GB590886A - Improvements in frequency responsive means particularly for use with frequency modulated radio transmitters - Google Patents

Abstract

5 9 0, 886. Frequency modulation. BRITISH THOMSON-HOUSTON CO., Ltd. Feb. 22, 1944, No. 3258. Convention date, Feb. 27, 1943. [Class 40 (v)] [Also in Groups XXXV and XXXVI] In a frequency-monitoring arrangement for frequency-.modulation signals, the signals and a standard-frequency signal are compared by alternate and periodic application to a frequency-measuring circuit. Switching valves 13, 14 (Fig. 1) with screens impulsed from the multivibrator 15 over the transformer winding 40 alternately connect the oscillators 10, 11 respectively to the limiter 65 and discriminator 12. If the frequencies differ, the discriminator output will include a pulsating component which is applied over the transformer 77 to the rectifier 18 together with impulses from the transformer winding 80 which determine the sense of the difference. The rectified output, smoothed by the network 93, is applied to the indicating instrument 90 and to a reactance valve 19 controlling the frequency of the oscillator 10. In a monitoring arrangement at a receiving station (Fig. 7) the output from the discriminator 131 is applied to the X-deflection plates of the cathode-ray tube 130, a sawtooth potential being applied to the Y-plates from the source 132. In order to distinguish the trace corresponding to the standard frequency the switch 134 is ganged to the switch 135 and causes the grid of the tube 130 to be modulated from the source 133 during the appropriate period. In another arrangement (Fig. 8) comprising a superheterodyne receiver in which the beat frequency is derived from -the comparison oscillator 143 over the frequency multiplier 144, switching is performed by a multivibrator 140 which produces`'alternate long and short impulses. The output from the discriminator 146, comprising a steady potential due to the long impulses with superimposed short pulses whose sign is determined by the sense of the frequency deviation, is applied to a detector (Fig. 9) comprising inverselyconnected rectifiers 190, 191 with the indicating instrument 147 in shunt with the common load resistance 196. The ganged switches 150-4 provide in position (1) for checking the pulse width (2) for the application of a standard frequency deviation, the relay 152 causing a shift of oscillator frequency by introduction of the condenser 157 when impulsed from the multivibrator 140, and (3) for normal operation. In a further arrangement (Figs. 5, 6 not shown) a differential indicating instrument is employed, the discriminator output being switched from the one coil to the other in synchronism with the switching of the oscillators. In a modification (Fig. 11) the switch 169 rendering the keying valves 160, 161 alternately conductive by means of impulses derived from the battery 168 also applies blocking bias to the diodes 174, 175 so that the discriminator output is deflected through the appropriate coils of the differential meter 180.