GB544945A - Improvements in or relating to the reception of frequency or phase-modulated waves - Google Patents

Abstract

544,945. Electric impedance networks; frequency and phase modulation. JAMES, I. J. P., and BEST, J. E. Oct. 31, 1940, No. 15898. [Classes 40 (iii) and 40 (v)] In the reception of frequency-modulated or phase-modulated waves, the pass width of a filter preceding an amplitude-limiter is varied in dependence on the frequency deviation, so as to reduce the pass width and diminish noise when the depth of the modulation is slight. The receiver circuit, Fig. 2, may comprise an input amplifier RF, frequency-changer FC intermediate frequency amplifier IF1, limiter L1, frequency-modulation detector D and low-frequency amplifier LF. An auxiliary frequency-modulation detector C is also fed through a limiter L2 from an IF amplifier IF2, and its output voltage is used to control the selectivity of the main amplifier IF1. Distortion produced by the control may be reduced by a negative feed-back connection NF from the detector D to the frequency changer FC. Delay networks such as TD ensure that the control is applied simultaneously with the arrival of the corresponding signal. The auxiliary channel feeding the control detector C may derive its input directly from the aerial, and in that case the control may be applied to the radio frequency amplifier RF as well as to the stage IF1 (Fig. 1, not shown). Alternatively the detector C may be fed from a first IF stage and may control a second IF stage (Fig. 3, not shown). When receiving television signals containing a DC component, the signal detected by C may be used directly as control voltage. For audio-frequency transmissions the negative half-waves are inverted before the signal is used as control voltage (Figs. 4 and 6, not shown). The signal after such inversion is shown in Fig. 8, and this signal may be arranged to operate as a stepped control voltage (shown in dotted lines), by feeding it to the grid of a cathode-follower 24, Fig. 9, the output of which operates a peak rectifier comprising a diode 25 and condenser 26. The control voltage is taken from the condenser 26 and rises when the input voltage is rising. - When, however, the input voltage begins to fall, the control voltage remains constant until the next stationary value of the input voltage is reached. For this purpose the voltage wave-form, Fig. 8, is first differentiated, then passed through a double limiter which may comprise reversely-connected diodes in parallel, and then differentiated again, giving a series of impulses which are applied in the positive sense to the grid of a pentode valve 27 connected across the condenser 26. The selectivity of the IF amplifier stages may be adjusted by applying the control voltage to the grid of a damping valve connected across the tuned circuit of the IF amplifier valve (Figs. 11 and 12, not shown), In Fig. 13 a resistance-coupled band-pass filter is interposed between two valves, and the bandwidth is varied by applying the control voltage to the grid of a valve 37. Variable selectivity may also be obtained by negative feed-back as in Specification 542,469. Specification 446,663 also is referred to in the Provisional Specification. Reference has been directed by the Comptroller to Specification 546,011.