Patent GB-1335211-A (GB)
Title: Demodulation systems
IPC: H04L27/233
Filed: 1970-01-14
Abstract: 1335211 Telegraphy; phase detectors; gate and hold circuits PLESSEY CO Ltd 8 Jan 1971 [14 Jan 1970] 1731/70 Headings H4P H3A and H3T A system for demodulating differential phaseshift signals comprises phase-detector means receiving the input signals and a phase reference independent of the input and identifying the one of a plurality of predetermined phase segments, fixed relative to the phase reference, in which the input signal vector lies, and output means comparing the phase positions of the segments identified in two successive data periods to detect the phase change between them and produce a data-indicating signal whose value is dependent on the change. In each data period, the phase of the input signal at 34 (Fig. 1) is Œ45 degrees or Œ135 degrees relative to that in the previous period, this phase difference representing the data value. A phase detector 30 produces between lines 42a, 42b an analogue signal indicating in amplitude and sign the phase difference between the input at 34 and the phase reference from 36. A phase detector 32 performs a similar function except that the phase reference is phase-retarded at 38 by 90 degrees. In quadrant detector and store 50, the two phase detectors 30, 32 each control a flip-flop via a differential amplifier receiving the two output leads of the phase detector, giving a two-bit indication of the quadrant in which the phase lies in the current data period which is decoded to partially enable one of four gates 56, 58, 60, 62, and select (in analogue detector and store 40) the output of one of the detectors 30, 32 for storage on a capacitor after comparison at 70 with the voltage stored on the capacitor in the previous data period. The voltage thus stored represents the phase relative to the beginning of the quadrant. Each data period, clock pulses at 74 are passed via clock gate 76 to increment two 4-state counters 64, 87 until counter 64 agrees with the output of quadrant detector and store 50 to gates 56, 58, 60, 62 which perform the comparison. If the comparison at 70 shows a decrease in the voltage selected for storage at 40, relative to the preceding period (indicating that the number of quadrant boundaries crossed by the input vector is one more than the number of phase angles of 90 degrees through which it has passed), line 72 increments counter 64 by one to reduce by one the number of pulses passed to counter 87, the output of which on one of four lines indicates the data value. Counter 87 is reset to its first output after each data period. Analogue detector and store 40 and comparator 70 are shown in detail in Fig. 8, a signal on line 66a or 66b enabling transistors at 99 or 100 to pass the voltage across lines 42a, 42b or 44a, 44b to line 108, a clock signal at 84 enabling transistors 126, 128 so that this voltage and that across capacitor 116 are compared in differential amplifier 130, a clock pulse at 83 then enabling transistor arrangement 110, 112, 118, 120 to pass the voltage on line 108 on to capacitor 116 for use in the next data period. An FET or metal oxide silicon transistor may replace the arrangement 110, 112, 118, 120. Phase detector 30 is shown in detail in Fig. 10, the output voltage between 42a, 42b across resistor 216 indicating the phase difference between the input at 34 and the phase reference 36, bias voltages V B , V B <SP>1</SP> being provided as shown. Modifications.-Quadrant detection 50 may be analogue. Detection of angle within quadrant 40 may be digital. The possible phase changes could be Œ90 degrees, Œ180 degrees. Multidifferential phase modulated signals can be demodulated by cascading extra units 40, 50.