Chapter 12 - The Other Receiver Circuits

Abstract

This chapter evaluates other receiver circuits, including noise blankers and automatic gain control circuits. Noise tends to amplitude modulate signals, regardless of the mode type. Even “noise free” FM will exhibit noise under the right circumstances if the receiver's limiter threshold is not exceeded by the signal. There are two basic types of noise circuit: noise limiters and noise blankers. Noise limiters basically are clipper circuits, because they clip the amplitude of the signal. Noise blankers poke a hole in the signal at the point where the noise occurs. Automatic gain control (AGC) stabilizes the gain of the receiver despite changes in input signal strength. The AGC network keeps the volume constant despite large variations in input signal level (in fact, it sometimes, in consumer electronic receivers, is called automatic volume control, AVC). If a transistor is used for the IF and RF amplifiers, then the bias from the AGC circuit is mixed with the fixed bias to produce the total bias on the transistor. By that means, the designer gets gain control. The AGC system will not respond directly to a change in input signal amplitude, rather, a short delay is built into the system. The fastest possible response time of the AGC circuit depends on the filtering ahead of the detector, filtering of the detector, and the IF amplifier itself. A squelch circuit (also called mute) is used to cut off the output of the receiver during times when no signal is being received. Automatic frequency control is used in some FM receivers to keep the tuning frequency stable.