AGC Design and PLL Comparison

Abstract

Automatic gain control (AGC) is used in a receiver to vary the gain to increase the dynamic range (DR) of the system. AGC also helps deliver a constant amplitude signal to the detectors with different radio frequency (RF) signal amplitude inputs to the receiver. AGC can be implemented in the RF section, the intermediate frequency (IF) section, in both the RF and IF portions of the receiver, or in the digital signal processing circuits. Digital AGCs can be used in conjunction with RF and IF AGCs. Most often the gain control device is placed in the IF section of the receiver, but placement depends on the portion of the receiver that limits the DR. The detection of the signal level is usually carried out in the IF section before the analog-to-digital converter (ADC) or analog detection circuits. Often the detection occurs in the digital signal processing (DSP) circuitry and is fed back to the analog gain control block. The phase-locked loop (PLL) is analyzed and compared with the AGC analysis, since both processes incorporate feedback techniques that can be evaluated using control system theory. The similarities and differences are discussed in the analysis. The PLL is used for tracking conditions and not for capturing the frequency or when the PLL is unlocked.