Patent GB-767483-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: 767,483. Phase - frequency and amplitude frequency measurements; variable capacitors. GENERAL ELECTRIC CO., Ltd. May 10, 1954 [May 15, 1953], No. 13711/53. Class 37. Apparatus for measuring phase distortion of an electric circuit over a frequency band comprises a generator of a test signal applied to the circuit consisting of an oscillation modulated by a low-frequency oscillation F 1  and periodically tunable through the test range at a frequency F 2  # F 1 ; a demodulator receiving the output signal of the circuit which derives an oscilation of frequency F 1 ; a phase comparator responsive to the latter signal and a reference oscillation of frequency F 1  to derive a signal dependent at any instant on the group delay of the test circuit at the appropriate frequency of the test signal; an indicator responsive to the phase comparator signal to display variations of the group delay as the test signal is tuned over the test range; and a correcting device operative to compensate the indications, as the test signal is tuned over the test range, for variations of the group delay of the measuring apparatus. In Fig. 1 a transmitter comprises an oscillator 1 (which may be of the kind described in Specification 767,486 [Group XL (c)]), mechanically scanned 50 times per second over a mean frequency range of 50- 70 mc/s. at a constant rate of variation by synchronous motor 5 and simultaneously frequency modulated over 100 kc/s. on either side of the mean frequency by the output of a crystal oscillator 2 at 120 kc/s. through phase shift network 3, which consists (Fig. 2, not shown) of a transformer energized from the crystal oscillator where recording is tuned to approximately 120 kc/s. by a fixed condenser and a parallel variable condenser of special law (described below) to modulate the oscillator over a cathode follower amplifier and filter 4. The compositely modulated output of oscillator 1 is supplied to the system under test 9 at constant level by an A.G.C. controlled amplifier and the resultant output passes to a receiver comprising a further A.G.C. controlled amplifier and limiter feeding a frequency discriminator 14, the 50 c/s. component of whose output is extracted by filter 15 and amplified to horizontally sweep a cathoderay oscillograph 17 in synchronism with the test signal frequency scan, while the 120 kc/s. component having phase-shift dependent on the group delay or rate of change of phase shift of the total system with frequency is extracted by bandpass filter 18 and supplied to a 10X frequency multiplier 19 whose 1200 kc/s. output is combined in mixer 22 with a 1080 kc/s. reference signal from a 9X frequency multiplier 21 supplied with an amplified reference 120 kc/s. signal of constant phase through a crystal bandpass filter 23 from the 120 kc/s. output of filter 18. The resultant mixer output signal at 120 kc/s., having increased phase shift, is compared with the amplified reference 120 kc/s. signal in phase discriminator 24 whose output vertically deflects the C.R. oscillograph 17 to display the group delay of the total system against the corresponding transmission frequency, scanned over 50-70 mc/s.; a blanking unit 55 being triggered from the horizontal deflection waveform to suppress the trace during each flyback. Variable condenser 28 of the phase-shift network 3 is driven by motor 5 synchronously with the scanning of oscillator 1 over a shaft 37 (Figs. 3, 4) bearing as one electrode an aluminium slip-ring connected rotor 36 rotatable between electrostatic guard plates 48, 49 relative to a fixed insulatedly mounted sector 34 spacedly carrying 50 screws 35 extending within slots 42, 43 of the rotor, as the other electrode. Initially, the residual group delay of the transmitter and receiver is compensated by shortcircuiting the test system 9 and sequentially adjusting the screws 35 until the law of the variable condenser is such that the phase shift introduced by the network 3 over the frequency scan cancels the residual group delay as shown by a horizontal trace on C.R. oscillograph 17 over the whole frequency range, so that the C.R.O. thereafter displays only the group delay due to the test system. A further C.R. oscillograph 56 horizontally swept by the same signals as C.R.O. 17 is vertically deflected by the amplified and rectified output signals of the test system to display the amplitude/frequency characteristic thereof. In modifications, the input test signal may be derived from a mixer energized by an oscillator mechanically scanned over the working range and another oscillator frequency modulated at 120 kc/s. through a phase shift network mechanically variable in synchronism with the scan; or the reference 120 kc/s. signal for the receiver may be derived direct from oscillator 2 when the transmitter and receiver are used in proximity; or the variable phase shift network may be connected alternatively in the 120 kc/s. reference signal input to phase discriminator 24 or in the 120 kc/s. signal input to frequency multiplier 19, with the drive motor locked to the 50 c/s. signal output from filter 15. The phase-shift networks may be duplicated, one in the transmitter and one in the receiver, each being adjusted to compensate the group delay of its associated apparatus. In a further transmitter modification (Fig. 5) a compositely modulated test signal from amplifier 6 is fed through an A.G.C. amplifier 61 and limiter 62 to a frequency discriminator 63 (which are respectively identical with corresponding units 11, 13, 14 of the receiver) to produce a 120 kc/s. signal having residual group delay equal to that of the similar signal produced by frequency discriminator 14 of the receiver. The amplified output of frequency discriminator 63 is compared with that of crystal oscillator 2 in phase discriminator 65, whose output signal controls the setting of phase shift network 3 in the 120 kc/s. reference signal from oscillator 2 frequency-modulating oscillator 1, so as to compensate for the residual group delay of the measuring system over the range of frequency scan.