Patent GB-504766-A (GB)
Title: Improvements in or relating to phase and frequency modulating circuit arrangements
IPC: H03J7/04
Filed: 1938-02-17
Abstract: 504,766. Phase and frequency modulation ; automatic control systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Feb..17, 1938, No. 5003. Convention date, April 13, 1937. [Class 40 (v)] [Also in Group XXXV] A phase or frequency modulated transmitter utilises automatic frequency control of the frequency-modulated oscillator to ensure a high degree of stability for slow frequency variations or drift, while allowing variations at signal frequency. The oscillator 10 supplies energy to antenna 6 through a power amplifier and /or an amplitude limiter and /or a frequency multiplier 5, and has connected across its tuned circuit 13 a thermionic reactance 20. Valve 20 acts as an inductance by virtue of the potentiometer R2, C3, which feeds to the grid 28 a voltage in quadrature with the oscillatory voltage on the anode 33 and across the tuned circuit 13. The reactive effect of valve 20 and thus the frequency produced by oscillator 10 is controlled by varying the bias on grid 28 (or some other control electrode), the modulating potentials being applied thereto by transformer 290 from a source MP. Correcting potentials derived from frequency-discriminating circuits are applied in series with the modulation to control valve 20 in the same manner. A portion of the output voltage from the oscillator 10 is fed to the frequency changer 30 with its own local oscillator circuits (crystal controlled) 14, 14', and a lower frequency which follows any changes in frequency of the oscillator 10 is produced in the primary 17 of the discriminator network 17, C, 18. The circuits 17, 18 are tuned and when the frequency is correct the voltages across them are in quadrature, the voltages fed to diodes 40, 41, which are the vector sums of the voltages across 17 and each half of 18, being equal. Under these conditions no resultant voltage appears across load resistances 24, 26. When the frequency is incorrect the phase quadrature relation is departed from and one diode is fed with a larger voltage than the other; the resultant rectified voltage being employed to apply through R1 an appropriate tuning correction by means of the valve 20. The time constant of R1 and its associated condenser C1 or C2 is such that only slow changes of the oscillator frequency are corrected. If the time constant of this control path is 'made somewhat less than <1>/6th second the degree of modulation of the lower frequencies is reduced and the resulting wave is in effect phase modulated. This distortion of the .modulating potentials to produce phase modulation may be accomplished by distorting networks 400. If the time 'constant is such that modulation frequencies are not attenuated, negative feedback with resultant improved linearity of modulation is obtained, but increased amplification of the modulation potentials is required. In a modification, Fig. 2 (not shown), the mixer valve 30 is not employed, the outputs from the oscillator 10 and a line-controlled local oscillator being applied to a diode rectifier. The diode has an amplitude limiting effect and produces the new frequency for application to the discriminator circuits. An audio monitoring amplifier is shown connected before the time constant network in the control circuit, and also the control voltage and modulation are shown applied to the suppressor grid of the reactance valve 20.