Patent GB-590901-A (GB)
Title: Improvements in or relating to means for comparing the frequencies of electric oscillations
IPC: H03L7/06
Filed: 1944-08-28
Abstract: 590,901. Automatic tuning correction. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Aug. 28, 1944, No. 16373. Convention date, Aug. 27, 1943. [Class 40 (v)] [Also in Groups XXXV and XXXVI] In a frequency comparison device both the wave under test and. the standard wave are applied to different electrodes, other than the cathodes, of each of a pair of electron discharge tubes, one of the waves being applied through a phase changer, whereby the beat frequencies produced in the respective tubes differ in phase by an amount and in a sense corresponding to the frequency deviation of the wave under test. The beat notes are applied respectively over cathode connected impedances to the control grids of a further pair of tubes arranged so that the average current in one of these tubes is greater dependent on the sense of the phase difference between the beat notes. As shown in Fig. 1, the frequency of a source B, for example a generator in. a radio transmitter or the local oscillator in a receiver is compared with a standard frequency source A in a phase detector PD comprising triodes VI, V2. The anodes of. these tubes are energized in phase from the generator B over condenser 6, and their grids are energized from the standard source A respectively over a phase advancing capacity C and a phase retarding reactance L. The beat note voltages produced across the cathode resistors R3, R4 are therefore equal in frequency but separated by a constant phase difference which is reversed in sense as the frequency of generator B passes through that of the standard A. These beat note voltages are applied over R.F. chokes 24, 26 and coupling resistances R5, R6 to the grids of triodes V3, V4 of a tripping circuit D of the type disclosed in U.S.A. Specification 1,844,950. The triodes V3, V4 have their grids and anodes reciprocally coupled by resistors R8, R9 and are so arranged that only one tube passes current at a time. The phase advancing and retarding elements C, L in the phase detector PD are preferably adjusted so that the phase difference between the two beat note voltages is 60 degrees. One of the triodes V3, V4 then conducts for one sixth of the beat cycle and the other for five-sixths. Connected in the anode circuits of triodes V3, V4 are the differential windings of a relay RL the armature 40 of which is rocked against contacts K1, or K2 dependent on which of the triodes V3, V4 is energized for the greater portion of the beat-cycle which again depends on whether the frequency of generator B is greater or less than that of the standard A. The armature 40 is energized over a line 50, as described below, and energizes one of the windings of a relay RL1 over the operative contact K1, K2 to operate a motor I energized from a source G to vary the tuning of the generator B in the corrective sense. A meter.or lamps may be used with the motor to indicate whether generator B is fast, slow or in step. Chattering of the armature 40 is prevented by large byepass condensers shunting the windings of relay RL and, preferably an inhibitor circuit is provided to prevent operation of the motor I 'until the beat frequency is substantial. The inhibitor circuit comprises triode amplifiers V5, V6 energized, respectively, over condensers 60, 62, by the beat-note voltages derived from the anodes of V3, V4. The secondary of transformer T connected in the anode circuits of V5, V6 is energized only when beat signals are present. The transformer output is rectified at X1, X2 to energize the armature 40 so that the relay RL1 and hence the tuning adjusting motor I are only operated when the generator B frequency differs substantially from that of the standard source A. The inhibitor circuit may be dispensed with in which case a D.G. source is connected to energize relay RL1 over armature 40. In a modification, Fig. 2 (not shown), the relay RL1 is dispensed with, and the inhibitor consists of a single triode normally blocked and adapted to pass current only when energized by the beat note voltage. A relay in the anode circuit of the inhibitor then operates a magnet, a contact of which energizes the armature 40 of relay RL to operate the motor and indicator I direct. In another modification, Fig. 3, the relay RL is included in the cathode return circuits of tubes V3, V4 instead of the anode circuits. In this case also the cathodes of the phase detector tubes V1, V2, are coupled to the grids of the tripping circuit tubes V3, V4 through D.C. filter condensers 29, 33, R.F. chokes 24, 26, potentiometers R5, R6 and low-pass condensers 31, 35, to isolate the R.F. and D.C. power in PD from the tripping circuit. A rectifier type A.C. meter M is connected across the filtered output of load resistor R4 by a switch 43 and may be calibrated to read the beat-frequency of the PD output. Since the standard frequency is known that of the transmitter B is also known. The operation of the tripping circuit, inhibitor and relay R.L. is substantially the same as that in Fig. 1. In this case the inhibitor tubes V5, V6 pass current only in the presence of a beat signal when a magnet RL 1 is operated to energize the armature of relay RL. For frequency indication purposes only, Fig. 4, milliammeters L, H are connected differentially in the cathode return circuits of the tripping tubes V3, V4 in lieu of the windings of relay RL (Fig. 3). When the generator B is not at the correct frequency, the two beat notes will cause one meter to read higher than the other according to whether B is fast or slow. As in Fig. 3 the meter M indicates the beatfrequency i.e. the cycles off frequency of generator B when switch 43 is closed. With B in synchronism, the deflection of M will be zero, but either meter L or H will show a deflection depending on the tripping circuit setting. U.S.A. Specifications 2,018,820 and 2,044,749 also are referred to.