GB581201A - Improvements relating to arrangements for frequency measurement, of particular application to obstacle detection - Google Patents

Abstract

581,201. Radiolocation. STANDARD TELEPHONES & CABLES, Ltd., and EARP, C. W. April 21, 1942,No. 5368. [Class 40 (v)] [Also in Group XXXVI] A signal, of which the frequency is to be measured, is applied to two electrical paths, either or both of which includes means for producing a frequency responsive differential distortion of a characteristic of the signal in the outputs of the two paths, the outputs of the two paths being applied to a measuring instrument which is automatically adjusted in accordance with this differential distortion and thereby gives an indication of the frequency. In the arrangement shown in Fig. 1 the signal is fed into two paths composed of resistance R1 condenser C1 and resistance R2 and condenser C2. The condenser C1 has small reactance compared with the resistance R1, while the condenser C2 has large reactance compared with the resistance R2. In one path the phase of the signal is advanced 90 degrees and in the other path the phase is retarded 90 degrees at all frequencies, so that the two outputs are always anti-phase and produce a straight line trace on the screen of a cathode ray oscillograph when applied to the plates X. Y. The network R1, C1 has high attenuation at high frequencies, while the network R2, C2 has high attenuation at low frequencies, so that the straight line trace has different angular positions for different frequencies, turning through a right angle over the frequency range from zero to intinity. In a modification, Fig. 2, a complete rotation or multiple rotation of the trace is obtainable. The signal is fed into two paths, one including a delay network DN and the other an attenuating pad T to adjust the outputs of the two paths to equal amplitudes. The phase of one output with respect to the other rotates uniformly with frequency on account of the phase distortion introduced by DN. The outputs are combined to produce their vectorial sum at A and their ectorial difference at B. The two outputs A, B are both variable in amplitude with frequency but are always in quadrature. The output A is subjected to an aperiodic phase shift of 90 degrees in the phase shifter PS1 so that the outputs A, B are rendered always in-phase or anti-phase and can be applied to the plates X, Y of a cathode ray oscillograph to give a line indication for all frequencies. In another arrangement, Fig. 4, the signal at IN is passed through a delay network DN and is then phasesplit by a resistance-capacity phasing network R1, C1, R2, C2 to provide a circular trace on the oscillograph screen. In the phasing networkthe resistances R1, R2 and condensers C1, C2 have substantially equal impedances over the frequency band passed by the delay network. The other path comprises an amplitude limiter L1, high pass filter F, and unidirectional limiter L2, from which a succession of short pulses is applied to a control electrode of the cathode ray oscillograph. The control electrode may be a focussing electrode by which the circular trace is brought into focus only at one point, corresponding to the frequency of the signal, thus giving a bright spot. Alternatively the deflection sensitivity of the oscillograph may be modulated by the pulse train, when a sharp radial kink is produced on the circular trace. In a further modification, Fig. 5, the signal at IN is passed in one path A through a delay network DN and is then fed to a phase-splitting network PS comprising resistances R1, R2 and condensers C1, C2. Four phases obtained from the corners of the network progress in equal steps of 90 degrees. Each output is combined with the non-delayed signal from path B, applied to separate rectifiers r1 .. r4, integrated in resistance-capacity circuits n1 .. n4, and applied to the plates X, Y of the oscillograph. As the phase difference between the signals in the paths A, B varies the spot deflects angularly, the deflection being a measure of the phase difference and hence the frequency. The deflection sensitivity of the oscillograph may be modulated at a frequency sufficiently high to avoid producing closed figure traces, as by applying an alternating voltage to an accelerator electrode AE. These arrangements may be used in an obstacle detector of the type which utilises a frequency-swept radio transmission. Relative velocity may be measured by converting the received pulses to an intermediate frequency and applying to a delay network having a delay equal to the transmitted pulse repetition period. If the obstacle is moving, frequency displacement due to the Doppler effect is measured on the oscillograph. In order to interpolate accurately between two fixed stable-frequency waves produced by a frequency standard the indication can be arranged to give any desired angle of rotation from one fixed frequency to the other, application of the intermediate frequency to be measured giving an intermediate angle of deflection on a calibrated scale.