Patent GB-686821-A (GB)
Title: Improvements in and relating to electric wave apparatus
Inventor: ECKERSLEY PETER PENDLETON
IPC: G01R27/32
Filed: 1947-10-08
Abstract: 686,821. Current and phase measurements. TELEPHONE MANUFACTURING CO., Ltd. Oct. 5, 1948 [Oct. 8, 1947], No. 27035/47. Class 37. [Also in Group XL (c)] The attenuation or phase-shift characteristic of a network over a range of frequency is tested or determined with simultaneous elimination of the effects of harmonics by mixing two waves, not necessarily purely sinusoidal, of variable fundamental frequencies fa, fb, but constant small frequency difference fd in a modulator 16, one wave passing through the test network 13, and passing the difference frequency through a filter 17, responsive only to fd, to an output meter 19, fd being so small that (M#a#N#b)##d is not satisfied for any values of the integers M, N, less than the integers m, n, where m and n are the orders of the lowest negligible harmonics of fa, fb. If m, n, are equal, fd should be less than 1/mth of the lesser of fa, fb. The network attenuation is measured by comparison with a calibrated attenuator 14 adjusted to give the same reading on the output meter 19. The modulation product may be made independent of the amplitude of one frequency by making the latter of much greater amplitude than the other. The frequencies may be obtained from resistance-capacity oscillators, as shown (Fig. 2), with equal resistors and capacitors R, C, the latter being variable and ganged, and with different capacities Cs1, Cs2 in series with the capacitors C so that the frequency difference is constant, or the resistors R may all be varied equally and be shunted by fixed different resistors Rp1, Rp2, respectively, in each oscillator, giving a constant frequency difference Alternatively, Fig. 3 (not shown), a wave free of harmonics may be passed from an oscillator of variable frequency f through a network producing two waves in phase quadrature to crossed coils of a transformer forming a rotating field therein, the transformer having a coil rotated at a constant frequency d within to give a pure output frequency (#1 + d) or (#1 - d) from the coils. This is mixed in a modulator with a frequency f2 filtered from harmonics to give one of the frequencies (f1-f2Œd). The oscillations f1, f2 are similarly beat to give a frequency (#1 - #2). The harmonic filters limit the permissible range of frequency variation of f 1 and f2 to less than an octave but the difference frequencies (#1 - #2) and (#1 - #2Œd) vary widely. A similar result may be obtained, Fig. 8 (not shown), by beating two different pure fixed frequency oscillators in modulators with an oscillator variable over a frequency range less than an octave, having a harmonics suppressing filter. In Fig. 4 (not shown), the resistancecapacity oscillators of constant difference frequency feed an output meter through a balanced modulator and a highly selective amplifier acting as a filter, a cathode bias thermistor fed from each oscillator output acting to maintain the oscillation amplitudes constant, as before a changeover switch inserting the test network or a calibrated attenuator in the output from one oscillator. One oscillator is automatically maintained of constant frequency f2 by beating in a further balanced modulator with f and passing the difference frequency fd through a discriminator with response maximum above and below the correct frequency differentially feeding a bias thermistor in the oscillator. In Fig. 5, the phase shift characteristic of a network 91 is measured by beating oscillators of constant frequency difference (#1 - #2) in modulators 94, 95, the latter feeding through a filter 96 a network 97 producing two waves leading and lagging by 45 degrees passed to two further modulators 98, 99 to which a filtered pure frequency (f1-f2) is also passed from modulator 94. The relative phase of the D.C. outputs from 98, 99 depends on the phase shift of the network 91 and the latter may be measured on an oscilloscope for varying frequencies or compared with a calibrated phase shift network 92. In the Provisional Specification, for phaseshift measurement, two oscillations are beat in a ring modulator and the frequency difference fd beat with the same frequency fd obtained by beating the two oscillations through a path including the test network. The resultant D.C. component is measured on a D.C. indicator. The difference in phase shift when the network is or is not in circuit is measured by balancing with a calibrated phase shifter to maintain the D.C. reading constant. Attenuation may be measured by phase splitting the frequency fd to produce a circular trace on an oscilloscope CRO and maintaining this radius constant when a calibrated attenuator is substituted for the network. In a modification, one oscillation and the test network are replaced by a wave to be analysed, the oscilloscope or D.C. instrument being used as a null indicator.