Circuit arrangements for influencing the modulation depth of an amplitude-modulated oscillation
IPC: G01R29/06
Filed: 1939-10-12
Abstract: 533,138. Modulating systems; impedance networks. PHILIPS LAMPS, Ltd. Oct. 12, 1939, No. 27785. Convention date, Oct. 15, 1938. [Classes 40 (iii) and 40 (v)] A circuit arrangement for varying the relative strengths of the carrier and side-bands in an amplitudemodulated wave comprises an oscillatory circuit tuned to the carrier and shunted by a rectifier operating as a peak detector and working into a lead the D.C. resistance of which is substantially different from its modulation-frequency impedance. As shown in Fig. 1 the tuned circuit 4 is connected in the plate circuit of a valve 1 of high internal resistance, the grid of which is fed with the amplitude-modulated wave. The shunt comprises a diode rectifier 5 in series with a load 7 across which is connected a condenser 6 which has low impedance at carrier frequency and high impedance at modulation frequency. If the resistance of 7 is low compared with its impedance at modulation frequency, then the circuit 5, 7 has low impedance at carrier frequency and the output voltage taken off at terminals 9, 10 has the side-bands accentuated. If the resistance of 7 is high compared with its impedanx at modulation frequency, then the circuit 5, 7 has high impedance at carrier frequency and the output at terminals 9, 10 has the carrier accentuated. The load 7 may comprise two parallel branches, one an ohmic resistance, the other a large condenser with or without a smaller resistance in series. Its A.C. impedance is therefore low compared with its D.C. resistance, and the output voltage is of lower modulation depth than the input. The output circuit may be inductively coupled to the tuned circuit 4 (Fig. 2 not shown). The damping of the circuit 4 may be reduced by a back coupling to the input grid (Fig. 3 not shown). The resistance constituting the D.C. load may be connected across the rectifier instead of across the condenser (Fig. 4 not shown). Fig. 5 shows a circuit for accentuating the side-bands. The tuned circuit 4 is inductively coupled to the tuned plate circuit of the valve 1, and the rectifier load comprises a resistance 13 shunted by a large condensor-17. The output, taken from a tuned circuit 19 coupled to a coil 20 in series with the rectifier 5, is of greater modulation depth than the input, i.e. the side-bands are accentuated. The same result is obtained with the circuit of Fig. 6, in which the tuned circuit 4 is connected through a small condenser to the rectifier 5 which is shunted by two parallel branches comprising (1) a resistance 13 and (2) a series arrangement of a choke coil 22 and a lower resistance 15. The rectifier load is high for modulation frequencies and low for D.C., and the output voltages at 9, 10 have the side-bands accentuated. In Fig. 7 the rectifier load comprises a choke coil 22 in series with a resistance 15, both being shunted by a condenser 6. The modulations-frequency impedance is high compared with the D.C. resistance, so that the shunt 5, 6, 15, 22 favours the passage of current of carrier frequency. The output is taken from the tuned circuit 19 coupled to a coil 20 in series with the rectifier, and is of reduced modulation depth. The circuit may be used for suppressing amplitude modulation in a frequency or phase modulated transmission system.