Patent GB-1010761-A (GB)
Title: Improvements in or relating to electrical frequency measuring equipment
Inventor: MORGAN ARTHUR GEORGE HENRY
IPC: G01R23/02
Filed: 1960-10-26
Abstract: 1,010,761. Frequency discriminators. ENGLISH ELECTRIC CO. Ltd. Jan. 26, 1962 [Oct. 26, 1960], No. 36781/60. Heading H3A. An electrical discriminator comprises a cyclically operative switch having a first contact operative to make connection one at a time to a plurality of other contacts, with a device producing an electrical output of value varying with the frequency of an AC input connected to the first contact, and plural A.C. sources connected to the other contacts, so that a rectangular waveform appears at the output of the frequency sensitive device, whose amplitude is a measure of the frequency difference between successive pairs of the A.C. sources. In Fig. 7, frequency sources f 1  and f 2  are connected to contacts 1, 2 of switch S cyclically engaged by a wiper connected through an amplitude limiter to a pump circuit (Fig. 2) wherein an input capacitance C is in series with a forward diode D2 and the emitter of a transistor connected in grounded base circuit; the capacitor/diode junction being earthed over a reversed diode D1 and the transistor collector being capacitatively shunted to earth and energized over a load resistor to develop a squarewave signal of amplitude proportional to the difference between the input frequencies. The diode D2 may be dispensed with (Figs. 3, not shown) or in modified transistor circuits (Figs. 4, 5, 6, not shown), and diode D1 may be reversed with NPN transistors. In Figs. 8, 8A, the transistor output is amplified and applied to the wiper of a switch S2 cyclically engaged with contacts 1, 2 in synchronism with switch S1 contact 2 being earthed and contact 1 developing an unsmoothed direct voltage representing the frequency difference between f 1  and f 2  in magnitude and sense. Frequency f 1  may be fixed to define the centre frequency of the discriminator and switch S2 is replaceable by a simple rectifier to give an output signal proportional to the modulus of the frequency difference. In Fig. 10, ganged 4 contact clockwise rotary switches S1, S2 have wipers connected to the input of the amplitude limaiter and to the output of the frequency detector respectively, with contacts 1, 3 of S1 connected to frequency source f 2 , contact 4 to frequency source f 3  and contact 2 to frequency source f 1 , while contacts 1, 3 of S2 are earthed, and contacts 2 4 are connected to outputs O1, 02. Fig. 11 shows the cycle of frequencies derived by S1, Fig. 12, the corresponding voltage cycle at the frequency detector output, and Fig. 13 the resultant waveform at the wiper of S 2 ; so that outputs O1, 02 each carry one pulse per switch revolution of magnitude and sign given by + (V 3 - V 2 ) and - (V 2  - V 1 ) filterable to give IC outputs representing (f 3 -f 2 ) and (f 1  - f 2 ) respectively, so that f 1  and f 3  are definable with regard to f 2  as a fixed reference frequency. The switches may have six, eight or more even number contacts with alternate contacts corresponding to the reference frequency and to plural comparison frequencies.