GB901972A - Frequency discriminator - Google Patents

Abstract

901,972. Frequency discriminators. MERO, P. G. S. Feb. 12, 1959, No. 4936/59. Class 40 (5). A frequency discriminator comprises two transistors 16, 17, the base electrodes of which are supplied in push-pull from the intelligence signal source 10 through a transformer and the emitters of which are supplied with a control signal from a frequency-responsive phase-shifting device in the form of a series-resonant circuit 27, 26 tuned to a reference frequency equal to the intelligence signal frequency, the control signal being of a frequency equal to the intelligence signal but shifted in phase by an amount determined by the intelligence frequency, the collector load circuit comprising resistances 20, 21 and the output of the discriminator at terminals 24, 25 consisting of a voltage, the amplitude of which indicates the intelligence signal frequency with respect to the reference frequency. When the signal frequency is equal to the reference frequency, i.e. the resonant frequency of the tuned circuit 26, 27, the net output of the discriminator will be zero, as shown at 46 in Fig. 2, where 42 (in solid lines) represents the output of transistor 31 for this frequency condition. The inputs to the bases of the transistors 16, 17 are as shown at 40, 41 in Fig. 2 and the output from transistor 31 applied in push-push to the transistor emitters will cause them to conduct current from the negative D.C. source B - to ground alternately, the currents through resistances 20, 21 (R 1 , R 2 ) being shown at 44, 45, respectively. The other case shown in Fig. 2 is that when the frequency of the intelligence signal is in such relationship with the reference frequency that the output of tuned circuit 27, 26 at 28 is shifted by a further 90 degrees, this being represented by 50 in the Figure. In this case transistor 16 is cut off all the time while transistor 17 conducts during each negative half-cycle of signal 41 so that there will only be a current through 21 (R 2 ) which is shown at 53, and the average voltage output at 24, 25 is as shown at 54, or, if the output at 28 is shifted in phase the other way, as shown at 55. A modification of the discriminator is described (Fig. 4, not shown) in which the control signal is used as a gating signal for the push-pull connected transistors rather than as a biasing signal for the emitters, while in a further embodiment (Fig. 5, not shown) the third transistor is omitted and the output tuned circuit connected to the transformer secondary input to the push-pull transistors, the emitters of which are earthed. In a further embodiment (Fig. 6, not shown), the control signal circuit is included in the load (or collector circuit) of the push-pull transistors. A detailed circuit diagram of the embodiment of Fig. 1 is given (Fig. 7, not shown).