Rf and if digitization in Radio Receivers: Theory, Concepts, and Examples

AGC is used to normalize the output signal level despite variations in input signal power. A 

typical AGC device operates using a feedback loop whereby the power of the output of a 

variable gain device (amplifier or attenuator) is sampled and used to adjust the gain of the 

variable gain device itself. In this way, the output power is maintained at a relatively constant 

level. The response time of the AGC loop is perhaps the most critical design parameter and is 

dependent upon the application. Generally the AGC device is adjusted to react fast enough to 

normalize the overall power but slow enough to maintain the desired information content of the 

signal. Depending upon the application, the control voltage for the variable gain device may be 

used to decompress the output signal after digitization.  

Typically AGC devices can have a total dynamic range of 80 dB. But because they operate by 

adjusting the gain of the system, the instantaneous dynamic range is much lower than that for 

logarithmic amplifiers. Therefore, these systems generally are better suited for narrower 

bandwidth systems where individual signals are isolated by appropriate filtering before the 

AGC. AGCs, unlike logarithmic amplifiers, are not effective in receivers where weak desired 

signals need to be detected in the presence of strong undesired signals.  

AGC devices can be purchased as discrete components or built into the front-end receiver 

circuitry. As with any feedback system, stability is an important consideration. In radio receiver 

applications that use an AGC device before an ADC, there should be enough gain in the system 

so that the system noise at the output of the AGC device prior to digitization is slightly greater 

than the quantization noise power of the digitizer. To maximize the SNR, the normalized output 

 

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of the AGC device should be set as large as possible without causing the ADC to overload or 

generate excessive spurious responses. A more detailed discussion of AGC devices can be found 

in [35].  

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