GB653652A - Improvements in or relating to noise suppression circuits in radio receivers - Google Patents

Abstract

653,652. Radio interference suppression. HUGHES, D. H., and PYE, Ltd. March 16, 1948, No. 7844. [Class 40 (v)] A noise limiting circuit for amplitude modulated signals comprises a noise limiting rectifier which is biassed according to the mean level of the modulated carrier and is fed through a resistance and a capacitor from a divided load resistor so that the higher frequencies are emphasized before limiting, and means are provided in the output circuit to restore the balance in the frequencies. In Fig. 2, audio frequency signals are fed through diode 4 to the audio frequency amplifier from the resistor load 2, 3, of signal detector 1. Diode 4 is normally maintained conducting by the negative voltage applied through smoothing circuit 5, 6, 7, the biassing voltage thus obtained being dependent on the mean carrier amplitude. Noise impulses above this mean carrier level cause diode 4 to become non-conducting and are therefore suppressed from the audio output. In order to increase this effect, diode 4 is fed from the load resistor 2 via a condenser 9 and a resistance 8, so that the higher frequencies are emphasized. The condenser resistance filter 11, 12 connected in the audio output circuit introduces a correcting emphasis of the lower frequencies after limiting has been effected, to restore the balance in the audio output. Diode 10 is connected across resistance 8 to remove impulses of reverse polarity which occur due to the effect of the condenser resistance circuit 8, 9. The invention may be applied to limiters which are connected in parallel instead of in series with the signal path, or to combinations of series and parallel limiters.