Patent GB-539655-A (GB)
Title: Improvements in diversity radio communication systems
IPC: H04B1/30
Filed: 1940-06-28
Abstract: 539,655. Wireless receiving systems. STANDARD TELEPHONES & CABLES, Ltd. June 28, 1940, Nos. 10998 and 10999. Convention dates, Aug. 23, 1939, and Aug. 31, 1939. [Class 40 (v)] The invention relates to an arrangement for reducing signal-to-noise ratio at the receiver of a diversity system of the type in which the signal receiving apparatus is at all times connected with all the paths and the energy contributions to the signal receiver by the various paths are controlled automatically in proportion to the varying amplitudes being received over said paths, and in which the carrier frequency for demodulation is generated locally or reconditioned at the receiver. In such a system the amplitude of the demodulated signal and noise contributed by each of the diversity branches is caused to vary with the amplitude of, and under the control of a selected portion of the energy received from the transmitting station over the corresponding space path. In the Fig. a single side-band system is described in which signals are received by aerials A 1 , A and pass to H.F. stages 11, 11<SP>1</SP> supplied from a common beat oscillator 20, and then through I.F. amplifiers 12, 12<SP>1</SP> and isolation amplifiers 13, 13<SP>1</SP> to second detectors 14, 14<SP>1</SP> which are fed with demodulating carrier frequency from a local oscillator 30 through local carrier amplifiers 18, 181. The audio frequency output is combined by transformers 21, 21<SP>1</SP>, amplified at 50 and received at 10. Part of the output of the I.F. amplifier passes to a band-pass crystal filter 15, 15<SP>1</SP> which selects a narrow band including the carrier which is passed to detectors 16, 16<SP>1</SP>, 17, 17<SP>1</SP>. Detectors 17, 171 control the potential drop across a common resistance 40 which is fed back to the I.F. stages to vary the gain such that the radio frequency outputs of the two branches have the same relative signal strengths as the radio frequency inputs. Detectors 16, 16<SP>1</SP> control the potential across resistances 19, 19<SP>1</SP> to vary the gain of amplifiers 18, 18<SP>1</SP> such that the amount of local carrier supplied to each channel is proportional to the signal strength. Thus when the signal in any branch fades, the amount of carrier supplied is reduced and hence the noise contribution of the branch is also diminished. The amplifiers 13, 131 are of the screen grid type to prevent feedback of the local carrier and its appearance in the other detector circuits. In a modification, the amount of local carrier supplied is constant and the demodulated output from the second detectors passes to audio frequency amplifiers in each branch, the outparts of which are combined and passed to the signal receiver. In this case the detectors 16, 16<SP>1</SP> serve to control the gain of the audio amplifiers so as to reduce the audio output of the branch in which fading is such as to produce an unfavourable signal-to-noise ratio. The demodulation carrier may alternatively be supplied by received carrier filtered and reconditioned. In cases where the carrier is entirely suppressed, a pilot frequency may be used for controlling the gains in the several signal branches. U.S.A. Specification 2,041,855 is referred to.