Patent GB-586278-A (GB)
Title: Improvements in or relating to means for measuring difference in frequency of alternating currents
IPC: G01R23/00
Filed: 1943-04-01
Abstract: 586,278. Radiolocation. STANDARD TELEPHONES & CABLES, Ltd., and JACOBSEN, B. B. April 1, 1943, No. 5267. [Class 40 (v)] [Also in Group XXXVI] A circuit for measuring in sign and magnitude the 'difference between 'the frequencies of two waves comprises means for applying the waves of both frequencies to each of two single-sideband modulators so arranged that a lower sideband is produced only in one modulator when' the frequency difference has one sign and only in the other when it has the other sign. Currents of frequencies F1, F2, the difference f of which is to be determined, are applied to two corresponding filters 1, 2, 'Fig. 1, for removing undesired frequencies. The frequency F1 is then applied to two similar branches of the circuit in parallel. The upper branch comprises two similar modulators 3, 5 connected in parallel and leading to a quadrature phase difference network 7. This comprises two phase-changing networks connected respectively in tandem with the modulators 3, 5 and producing a phase difference of 90 degrees between the two output currents, which are then combined and applied through a low-pass filter 9 and shaping network 11 to an output rectifier or detector 13 having in its output a resistance R1, across which it produces a continuous difference of potential. The lower branch similarly comprises modulators 4, 6, quadrature phase difference network 8, filter 10, shaping network 12, and detector 14 connected to a second resistance R2 equal to R1. The current of frequency F2 is used as a carrier current for the modulators 3, 4, 5, 6 and is supplied to them in pairs through a quadrature phase-difference network 15, so that the modulators 5, 6 are supplied with carrier current in quadrature with that supplied to the modulators 3, 4. An instrument 16, such as a centre-zero D.C. voltmeter, is connected across the resistances R1, R2. The shaping networks 11, 12 cause the voltage applied to the detectors 13, 14 to vary with the difference frequency f in any desired way. The direct current outputs 'from the detectors 13, 14 flow in opposite directions in the resistances R1, R2. Each branch of the circuit forms a single-sideband modulator and the arrangement is such that the upper branch produces the lower sideband F1-F2 only if F1>F2, and the lower branch produces the lower sideband F2-F1 only if F2>F1. A rectified current is therefore obtained in R1 proportional to the frequency difference f in the first case and in R2 in the second case so that the instrument 16 reads on one side of zero when f is positive and on the other side when f is negative. The modulators 3, 5 and the quadrature network 7 may be omitted as shown in Fig. 3. The two components at the output of the network 8 are applied respectively to two conjugate pairs of input terminals of a four-wire terminating set TS. The other two conjugate pairs of output terminals are connected to the input sides of the filters 9, 10, the remainder of the circuit being as shown in Fig. 1. The two components derived from the network 8 are added at the input of one of the filters 9, 10 and subtracted at'the input of the other, so that the effect on the instrument 16 is the same as in Fig. 1. Fig. 4 shows a circuit adapted for the aircraft locating system described in Specification 582,406 in which an aeroplane pursuing another machine carries a transmitting aerial Z, which emits a carrier wave of frequency F, and two receiving aerials X, Y placed some distance apart. The receiving aerials pick up waves reflected from the pursued machine, which waves have frequencies F+F1 and F+F2 slightly different from that transmitted and from each other on account of the Doppler effect. An oscillator 20 generates waves F which are supplied to the transmitting aerial Z and also to three similar frequency changers 21, 22, 23, the last being supplied through a network 30 which introduces a change of phase of 90 degrees. Filters 24, 25, 26 remove unwanted frequencies derived from the frequency changers, their outputs being connected to amplifiers 27, 28, 29, which are provided with means for maintaining a constant output level. The outputs of the amplifiers 27, 28, 29 are applied to the primary windings of three transformers TC, TB, TA, of which TA, TB have two equal secondary windings and TC has four equal secondary windings. The measuring circuit comprises two valves V1, V2 arranged as rectifiers in opposition. These correspond to the output rectifiers 13, 14, Fig. 1. The resistances R1, R2 are connected in series with the anodes, the anode battery BI being connected to their junction. The instrument 16 is connected across the resistances R1, R2 and is shunted by a condenser C for removing A.C. components. Two equal resistances R5, R6 are connected in a portion of the grid circuit common to both valves. Equal resistances R7, R8 shunt the control grids, and the potential drop across two equal resistances R3, R4 is applied between the control grids through equal condensers C7, C8. The resistances R3, R4, R5, R6 form the loads of four similar modulators M1, M2, M3, M4, each of which comprises four copper oxide or selenium rectifiers arranged in a bridge in which one pair of diagonal corners are connected to input terminals 1, 2 and the other pair are connected to output terminals 3, 4. The modulators M1, M2 are supplied with frequency F2 in the same phase, but with F1 in opposite phases, and similarly for M3, M4, but the phase of F2 as supplied to M1, M2 is 90 degrees in advance of that supplied to M3, M4. The outputs of both pairs of rectifiers are respectively connected in series opposing. The L-type networks C7, R7 and C8, R8 perform the combined functions of the elements 7, 11 and 8, 12, Fig. 1. The voltage applied to the control grids of the valves VI, V2 is equal to the voltage drop across the resistances R5, R6 plus or minus a voltage proportional to the drop across R3, R4 advanced in phase by 90 degrees. In a modification, Fig. 6, two ring modulators RM1, RM2 are used instead of four bridge rectifiers. The transformers TTA, TTB which supply the frequency F2 each have a centre-tapped secondary winding, and the transformer TTC which supplies F1 has two similar centre-tapped secondary windings. Each of the ring modulators RM1, RM2 is supplied from one of the windings of the transformer TTC. The modulator RM1 is also supplied from the transformer TTA, and RM2 from TTB. The two control grids are supplied in series from the secondary winding taps of the trans. formers TTA, TTC, and in parallel from the taps of the transformers TTB, TTC. Specifications 260,067, [Class 40 (v)], and 547,601, and U.S.A. Specification 2,248,250 are referred to,