Patent GB-626154-A (GB)
Title: Frequency-modulation radio receivers
IPC: H03D11/02
Filed: 1947-03-11
Abstract: 626,154. Frequency modulation. HAZELTINE CORPORATION. March 11, 1947, No. 6783. Convention date, March 19, 1946. [Class 40 (v)] Relates to frequency-modulation detector systems employing two circuits tuned respectively above and below the centre frequency of the F.M. wave, and combining differentially the outputs of rectifiers associated with the respective circuits ; and consists in arranging the two circuits so that their frequency-response characteristics are in the form of the probability curve (log y=a-bx<2>), and their amplituderesponse characteristics are in the form of a logarithmic curve (y=c log x). Under these conditions, the detected output signal varies linearly with the frequency deviation of the input wave, and is independent of amplitude variations. The invention is described in connection with the F.M. receivers of Specification 589,153, in which the tuned circuits are back-coupled to valve electrodes, and are given alternately positive and negative resistance by a quench oscillator so as to operate as superregenerative receivers. In some forms, only a single circuit is used, its resonant frequency being alternated between two values by a thermionic reactance. In Fig. 1, the incoming wave is applied through amplifiers 19, 19<1> to the tuned circuits 10, 11 ; 10<1>, 11<1> which are in back-coupling connection with valves 12, 12<1>. The tuned circuits are associated with oppositelypoles rectifiers 24, 24<1> and load resistors 25, 25<1>, and the difference between the rectified voltages is taken through resistors 28, 28<1> to an instrument 27. The valves 12, 12<1> have their anodes connected to a pulse generator 20 which renders the valves operative intermittently, either simultaneously or alternately according to the setting of a switch 22. In Fig. 2a, the upper curve shows the variations of the conductance of one of the tuned circuits 10, 11 under the influence of pulses of quench frequency applied to the valve 12. The desired probability curve for the frequency response is attained by giving a suitable slope D to the change from positive to negative conductance during the period t0, t1. The slope must, however, be such as to leave a considerable fraction t1, t2 of the quench period during which the negative conductance is uniform. In this way signals F<1>, F, F" of large, medium and small values are all able to reach the same saturation value during the period t1, t2, as indicated in the two lower curves. This condition is necessary in order that the amplitude response characteristic may have the desired logarithmic form. The Specification shows by analysis that the output across resistor 25 has the form k1 (log E-x1<SP>2>) +k2, where E is the amplitude of the input carrier, and x1 is the deviation of frequency from that of the tuned circuit. For the other tuned circuit a similar relation is obtained, except that the deviation x2 is from the other resonance frequency. When these are combined differentially, the final output is represented. by k1 (x2<SP>2__>x1<SP>2>), which, when x1 and x2 are referred to a datum frequency equal to the centre frequency of the input wave, reduces to a. linear function of the deviation. The quench pulses corresponding in shape to the upper curve of Fig. 2a are produced by clipping a sine wave or a sawtooth wave. They may be adjusted to the desired shape by trial and error after experimentally plotting the curves of frequency response and amplitude response of the respective tuned circuits. The circuits employed may be modified in the manner shown in Figs. 3, 5 and 6 of Specification 589,153. Specification 571,580 also is referred to.