Patent GB-1474084-A (GB)
Title: Heterodyne frequency measuring system
IPC: G01R23/14
Filed: 1974-05-24
Abstract: 1474084 Measuring frequency TAKEDA RIKEN KOGYO KK 24 May 1974 [28 May 1973]  23280/74 Heading G1U In a system for determining the frequency fx of an input signal 1, Fig. 1, the input signal is mixed 2 with a swept reference signal fr(t) and all its harmonics 2fr, 3fr, etc, a particular intermiediate frequency fi is selected by a filter 4 from the resulting modulation products, so that as the reference frequency fr(t) rises with increasing time a series of output pulses, each being a very short burst of frequency fi, are produced by the filter 4, and that output pulse which follows an earlier output pulse by a pre-set time duration is selected. This pulse is then used to terminate the sweep of the reference signal fr(t), and it transpires that the frequency fx of the unknown input signal can then be deduced from (a) the pre-set time duration, (b) the intermediate frequency fi and (c) the final value of the reference frequency fr(t). In fact, as will be seen below, selecting an intermediate frequency output pulse that follows, by a pre-set time duration, an earlier output pulse, is equivalent to selecting a particular order of harmonic of the reference frequency fr(t). It then follows that if fr(t2) is the reference frequency when such a pulse occurs, and if n is the order of the selected harmonic, The theory on which the invention is based is as follows. Let t1 be the first time that the nth order harmonic of the reference frequency fr(t) beats with the unknown input frequency fx to produce an intermediate frequency output, Assume that the reference frequency fr(t) increases with time at a rate of R hz/sec, then from (1) and Thus, from equations (1) and (2) each harmonic of the reference frequency fr(t) produces a pair of intermediate frequency outputs (or beats), and from equation (3), regarding fi and R as given, any particular order of harmonic, namely n, can be chosen by appropriately choosing #t. In theory, however, a pair of intermediate frequency outputs separated by a period of #t, may result not only from the same order of harmonic, namely n in equation (3), but the first may result from one order of harmonic, say n1 and the second from another order, say n2, and this can occur in four different ways, one being - However either the equations in question have no solution, or, as for equations (4) and (5) nl is, in practice, so large that the intensity of the harmonic of that order is substantially zero. Hence, as stated above, by using equation (3) any order of harmonic can be selected, and clearly to vary the order that is selected any one of #t, R and fi can be appropriately changed. In the preferred embodiment #t and fi are maintained constant and successive sweeps of the reference frequency are made at different speeds. As described the unknown frequency 1 is applied to mixed 2 with the reference signal, the intermediate frequency output pulses are extracted 4 and detected 5, and passed via a noise reducing circuit 6 to a gate 7 which looks for pairs of pulses separated by the pre-set time #t. This is effected by causing each intermediate frequency pulse to reset a counter 15, so that it starts to count clock pulses 24 from zero, and by causing the counter via decoders 17, 18 and a bi-stable to open a gate 7 for a narrow time window commencing a little less than #t and ending a little more than #t after the zeroizing of the counter, so that if after any intermediate frequency output pulse another output pulse occurs a time #t later, it finds gate 7 open and passes through to reset bi-stables 20, 22 and stop a counter 13 which controls the sweep of the reference frequency. Of course, if any intermediate frequency output pulse follows its predecessor by less than the period #t then it merely resets the counter to zero before the time window arrives. When the sweep of the reference frequency is stopped, the final reference frequency is measured 41, multiplied 42 by the order of the harmonic and combined 44 with a measurement 40 of the intermediate frequency. As described successive sweeps follow each other automatically the sweep rate being changed 12 for each by a counter 43 until the unkown input has been measured.