Patent GB-704929-A (GB)
Title: Arrangements for altering the phase of the modulation of amplitude-modulated a.c. signals
Inventor: MEREDITH FREDERICK WILLIAM
IPC: G05D3/14
Filed: 1952-04-15
Abstract: 704,929. Alternating current signal transmission systems. SMITH & SONS (ENGLAND), Ltd., S., and FURZEHILL LABORATORIES, Ltd. April 15, 1952, No. 9396/52. Class 40 (4) [Also in Groups XXXV and XXXVI]  Signals conveyed as amplitude modulation of an A.C. carrier are altered in amplitude and phase in a frequency-dependent manner without subjecting them to demodulation and subsequent re-modulation. The modulated carrier is applied to a phase-sensitive rectifying system in which alternate half-cycles of the modulated carrier are switched to opposite ends (or to the same ends of two opposed windings) of a load impedance in the form of an inductor, the switching means preferably comprising dry-plate rectifiers blocked and unblocked in turn bv a reference A.C. The inductor is connected in series with a resistor external to the rectifying system, and the output taken across the resistor comprises an amplitude-modulated A.C. carrier in which the modulation is delayed in phase by an amount dependent on its frequency. In Fig. 1 four rectifiers 105 ... 108 are so connected across the secondaries 109 ... 112 of the transformers 119, 120 that one half-cycle of the reference voltage C unblocks the rectifiers 105, 107 and blocks the rectifiers 106, 108, whilst the other half-cycle produces the opposite effect. The modulated voltage applied at AB, if in phase with the reference voltage C, is thus enabled during one half-cycle to drive a current through the coil 103 of an inductor 104, and in the other half-cycle through the coil 102. The two coils are so wound that these two current pulses produce flux in the same direction in the core. If the modulated voltage AB is in anti-phase with C, the flux in the inductor core is in the reverse direction. The effect is to impose a time-lag on any changes in the amplitude of the A.C. passing between the terminals AB, and if this current is passed in series through the inductor L, Fig. 4, and an external resistor R, the alternating voltage across this resistor has an amplitude variation differing in a frequency-dependent manner from that of the applied modulated voltage. Limiter resistances 113 ... 116 are connected between the transformer secondaries and the rectifiers. In a modified circuit, Fig. 2 (not shown), four pairs of rectifiers are used, each pair being connected back to back in one arm of a bridge which has the modulated input applied across one diagonal, and the inductor, which in this case has a single winding, is connected across the other. In Fig. 3, the four arms of a bridge each comprise a secondary bridge 304 ... 307 of four rectifiers, the four secondaries of the reference input transformer 316 being each connected across a diagonal of the corresponding secondary bridges so as to unblock one pair and block the other pair alternately. The modulated input is applied to one diagonal AB of the main bridge and the inductor 302 connected across the other, so that current always passes in one direction through the inductor, this direction depending on whether the modulated input AB is in phase or in antiphase with the reference voltage C.