Patent GB-581202-A (GB)
Title: Improvements relating to arrangements for frequency measurement
IPC: G01R27/32
Filed: 1943-12-08
Abstract: 581,202. Frequency measurements. STANDARD TELEPHONES & CABLES, Ltd., and EARP, C. W. Dec. 8, 1943, No. 20558: Addition to 581,201. [Class 37] An arrangement for measuring the frequencyof a signal wave comprises a potential divider which includes two series-connected network portions solely composed of reactive elements and having different reactance characteristics, means for applying the signal wave to the potential divider, and means for separately applying the potential differences obtained across the network portions to a cathode ray oscillograph in such a manner that it gives an automatic indication of the frequency. In the arrangement shown in Fig. 1, a signal the frequency of which is to be indicated is applied through a transformer T to a potential divider comprising an inductance L and condenser C in series, the junction point being connected to the deflecting plates X1, Y1 of the cathode ray oscillograph K, which plates may be connected to earth as indicated. The plates X, Y are connected to the other terminals of the inductance L and condenser C, these terminals also being connected, to the transformer T. The resulting trace on the oscillograph is a straight line confined to two opposite quadrants, the orientation being dependent on the frequency. At very low frequencies the potential of the plate X approaches zero and the trace approaches the vertical axis, whilst at' very high frequencies the potential of the plate Y approaches zero and the trace approaches the horizontal axis. The whole frequency range is thus indicated over an angle of 90 degrees. This range may be extended by forming the two parts of the potential divider as series resonant systems L1, C2, L2, C1, Fig. 3, which resonate at different frequencies. The scale may be improved by the addition of reactive components C3, L3 shunting the two portions of the potential divider. The primary or secondary winding of the transformer T may be shunted by a series resonant circuit L4, C4 to counteract the resonance of the potential divider as a whole. The scale of the trace may be further extended by the use of reactance networks having more than two resonance frequencies in place of the systems L1, C2, L2, C1. Additional resonant shunts may be connected across the transformer to deal with any additional resonances of the whole potential divider. The arrangements described may be used in altimeters and obstacle detectors. If the oscillograph screen is provided with a polar diagram scale the length and position of the trace may be used for checking amplitude-frequency characteristics of amplifiers &c. Where a number of different signal frequencies are simultaneously present, the arrangement shown in Fig. 6 may be used. Incoming signals are applied to a modulator M1 supplied from a variablefrequency oscillation generator G. The sidebands are passed to a narrow band-pass filter F and thence to a second modulator M2 supplied in parallel with the modulator M1 from the oscillator G. When the generator G is adjusted so that a side-band frequency is produced that will pass through the filter F, the frequency component that produces-the side-band will be recovered from the modulator M2, all other components being excluded. The recovered component is applied to a network N for providing the deflecting voltages for the oscillograph K as previously described. If the generator G be caused to scan continuously over a range of frequencies simultaneous indications of all the frequency. components present will be obtained on the oscillograph. A number of different' potential dividing networks may be provided and selected by switches so that various frequency scales may be obtained.