Patent GB-767484-A (GB)
Title: Improvements in or relating to apparatus for the measurement of phase distortion in electric circuits
Inventor: BRYDEN JOSEPH EASTO
IPC: G01R25/00
Filed: 1953-05-15
Abstract: 767,484. Phase shift measurements. GENERAL ELECTRIC CO., Ltd. May 17, 1954 [May 15, 1953], No. 13712/53. Class 57. Apparatus for measuring phase distortion in an electric circuit over a frequency band comprises a generator of a test signal consisting of an input oscillation modulated by a relatively low frequency F 1  and periodically tuned over the frequency band at a frequency F 2  # F 1 , a demodulator responsive to the output signal of the test circuit to produce an oscillation of frequency F 1 ; a phase discriminator comparing the latter signal with a reference oscillation, and an indicator responsive to the phase discriminator output to display variations of the test circuit group delay with the tuning of the test signal; the generator of the reference oscillation being controlled by the smoothed output of the phase discriminator to lock the respective frequency and phase of the reference signal to the frequency and mean phase of the modulation of the output signal of the test circuit. The output frequency of an oscillator 1 is swept sinusoidally at 50 c/s. over the range 100- 120 mc/s. about a mean of 110 mc/s. by a tuning condenser driven by synchronous motor 3, and is combined in mixer 5 with the output of an oscillator 2 at 170 mc/s. frequency-modulated over a range of Œ100 kc/s. by a crystal controlled oscillator 4 operating at 120 kc/s. The amplified mixer output comprising a test signal varied sinusoidally at 50 c/s. over the range 50-70 mc/s. and frequency-modulated at 120 kc/s. is applied to a transmission system 7, which may be a radio link involving modulation of a carrier frequency of 2000-4000 mc/s. At the receiving end the limited test signal energizes a frequency discriminator 9 from whose output a 50 c/s. signal is filtered to horizontally sweep (after amplification) the trace of a C.R. oscillograph 13, and to operate a blanking unit 21 suppressing the trace at each flyback. A 120 kc/s. signal in phase dependent on the group delay (the rate of phase shift change with frequency) of the test transmission system is filtered from the output of discriminator 9 and applied through N times (e.g. 10) frequency multiplier 16 to mixer 18 together with the output of (N - 1) times frequency multiplier 17 energized by a reference signal at 120 kc/s. from oscillation generator 22, to produce a 120 kc/s. signal having increased phase shift, which is compared with the reference signal in phase discriminator 19, whose output vertically deflects the C.R. oscillograph trace to display the variation of phase shift over the test frequency sweep. Oscillation generator 22 comprises a crystal-controlled oscillator 24 operating at approximately 120 kc/s. in combination with a controlling reactance valve 23, regulated by the output of the phase discriminator 19 after smoothing to represent the mean phase difference of the signals applied thereto, so that the reference frequency is held identical with that of the phase shifted 120 kc/s. signal, while its phase is determined by the mean phase shift of the latter signal, so that the mean output of the phase discriminator is zero and the displayed trace is self-centring to show the group delay over the test frequency sweep. The apparatus is adaptable for two-way measurement by using the reference frequency oscillation generator for one direction of measurement to frequency modulate the test signal for the other direction of measurement. The test signal may alternatively be generated by an oscillator of the kind described in Specification 767,486, [Group XL (c)], and the apparatus may be compensated for its residual group delay as set forth in Specification 767,483.