Patent GB-895230-A (GB)
Title: Improvements in or relating to impedance measuring apparatus
IPC: G01R27/02
Filed: 1959-07-10
Abstract: 895,230. Valve amplifying circuits; phase detectors. ELECTRONIQUE APPLIQUEE. July 10, 1959 [July 17, 1958], No. 23771/59. Classes 40(5) and 40(6). [Also in Group XXXVI] In an impedance device an A.C. source Ve (Fig. 1) is connected to earthed potential dividers R11, R12 of ratio P and R21, R22 of ratio Q, to contact b of three-way switch C22, and to a selected resistance R4 connected over switch Cl to contacts c of three-way switches C23, C24. The top of divider R11,  R12 is returned to contact c of three-way switch C21 and that of R21, R22 to contact b of switch C24, while an unknown impedance Z is connected between the sliders of switches C22, C23; the contact c of switch C22 being earthed, all the switches C2 being ganged and the sliders of switches C21, C24 being connected to feedback amplifiers Al, A2 having bridge rectifiers in the feedback paths (Fig. 3), supplying unidirectional currents to the coils of a logometer MZ; a series galvanometer G detecting overloading. For low values of Z, swithces C2 are placed in position c so that amplifier Al is fed from potential divider R11, R12 and amplifier A2 from a potential divider having R4 in series with Z (Fig. 2c not shown) so that logometer MZ reads proportionately to the modulus of the impedance; R4 being switchable to introduce a variable multiplying constant. For high values of Z, switches C2 are placed in position b so that amplifier A1 is fed from potential divider having Z in series with R3 and amplifier A2 from potential divider R21, 22 (Fig. 2b not shown) the logometer as before reading proportionately to the modulus of Z. For channel balance, switch Cl is placed in position E whereby the inputs of A1, A2 are connected to the taps of the potential dividers R11, R12, R21, R22 and the amplifier feedback gains are adjustable for equality. The amplifier outputs are also applied to clipper-limiter stages El, E2 and Schmitt bistable circuits B1, B2 (Fig. 3 not shown) to produce rectangular waveforms in phase with the respective outputs (Fig. 4a, b, e, f) which are differentiated, pulses of predetermined polarity are suppressed (Fig. 4c, d, g, h) and the remaining pulses operate a flip-flop circuit B# generating rectangular waveforms of opposed polarities (Fig. 4i, j) which are applied to opposite poles of a voltmeter M# measuring the combined rectangular waveform (Fig. 4k) whose half-cycles have widths varying with the relative phase shifts between the respective actuating signals; the net D.C. component and the deflection of the meter representing the unknown impedance in magnitude and sign. Accuracy is ensured at values of phase shift approaching zero by allowing the introduction of a switchable 180 degrees phase shift into the Schmitt circuit BI of one channel only by switch C31 (Fig. 3). In the position a of switch C2 the inputs of amplifiers Al, A2 are connected directly through calibrated attenuators el, e2 to inputs for comparing directly the amplitudes and phase shifts of two input signals or for determining the amplitude and phase response with frequency of a quadripole fed with a reference signal. The potential dividers R11, 12 and R21, 22 may be variable.