SIGNAL HANDLING ARRANGEMENTS AND FREQUENCY DIVERSITY COMBINING ARRANGEMENTS UTILISING THE SAME

摘要

1408423 Diversity reception MARCONI CO Ltd 16 Jan 1973 [11 April 1972] 16738/72 Heading H4L A known arrangement for bringing received signals into a predetermined phase relationship for use in a frequency diversity predetection combining system, is modified to minimize unwanted phase-shifts arising from the use of filters. In the known arrangement, Fig. 1, terminals A, B, receive respective frequencydiversity signals to be combined at OP. If the transfer characteristics of the combining circuits are square law the A.G.C. circuit 15, 16 controlling amplifiers 13, 14 provides ratio-squaring in known manner. Assuming that the signal at A is 70 MHz with phase angle α and the signal at output OP is 59À3 MHz with phase angle #, these two signals are supplied to a balanced mixer 5 whose output via a narrow band filter 7 is a difference signal output of 10À7 MHz without modulation and with phase angle α - # which is supplied to a balanced mixer 6 receiving also the 70 MHz input. The difference signal from mixer 6 is 59À3 MHz with modulation and instantaneous phase angle # so that the instantaneous phase angle of the incoming signal has been removed. Channel B operates similarly and is assumed to have an input of 70 MHz + #F so that the output of filter 7B will be 10À7 MHz+ #F and the output from mixer 6B via filter 8B will be 59À3 MHz as before. However, due to the difference in frequencies of the signals passing through filters 7 and 7B, the output signals applied to terminals 9, 9B will acquire relatively different phase shifts due to the phase/ frequency characteristics of the filters and the present invention reduces these phase errors. In one arrangement, Fig. 2 (not shown), the filters 7 are replaced by a phase-locked loop circuit which acts as a tracking filter having a narrow bandwidth of the same order as that of filter 7 and trackable over a range corresponding to the difference in input signal frequencies which may be expected in use. This arrangement provides a constant output level which does not permit ratio-squaring. In a modification of this arrangement, Fig. 4, the phase-locked loop 24 is connected between mixers 5 and 6 as before but in addition, a synchronous-detector 22, 23 provides an output to a low-pass filter and amplifier 26 which is linearly proportional to the output of mixer 5, i.e. is proportional to the amplitude of the input signal. The output of filter 26 controls a PIN diode attenuator 25 providing an output which is proportional to the square of the input signal which is the condition required for ratio-squaring. In a further arrangement, Fig. 3, the filter 7 is replaced by a tracking filter 18 controlled via an active low-pass filter 20 by a frequency discriminator 19 deriving one input from the mixer 5 and the other from an oscillator 21 whose output frequency is the centre frequency of the tracking filter. The output of filter 18 is amplitude linear, permitting ratiosquaring.