Patent GB-762753-A (GB)
Title: Automatically frequency selective indicating or measuring apparatus
Inventor: BORDEWIJK JAN LOURENS
IPC: G01R23/00
Filed: 1952-10-13
Abstract: 762,753. Measuring-bridges; voltage measurement; frequency analysers. STAATSBEDRIJF DER POSTERIJEN, TELEGRAFIE EN TELEFONIE. Oct. 13, 1952 [Oct. 13, 1951], No. 25629/52. Class 37. Apparatus for indicating or measuring alternating voltages or currents derived from a bridge network energized by frequencies lying within a selected band comprises a frequency generator energizing the network, a first modulator energized by the bridge unbalance signals wherein they are single sideband modulated with a fixed frequency, a second modulator wherein the resultant signals are further modulated by the bridge energizing generator frequency to produce a range of output signals of different frequencies, including the fixed frequency, and frequency selective means for selecting therefrom the voltage or current signals at the fixed frequency which are amplified to operate measuring and indicating devices as a function of the bridge output signals, with discrimination against spurious unwanted frequencies therein. In Fig. 3, a bridge 2 energized by a variable frequency f at 1 measures the impedance/frequency characteristics of a properly terminated coaxial cable forming one arm, with the comparison characteristic impedance of the cable forming another bridge arm (Figs. 1, 2, not shown). The unbalance voltage is combined in modulator 4 with a signal of e.g. 100 c/s. from oscillator 5, and the resultant frequencies f, f + 1000 are combined in demodulator 6 with frequency f from the bridge input to produce signals of frequencies f, 2 f, f + 1000, 2/+ 1000, 1000 c/s. of which the latter component is extracted by narrow band filter 7, selectively amplied at 8, and applied to a measuring instrument and an audible detector; the apparatus being thus made responsive only to bridge unbalance signals of the excitation frequency f and unresponsive to unwanted signals not producing a 1000 c/s. signal after modulation and demodulation. Fig. 4 shows a modification giving a true indication of the unbalance voltage irrespective of the phase variations thereof in the neighbourhood of balance in which the unbalance output is applied to wideband networks 4, 5 giving phase shifts of + 45 degrees whose outputs differing in phase by 90 degrees are respectively modulated by the 1000 c/s. output of oscillator 8 applied after respective phase shifts of 45 degrees in net. works 6, 7, to frequency converters 6A, 7A whose single sideband modulated outputs are combined at the input of demodulator 9 and demodulated by the excitation frequency f of the bridge. The resultant output passes through bandpass filter 10 and selective amplifier 11 which suppress all unwanted frequencies, leaving only a 1000 c/s. signal corresponding to the bridge unbalance at frequency f, which is shown to be independent of the phase angle of the latter, and which operates a measuring instrument and an audible detector, so that errors due to phaseangle variations of the bridge unbalance signal giving zero indication for non equilibrium are removed. To avoid interference due to the presence at the demodulator of a spurious frequency St from the bridge output together with its modulation product St+1000 c/s. which combine in the demodulator non-linearity to give a spurious 1000 c/s. signal, the unbalance output of bridge 2 is modulated with 1000 c/s. in a balanced modulator 3 (Fig. 5) and the resultant signal is applied to a single sideband demodulator 4 supplied with the bridge energization frequency through a pair of phase shift networks P giving a shift of + 45 degrees to produce an output voltage at 1000 c/s. representing the bridge unbalance voltage which is phase shifted by + 45 degrees in further networks P, passed through a 1000 c/s. bandpass filter 5 and selectively amplified at 6 to operate a measuring instrument and an audible indicator as before. The apparatus may be utilized as a selective wide-range vacuum tube voltmeter, and threshold A.V.C. may be applied to an amplifier succeeding the bridge unbalance output to reduce over-excitation of the demodulator on large unbalance (Fig. 6, not shown). In detection of bridge unbalance the frequency response curve of the apparatus may be made complementary to the bridge response curve, the local oscillator outputs may be provided with distortionless limiters (e.g thermistors or lamps) and the output may operate a recording device.