GB1178539A - Improvements in Timing Arrangements for Electrical Signalling Systems - Google Patents

Abstract

1,178,539. Homodyne receivers; automatic frequency control. PLESSEY B.T.R. Ltd. 13 Dec., 1967 [14 Dec., 1966], No. 56067/66. Headings H3A and H3Q. To generate a signal locked in phase and frequency with an incoming signal (e.g. for coherent detection of a modulated wave or a digital signal), the input signal is combined with (a) a locally generated signal of the same frequency, and (b) a phase-quadrature version of such local signal, to give two outputs whose magnitudes represent the phase difference between the respective pairs of signals combined; the required signal is then obtained by adding the combinations of these two outputs with the two respective locally generated signals. In the simplest form (Fig. 1) the input at 1 is combined at 3 with alocal oscillator at 2 and with a 90 degree shifted version of this at 6. After passage through low-pass filters 8 and 9 the resultant phase-difference signals A and B are re-modulated on to the two locally generated signals at 4 and 7, and the outputs additively combined at 10. To provide a phase-memory for long interruptions of the input signal (Fig. 3, not shown) the A and B signals are combined to produce (A<SP>2</SP> + B<SP>2</SP>) which opens electronic switches (15, 16) between the filters and the modulators 4 and 7 when its amplitude falls below a certain level. The previous values of A and B are held by storage devices at the inputs of the modulators 4 and 7. These storage devices may be capacitors (17, 18) which normally form part of the filters 8 and 9, or for longer interruptions may be analogue-to-digital converters with digital stores. Separate stores may be switched in for different types of incoming signals. To stabilize the amplitude of the output at 11 this may be amplified and sliced to produce a square wave, or a gain-controlled amplifier may be provided at the input 1. Alternatively the inputs to the modulators 4 and 7 can be A(A<SP>2</SP> + B<SP>2</SP>)<SP>-¢</SP> and B(A<SP>2</SP> + B<SP>2</SP>)<SP>-¢</SP> instead of A and B respectively. In another form of amplitude stabilization (Fig. 4, not shown) the filters 8 and 9 are replaced by integrators whose decay times are inversely proportional to the amplitude of the envelope of the signal at 11. Frequency-look circuits.-To minimize the phase error when an interrupted signal reappears, the frequency of the local oscillator 2 can be controlled by a voltage fed back through a low-pass filter. In Fig. 5 (not shown), this voltage represents which can be shown to be proportional to the frequency error and to the square of the amplitude of the incoming signal. Thus rapid frequency correction occurs for a large signal while "frequency memory" is held during interruptions. As before, the low-pass filter can be replaced by a switched store. Phase-lock circuit (Fig. 6, not shown). The circuit of Fig. 1 is modified by controlling the local oscillator frequency from the output of unit 6 through a low-pass filter. The average value of this output is V sin #, where V is the amplitude of the incoming signal and # is the phase difference between the signals at 1 and 2. The features shown in the different figures may be used in combination.