GB1112448A - Improvements in and relating to electric frequency measuring apparatus - Google Patents

Abstract

1,112,448. Digital frequency measurement. ADVANCE ELECTRONICS Ltd. 7 Oct., 1966 [7 Oct., 1965]. No. 42580/65. Heading G4H. In apparatus for measuring the frequency of an electric signal, a further signal is derived having a frequency equal to that of the input divided by n, which signal is applied to a counter 4 having a controllable counting period, and an oscillator 8 is caused to develop an output signal of which the frequency of the nth harmonic differs from the input frequency by a standard frequency (10 mo/s.), the oscillator output being mixed with the further signal to obtain a signal having a frequency equal to the sum or difference of the frequencies of the mixed signals, this signal being applied to control the counting period of the counter to a period n times the standard frequency so that the number counted at the end of each measuring interval represents the frequency of the input signal. The input signal frequency on terminal 1 is applied to a multivibrator 2 which divides the frequency by n. The standard frequency of 10 mc/s., obtained from terminal 7 of the counter, is applied to a phase comparator 21. The oscillator 8 passes its output via a harmonic generator 16 to produce a frequency n times the oscillator frequency. This is mixed at 15 with the input frequency, and the sum frequency is compared at 21 with the standard frequency. The output of the comparator adjusts the oscillator until equality is obtained, the oscillator frequency then being #+ 10 mc/s. divided by n. This signal is then mixed with the output signal of multivibrator 2 (frequency #/n) and the result, after passing through band-pass filter 25, is 10/n mc/s. This is applied to terminal 6 of the counter to set the counting period. In a modification (Fig. 1, not shown), means are provided for first bringing the frequenoy of oscillator 8 to a value from which the phase comparator is capable of adjusting it to the required value. This may be done manually or automatically by a 3-position selector switch providing two preliminary adjustment steps and the frequency measuring operation described.