Automatic adjustment circuits Automatic gain control (AGC)

When receiving signals with varying amplitude, such as BPSK, DQPSK or QAM, it is necessary to maintain the amplitude of the signal at the input of the demodulator in certain predetermined limits. This task in radio receiving devices of mobile radio systems is solved by automatic gain control systems.

Usually the level of the useful signal at the input of the receiver varies depending on the conditions of propagation of the signal. Therefore, to ensure the level of the signal required for optimal operation of the quadrature detector multipliers, it is necessary to change the gain of the RF amplifier. For this purpose, an automatic gain control (AGC) circuit is used in the input device. The use of the AGC circuit increases the dynamic range of the receiver. The block diagram of the receiver with the AGC system is shown in Figure 1.

Figure 1. Block diagram of a radio receiver with AGC

In this scheme, the signal from the output of the high-frequency amplifier is fed to the AGC detector. At its output a constant voltage is formed corresponding to the level of the input signal. It is amplified by a low frequency amplifier (ULF) to the value required for the operation of the attenuator. As a result, when the input signal exceeds a predetermined threshold, the attenuator attenuates it to this threshold. As a result, the overall gain of the circuit is reduced and no distortion of the input signal occurs.

Figure 2. Amplitude response of the receiver covered by AGC

When implementing a direct gain receiver in communication systems that use signals with a constant envelope, instead of the amplifier circuit covered by automatic gain control, a limiting amplifier is usually used. This amplifier implements the same functions as the radio frequency amplifier covered by the AGC, but its implementation is much simpler and the parameters are more stable.