Variable attenuator for short wave gain control at receiver input - allows automatic gain control at high frequency stage

摘要

The input and output characteristic impedance is held constant regardless of the attenuation. This is for use in an automatic gain control circuit applied to the high frequency stage of a short wave radio receiver. The attenuator consists of a bridge 'T' network of which the leg and bridge links are heat dependent resistors. The attenuator output drives a broadband amplifier and thence a detector and a voltage comparator, from which a control voltage is derived to heat the heat dependent resistor to prevent circuit saturation in the high frequency stage of the receiver. Teh effect of this alone is to alter the input and output impedance. A circuit monitors the resistance change in the leg element and alters the temperature and impedance of the bridge element to compensate for this.