GB517845A - Improvements relating to radio receiving devices - Google Patents

Abstract

517,845. Valve circuits for wireless reception. AGA-BALTIC RADIO AKTIEBOLAG. Aug. 10, 1938, No. 23627. Convention date, Aug. 10, 1937. [Class 40 (v)] In a superheterodyne receiver in which the intermediate frequency is displaced relatively to the band-pass filter in accordance with the strength of incoming signals so as to include more of one side-band, and in which a linear detector is employed which produces distortion when working on a single side-band, the amount of displacement is limited so that the maximum frequency received on both side-bands is always greater than one-seventh of the highest frequency passed by the audio-frequency channel. Seventh and higher harmonies, which are produced by the linear detector working on a single side-band and which are stated to be the main cause of distortion, will then always be above the limit of the audio-frequency channel and will not be reproduced in the loud-speaker. In the system shown, the band-pass filter comprising coils 26-30 and condensers 27-31 passes a band of 9 kc. represented by curve A, Fig. 1, and for weak signals both side-bands up to a width of 4À5 kc. on either side of the centre line B are passed. With a stronger signal, a bias voltage produced in a diode 32, 34 and used also for automatic volume control purposes is applied over resistance 51 to a valve 47 to vary the impedance of the tuned circuit 20, 21 of the local oscillator. The values of the inductance and capacity of this tuned circuit are so calculated that with the maximum inductance or capacity effect of the valve 47 the displacement of the carrier frequency will not be beyond B<1>, Fig. 1, so that modulation frequencies up to 1 kc. corresponding to curve C will be received on both side-bands, it being assumed that the audio-frequency channel is limited, e.g., by a filter 54 ... 56 to cut off at 7 kc. The displacement may be in either or both directions and means may be provided to vary the band-width passed. Automatic or manual tuning control may be employed and in the latter case the displacement-control may be cut out during tuning by connecting the control grid of valve 47 to ground over a switch 53 which may be coupled to the manual tuning means.