Patent GB-1326953-A (GB)
Title: Curcuit system for the comparison of two frequencies
IPC: H03J3/14
Filed: 1970-09-23
Abstract: 1326953 Frequency detecting; pulse frequency changer WANDEL U GOLTERMANN 23 Sept 1970 [8 Oct 1969] 45284/70 Headings H3A and H3T [Also in Division G1] In order to determine both whether the frequency f<SP>1</SP>m of an unknown pulse train is greater or less than a reference frequency f ms , and also the magnitude of any difference, especially for small differences in frequency of the order of 1c/s, time is notionally divided into a sequence of time slots occurring in groups of p  numbered from 0 to p-1 and repeating at the reference frequency f ms , the time slots are initially phased so that a pulse of the unknown pulse train occupies a time slot 0, and subsequently occurring pulses-all of which should occur in subsequent time slots 0 but which will in fact drift either towards slots p-1 or slots 1 if f<SP>1</SP> m  is slightly larger or smaller than f ms  respectively-are examined to detect when the drift has become sufficiently large for a pulse actually to occur in a slot p-1 or 1, and each time such an occurrence is detected the pulse in question is routed to flash one of two lamps in accordance with the direction of the drift, i.e. the sign of f<SP>1</SP> m -f ms , and the notional time slots are rephased so that time slot 0 again coincides with a pulse, namely the pulse that occurred in slot  p-1 or 1 prior to rephasing. Thus the particular  lamp flashed indicates whether the unknown frequency f<SP>1</SP>m is larger or smaller than the reference frequency f ms , and the frequency of flashing is equal to p times the difference frequency. As described p = 4, and clockpulses of frequency 4f ms  are applied to a twostage binary counter 6, 7, Fig. 1, thereby establishing the notional time slots which are numbered repeatedly from 0 to 3. The unknown pulse train f<SP>1</SP>m, which as described, is derived from a waveform f m  by squaring and differentiating, is applied to two AND gates 3, 4, which are arranged to be open respectively during time slots 3 and 1. Thus, if f<SP>1</SP> m  = f ms  and if the counter 6, 7 is reset to zero by one of the pulses of the train f<SP>1</SP>m, then every pulse of the train f<SP>1</SP> m  will occur in a slot 0 and none will pass through either gate 3 or gate 4. If however f<SP>1</SP> m  departs from f ms , then eventually, as explained above, a pulse of f<SP>1</SP> m  will occur in a time slot 3 or a time slot 1 and will pass through one of gates 3, 4 to flash a corresponding lamp 10, 11 and also, via an OR gate 5, to reset the counter 6, 7 to zero thereby effecting the necessary rephasing of the time slots. Although the lamps will flash at four times the difference frequency, the duration of each flash will depend upon the duration of each pulse of the train f<SP>1</SP> m . In a modification (Fig. 2, not shown) in order to increase the flash duration and to render it independent of the train f<SP>1</SP> m , triggers are interposed between the gates 3, 4 and the lamps 10, 11 and a monostable circuit is provided to reset either trigger a fixed time after it is turned on. In order to ensure that the counter 6, 7 is correctly phased when the arrangement is initially turned on, gate 3 is also open during time slots 2, and therefore the first pulse of the train f<SP>1</SP> m  that occurs will reset the counter to zero, unless, by chance, it already is set at zero. The pulses applied to either lamp could also be applied to an integrator to provide an amplitude signal indicative of frequency difference.