Patent GB-1296296-A (GB)
IPC: G01R25/00
Filed: 1971-06-23
Abstract: 1296296 Phase comparators; ramp generators; radio direction-finders SINGER CO 23 June 1971 29470/71 Headings H3A H3T and H4D A system for measuring the relative phase difference between two signals comprises: (a) A pair of frequency changers 20, 30 each having a first input 13, 14 fed with the two signals respectively; (b) A local oscillator 40 coupled by transmission lines 41, 42 to the second inputs of the two frequency changers 20, 30; (c) A signal generator 60, 90 coupled to the outputs 16, 19 of the frequency changers, and providing an output signal which alters the frequency of generator 40 to cause a continually varying phase difference between the outputs from the two frequency changers, through a point of phase coincidence; and (d) A sampler 95 samples the output from 90 at the instance of phase coincidence to produce an output proportional to the phase difference between the two input signals. The signal generator comprises a phase comparator 60 in which the two signals from the frequency changers are combined in quadrature by a hybrid 50 to give signals A + jB and B + jA and these signals are then fed to sum amplifier 70 and difference amplifier 80. When the output from sum amplifier 70 exceeds a threshold generator 90 starts a ramp signal which sweeps oscillator 90. The output from the difference amplifier is differentiated, and when the phase difference between the signals on 16, 19 passes through zero in one direction the output from the differentiator 84 causes circuit 95 to sample and store the value of the ramp signal from 90. The two signals being phase compared may be derived from the two aerials of a direction finder system as shown in Fig. 1, and in order to avoid ambiguities, a third co-linear aerial may be included, Fig. 2 (not shown), to produce a third signal which is phase compared with one of the others in a similar manner. The system may also be used for frequency measurement, Fig. 3 (not shown). In such an arrangement, the signal to be measured is fed directly to one input of the phase comparator, and is fed to the other input via a device producing a phase shift which is dependent on frequency. In the arrangement shown, a second comparator is used, being fed with the direct signal and with a signal fed via a different phase shifter. Further frequency changers fed from a further local oscillator may be included in lines 16, 19 for operating in two frequency bands, Fig. 4 (not shown). The ramp generator, Fig. 6, comprises a blocking oscillator Q2 which is triggered by the output from the sum amplifier applied to terminal 91, to produce an output pulse which turns off transistor Q4. Capacitor 152 then charges from constant current source Q6 to produce the ramp signal which is amplified by Q5, Q7, Q8, Q9 to provide a signal for sweeping the local oscillator and for feeding the sample and hold circuit. The sample and hold circuit. Fig. 7.-Pulses from blocking oscillator Q2 turn on Q11 to allow capacitor 164 to produce a ramp signal which is synchronized with that of the ramp generator of Fig. 6, by applying the output of the latter to transistor Q12. When differentiator 84 of Fig. 1 produces an output pulse, this is fed over lead 162 to stop the charging of capacitor 164, the voltage across it at that instant is compared with a slow ramp voltage from generator 170 by comparator Q13A, Q13B which produces the required output signal in dependence on the phase difference between the two signals to be compared.