Patent GB-651317-A (GB)
Title: Improvements in or relating to systems for measuring the difference of delays or thedifference of phase-changes to which two trains of electrical oscillations are subject
IPC: G01S1/24
Filed: 1948-10-21
Abstract: 651,317. Phase-difference measurements. SPERRY CORPORATION. Oct. 21, 1948, No. 27383. Convention date, Jan. 22. [Class 37] [Also in Group XL (c)] In a, cycle-matching system, particularly in a " Loran " hyperbolic navigation system, errors of one or more complete cycles are avoided by phase or frequency modulating the two trains of oscillations to be matched and comparing a plurality of successive cycles of each train of oscillations. Fig. 1a shows small portions of two pulse-modulated radio-frequency signal oscillation trains that are phase-modulated. The radio-frequency waves shown are those at the leading edge of the pulse and are therefore gradually increasing in amplitude. The two phase-modulated pulses are mis-matched or phase-displaced by one complete cycle of the radio carrier frequency, although being apparently in phase at X. At Y, however, the mis-match or phase-displacement is apparent. Fig. 1b shows ordinary pulses similarly mismatched by one complete cycle but not phasemodulated. In this case there is an apparent identity of phase both at X and Y. The radio frequency, phase-modulation frequency, and pulse repetition frequency may be so chosen that the phase-modulation comparison resolves cyclical ambiguity of the radio-frequency cycle comparison, and the pulse modulation comparison resolves cyclical ambiguity of the phasemodulation comparison. Fig. 3 shows transmitting apparatus embodying the invention comprising a pair of "Loran" transmitting stations A, B. A master station A includes a radio-frequency oscillator 1 and a low-frequency modulation generator 2, which phase-modulates the output from the oscillator 1 in a phasemodulator 3. Frequency-modulators may be used in place of phase-modulators. The phasemodulated output of the modulator 3 is applied to a transmitter 4, the output of which is pulsemodulated by a pulse-modulator 5 at the pulse repetition frequency generated by the pulsefrequency generator 6. Alternatively the phasemodulating and pulse-repetition frequencies may be obtained by dividing the frequency of the oscillator 1, as indicated at 8, 9. A slave station B is identical with the master station A except that its oscillator 1<SP>1</SP> is synchronized with the oscillator 1. The phase-modulators 3, 3<SP>1</SP> and pulse-modulators 5, 5<SP>1</SP> are similarly synchronized. In order to identify the respective pulses an arbitrary delay between the master and slave pulses is effected in a delay circuit 7. Thus the outputs of the two stations will be waves as shown in Fig. 1a, i.e. synchronized waves phase-modulated and also pulse - amplitude - modulated. Fig. 4 shows receiving and timing apparatus for co-operation with the transmitting apparatus of Fig. 3 and is similar to the conventional " Loran " receiver with the addition of a selector switch 12 and an additional " very fast " sweep voltage output for the usual cathode-ray oscillograph 14 employed for pulse comparison. With the sweep generator 29 in the first or "slow" position the signals are received by the radiofrequency amplifier 11 and applied through the pulse detector 10 to the oscillograph 14, so that the two pulse envelopes from the stations A, B produce separate traces. A drift-control 32 is then adjusted until the master pulse is located on the upper pedestal, and a delay control 30 is adjusted until the lower pedestal is under the slave pulse. The sweep generator 29 is turned to the second position and the drift and delay controls 32, 30 are adjusted until the respective pulse envelopes are near the left-hand end of the sweep traces. The sweep generator 29 is then turned to the third position, which provides a much faster sweep, and the controls are again adjusted to match the pulse envelopes, the trace separation being eliminated in order to superimpose the sweeps to provide better matching. The sweep generator 29 is then turned to the fourth or " very fast " sweep and the switch 12 is turned to the left-hand position, which applies about five radio-frequency cycles to the oscillograph. The delay control 30 is adjusted until the two radio frequency voltages are perfectly matched. A time-difference meter 26, which is geared to the control knob 30, then reads the time delay between the signals.