Patent GB-652464-A (GB)
Title: Improvements in or relating to circuit-arrangements for controlling the amplitude and the frequency of an electrical oscillation
IPC: H03B21/02
Filed: 1948-08-20
Abstract: 652,464. Valve circuits; frequency modulation. PHILIPS ELECTRICAL, Ltd. Aug. 20, 1948, No. 21984. Convention date, Aug. 25, 1947. [Classes 40 (v) and 40 (vi)] Two mixing valves are cross-coupled so as to be self-oscillatory; various embodiments are described, the first, Fig. 1, being a frequencychanging circuit of constant gain. Local oscillations of frequency f0 are mixed in the valve 2 with the signal fa and in the valve 4 with the first beat product (fa - f0), the output circuit being tuned to the frequency (fa - 2f0). In addition, the valves form a self-oscillatory circuit generating a pilot tone f1, being backcoupled over a circuit 13 tuned to this frequency, which is converted in the valve 2 to the frequency (f0+f1) and regenerated in the valve 4, the intervalve coupling including a circuit 11 tuned to the frequency (f0+f1). A D.C. gain control potential is applied to the inner grids of the valves 2, 4 from the load circuit 8 of a rectifier 7 in order to maintain constant the amplitude of the pilot tone f 1 across the circuit 13; a mathematical demonstration is given in the Specification, showing that this renders the conversion, gain of the circuit for the signal fa constant and independent of valve characteristics provided the various impedances are constant. In a modification,Fig. 2 (not shown), the signal fa is applied to both valves in parallel, an output (fa - f 0) being taken from the anode circuits in series, or alternatively, Fig. 2a (not shown), in parallel; the feedback path may also comprise a resistance-capacity phaseshifting network, Fig. 4 (not shown). Specifications 625,915 and 646,118 are referred to. Frequency-changing circuit giving an output of constant amplitude. Signals of frequency fa, Fig. 3, are mixed in the valve 2 with local oscillations f0, and again in the valve 4 with the first beat product (f0-fa), the valve 2 acting also as an amplifier for the frequency fa which is applied to the inner grid of the valve 4 via a tuned circuit 14. The anode circuit of the valve 4 contains an output circuit 12 for the frequency (f0 - 2fa) and in addition a feedback circuit 13 tuned to the pilot frequency f1, which is regenerated in the valves 2, 4 utilizing the signal fa as beating oscillator, the circuit 11 in the intervalve coupling being tuned to the frequency (fa-f1). A D.C. control potential proportional to the amplitude of the pilot tone is produced as in the arrangement of Fig. 1, and is shown in the Specification to render the amplitude of the output (f0 - 2fa) independent of the amplitude of the signal fa. In a modification, Fig. 3a (not shown), the valve 4 comprises a diode. Frequency - modulation limiter and discriminator; means for varying the depth of modulation of an F.M. wave. Amplitude modulation of the output (f0-2fa) is produced in the arrangement of Fig. 3 described above from a frequency-modulated signal fa by making the transmission ratio of the circuit 10 vary suitably with frequency; an output which is constant in frequency may be obtained by tuning the output circuit to the frequency f0. Frequency-modulation of the self-generated pilot tone f is produced if circuits 11 or 14 and 13 are given frequency-dependent transmission ratios, the relationship of the deviation to the deviation of the signal fa being determined by the circuit constants; if amplitude modulation of the tone f1 is also produced proportional amplitude modulation of the output ( f0 - 2fa) results, which is in addition to the amplitude modulation produced by the circuits 10. In arrangements in which the local oscillator f0 is used, i.e. other than that of Fig. 4, the signal and pilot tones may be kept separate by using separate mixers, each stage comprising a pair of valves which are, however, connected in parallel as regards the D.C. control potential. The Specification as open to inspection under Sect. 91 also comprises a simplified circuit, Fig. 4 (Cancelled; not shown), based on that of Fig. 3, in which the local oscillator f0 and circuits 10, 12 are omitted. A frequencymodulated output is obtained at the frequency f exhibiting a deviation proportional to that of the input fa, by using circuits 11 or 14 and 13 in which the phase-shift is linearly proportional to frequency deviation. This subjectmatter does not appear in the Specification as accepted.