GB747489A - Improvements in or relating to apparatus for electrical frequency measurement - Google Patents

Abstract

747,489. Frequency measurement. CINEMATELEVISION, Ltd. Feb. 9, 1954 [Feb. 19, 1953], No. 4683/53. Class 37. An electrical frequency measuring apparatus comprises a frequency determining circuit of limited range supplied from the difference frequency output of a mixer circuit over a lowpass filter cutting off at the upper limit of the range of the frequency determining circuit; the mixer being supplied with the input signal to be measured and with one of a plurality of signals of differing and accurately known frequencies generated by an oscillator circuit; and circuit means responsive to the absence of a signal at the output of the low-pass filter to control the operation of a selector which successively applies the several known frequency signals to the mixer, until a signal appears at the filter output and the difference frequency is thus brought within the range of the frequency determining circuit, whereby the measured frequency is equivalent to the sum of the selected known frequency and the indication of the frequency determining circuit. In Fig. 2, an input signal of unknown frequency is supplied to a mixer circuit 2 whose difference frequency output passes through a low-pass filter 5 to a gating and shaping circuit 8 producing sharp pulses from the cycles of the applied waveform, which are registered by an electronic six decade visually indicating counter circuit 9 over a period of e.g. 1 second determined by pulses from a timing circuit 10 comprising a six decade counter operated from the output of a 1 Mc/s. crystal oscillator, so that frequencies of 0 to 1 Mc/s. are measurable. Filter 5 is given a cut-off frequency of 1 Mc/s. and the input signals are converted to the measurable range by supplying the mixer with one of the harmonic frequencies of the crystal oscillator, separated by a circuit 20 and selected by an automatic stepping switch 3 cyclically rotated by a drive magnet 16 energized from D.C. source 17 over relay contacts 19 and break contacts 18 operated by the switch itself. When no signal appears from the output of filter 5 on the grid rectifier valve 11, relay 14 is energized from D.C. source 15 to close contacts 19 and operate the stepping switch 3 continuously until it selects a local frequency which brings the difference frequency within the filter range. The current in valve 11 then falls to de-energize relay 14 and arrests the stepping switch, and the counter 9 measures the difference frequency, which is added to the selected local frequency to determine the unknown input frequency. The stepping switch may be arranged to operate display means for the selected frequency similar to those of the counter 9 and to home to its zero position on cessation of the filter output until restarted by an operator's switch. In a modification (Fig. 3), the mechanical stepping switch is replaced by gate circuits 22, 23, 24, 25 inserted in the respective frequency channels from the harmonic selector 20 to the mixer circuit 2, which gates are sequentially and continuously opened by a counting-ring circuit 21 stepped by pulses from timing circuit 10 and arrested by a unidirectional potential derived from rectifier 7 on arrival of a signal on the output of filter 5.