Patent GB-779233-A (GB)
Title: Improvements in or relating to frequency monitoring apparatus
Inventor: WALLS WALTER RICHARD
IPC: G01R23/00
Filed: 1954-02-11
Abstract: 779,233. Frequency measurement. CINEMATELEVISION, Ltd. Feb. 11, 1955 [Feb. 11, 1954], No. 4036/54. Class 37. [Also in Group XL (c)]  Apparatus measuring the frequency of periodic electric signals comprises a frequencydetermining apparatus of upwardly limited range, an input of test signals, a mixer fed therewith wherein they may or may not be mixed with other signals to yield respectively a difference frequency component or a component of the input frequency, means applying the mixer output signals to the frequency determining apparatus, means responsive to the presence of a signal at the input and the absence at the mixer output of a signal within the range of measurement of the frequencydetermining apparatus which is operative to apply to the mixer a locally generated signal of frequency not greater than the upper limit of the range of measurement to produce a mixer output comprising a difference frequency component, and other means responsive to the presence of the locally generated signal and the continued absence of a mixer output signal within the range of measurement operative to replace such locally generated signal by its second and higher harmonics in succession until there is derived a mixer output containing a frequency component within the range of measurement, whereby the input frequency is determined as the sum of the indication of the frequency determining apparatus and the locally generated signal frequency. In Fig. 1 the signal whose frequency is to be measured is applied over terminal 1 to mixer 2 supplied over gating circuits 11, 12, 13 with the fundamental, 2nd and 3rd harmonics of a constant frequency oscillator 14 and harmonic generating circuit 15 through appropriate selective amplifiers 16, 17, 18; the fundamental being equal to the maximum frequency of their measuring circuit 4 and the mixer output containing the input frequency plus the difference frequency being supplied to the frequency measuring circuit 4 (which may count the cycles of the applied signal over a fixed time interval to operate a direct display), through a low-pass filter amplifier 3 of cut-off equal to the maximum frequency of the measuring circuit. A selector circuit 9 (which may be a homing uniselector) is stepped by a single stroke relaxation oscillator 8 under control of interacting trigger circuits 6, 7 (Fig. 2, not shown), to sequentially open gate circuit 10 in the " home " position followed by gate circuits 11, 12, 13. Initially, the frequency measuring circuit 4 indicates zero and selector 9 in the home position opens gate 10 and closes gates 11, 12, 13 so that the applied input signal at the output of gate 10 is rectified at 20 to a negative bias potential for trigger circuit 7, preparing the relaxation oscillator 8 to make one stroke. If the input frequency lies within the range of measuring circuit 4 and the pass-band of amplifier 3, the signal frequency appears in the mixer output, is measured and displayed, and is also rectified at 5 to negatively bias trigger circuit 6, whereby operation of the relaxation oscillator is inhibited notwithstanding the preparation bias on trigger circuit 7 from rectifier 20. If the input frequency is greater than the range measurable by circuit 4, zero rectified bias voltage is applied to trigger circuit 6, which, in conjunction with trigger circuit 7, allows the relaxation oscillator 8 to perform one stroke and pulse the selector 9 one step, so that gate 10 closes and gate 11 opens and supplies the fundamental frequency of oscillator 14 to the mixer to develop a difference frequency at the output which if within the range of measurement of circuit 4 is measured and displayed thereby, and is also rectified to a negative bias inhibiting the relaxation oscillator 8 over trigger circuit 6 as before. If the difference frequency is outside the measuring range of circuit 4, amplifier 3 produces no output signal and trigger circuit 7 is operative in response to the output from gate 11 rectified at 19 to allow the relaxation oscillator to perform one more stroke and pulse selector 9 to open gate 12 and close gate 11. A similar cycle of operations closes gate 12 and opens gate 13, and if the input frequency is higher than the measuring capacity of the equipment with gate 13 open, the selector 9 returns to the initial home position and thereafter recycles indefinitely or it may be stepped to a further position operating an alarm.. The range of fixed frequencies available may be extended indefinitely and may be derived from separate oscillators; while removal of the input signal removes the rectified bias from trigger circuit 6 and allows the relaxation oscillator to repeatedly pulse the selector 9 until it reaches its home position, at which the absence of a rectified bias on trigger circuit 7 from gate 10 arrests the pulse sequence. The input frequency is measured as the sum of the reading of the measuring circuit 4 and the frequency of the operative fixed local signal source. Trigger circuit 7 can only operate in the presence of the rectified output of gate 10 or the rectified output of gates 11, 12 or 13, so that no higher local frequency may be selected in the absence of the next lower local frequency, thus preventing false readings under fault conditions. Selector 9 may comprise a thermionic valve counting chain. Specifications 747,489 and 769,698, [Group XL (c)], are referred to.