GB685358A - Improvements in or relating to electric carrier wave modulators - Google Patents

Abstract

685,358. Valve modulating circuits. STANDARD TELEPHONES & CABLES, Ltd. Aug. 15, 1951, No. 19244/51. Classes 40 (v) and 40 (vi). A modulating circuit comprises a pair of similar valves 1, 2 with their grids excited in push-pull by an input wave of frequency q, and with their anode-cathode circuits in parallel with each other and in series with the anode-cathode path of a valve 4 to whose grid a second input wave of frequency p is applied and whose anode current follows the instantaneous grid voltage; output being taken at frequencies p, q, p Πq, from the anode of either or both of the valves 1, 2. The carrier wave (which may be the input p) is amplified in a valve 7 before being applied to the grid of the valve 4, and negative feed-back is applied from the cathode resistor 46 of the valve 4 to the grid of the valve 7. The signal wave (which may be that of frequency q) has one side of its input transformer 3 connected to the grid of a further valve 11, and voltage of frequency q is transferred from the anode of valve 11 to the anode of valve 2 to supplement the voltage of frequency q at the latter point. Negative feed-back is provided from the junction of cathode resistors 33, 34 to the grid of the valve 11, and the amount thereof may be adjusted by a variable resistor 37 switchable into parallel with the resistor 34, thus varying the gain of the valve 11. A network comprising two condensers 39, 40 and an inductor 41 is provided between the cathodes of the valves 1, 2, the network giving series resonance at frequency q and parallel resonance at frequency 2q. Adjustment of condenser 40 and resistor 37 enables the q output at terminals 28 or 32 to be reversed in phase by operating the switch 38, without altering the q output amplitude. The valves 7, 11 may be connected in series across the H.T. supply, interaction being prevented by holding the lead 16 at zero alternating voltage by condensers 27, 26. The modulating wave q may be a speech wave, but the invention is particularly intended for use in measuring the delay of transmission in a wide-band frequency modulation communication system. For this purpose the modulation products p, p Πq, and the reference wave q are transmitted together over the system, and at the receiving end the reference wave q is recovered from the received modulation products and its delay compared with that of the reference wave q received directly. This is repeated over a wide-band of variation of the p frequency, e.g. from 250 kc/s. to 5 mc/s., while the reference frequency q may be fixed at 60 kc/s. It is necessary that the harmonics of p and q, and the higher order side-bands should be of very small amplitude; and the modulator is designed to meet these requirements.