Patent GB-973418-A (GB)
Title: Diversity receiving system
IPC: H04B7/08
Filed: 1961-03-03
Abstract: 973,418. Radio signalling. STANDARD TELEPHONES & CABLES Ltd. March 3, 1961 [March 7, 1960], No. 7859/61. Heading H4L.  Relates to a diversity receiving system including an arrangement for bringing the received signals into phase for subsequent combination and is particularly applicable to an angle diversity system in which a number of directional aerials are energized by means of independent transmitters at different frequencies which co-operate with a corresponding number of receiving aerials appropriately orientated at the receiving station. As shown, a plurality of diversity signals of differing frequency are supplied via a wideband receiver 1 and I.F. amplifier 2 to the pre-detection combining circuit 3. At the input to the circuit 3 a mixer 5 receives the signals from the I.F. amplifier 2 together with a reference signal from an amplitude limiter 4 and the resultant frequency-spaced beat frequency signals are supplied to narrow band-pass filters 6 which are tuned to the respective signals. The outputs of the filters 6 together with the output signals from the I.F. amplifier 2 are supplied to a circuit 7 which is a heterodyning device and an amplitude product-taking device which supplies an output signal corresponding to the frequency of the reference signal. Each signal from the mixer 5 produces an output from its corresponding filter 6 in proportion to the degree of correlation between the associated input signal and the reference signal, and since each signal from the mixer 5 will have the same phase relationship with the reference signal as its associated input signal and the reference signal is supplied by the output of circuit 7, the combination of the signals from the filters 6 with their associated input signals in the circuit 7 will produce resultant beat frequency signals which are cophasal. Each individual output signal from the circuit 7 will have an amplitude proportional to the product of the corresponding carrier amplitude and its associated beat frequency signal from the filter 6 and is therefore proportional to the square of the amplitude of that particular carrier, thus giving a ratio-squared type of combination. If linear combining is required a limiter 9 is switched into circuit or alternatively the limiters 12 may be used. In a further arrangement a conventional mixer is substituted for the product multiplier 7. The output of the circuit 7 is supplied to an amplifier limiter 4 which has a pass-band such that only the desired beat frequency signal is passed and this combined signal provides the local oscillator signal for the mixer 5 and is also demodulated in the discriminator 15 and supplied to the utilization device 16. A further receiver 17 and combining circuit 3<SP>1</SP> operating under similar conditions with the mixer 5<SP>1</SP> utilizing the common reference signal, provides space diversity, the outputs from the circuits 7, 7<SP>1</SP>, being combined in the amplifier limiter 4. The combined output is also used to provide automatic gain control of the I.F. amplifiers 2, 2<SP>1</SP>. The signals supplied by the receiver 17 may be of the same or different frequency to those supplied by the. receiver 1.