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Typically AGC is used to increase the noise level. If my receiver sensitivity is -100 dBm with 60 gain After AGC it becomes -40 dBm. If the noise figure of AGC is less. So the noise also increased by 60 dB. High bandwidth has higher noise floor? How does it affect after ADC?

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All noise that reaches the power detector that is in the AGC loop will control the AGC level, so the important consideration is filtering prior to this power detector. This also applies to narrow band interference---- consider a strong interference that is outside the signal bandwidth but inside the bandwidth that reaches the power detector--this interference would then control the signal level! Often AGC's are implemented in two stages for this reason: there is an analog AGC to ensure the signal (which would be signal plus all noise and interference) presented to the ADC input does not saturate the AGC and optimizes quantization noise considerations. Since there is often considerable additional digital filtering to reduce the spectrum to be just the signal of interest and in-band noise, an additional digital AGC after filtering is also required (especially so for multi-band systems where the ADC is quantizing multiple channels).

For all considerations of setting the AGC level prior to the AGC for optimum performance, please see this post: How to adjust receiver gains to avoid saturation and quantization noise to optimise post digital processing?

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  • $\begingroup$ I went thru the post and was in great detail. Just confused a little bit. Assuming my signal band width is 100khz So if my receiver sensitivity is -100dBm and ktb noise floor is ( -174+10log(10e3))= -134 dBm So SNR is 34dBm. If there is an amplifier with 60 dB in the RF path then the signal and noise will be increased by 60 dB? Similarly if a strong received signal is attenuated by 60 does the signal and noise get decreased by 60? Assuming I am going to operate adc input between -5 and -30 dbm. I have to map all the signal strengths in this range ? Do these signals will have the same snr? $\endgroup$ – mark Jun 11 at 11:00
  • $\begingroup$ @mark how familiar are you with cascaded noise figure calculations? I ask as maybe that is the source of your confusion - the answer to how the SNR will change with changing gain depends very much on this $\endgroup$ – Dan Boschen Jun 11 at 11:01
  • $\begingroup$ What you want to calculate for your specific system is what is the noise figure versus gain over the operational gain range of your AGC. That by definition will be the change in SNR you are looking for. $\endgroup$ – Dan Boschen Jun 11 at 11:07
  • $\begingroup$ I do know about the cascaded noise figure which will be dominated by lna. So based on the gains and attenuations different signal strengths will have different SNR before AGC?. Do digital AGC provide constant level so that thefixed point blocks after adc will get signal at constant level? $\endgroup$ – mark Jun 11 at 11:25
  • $\begingroup$ @mark From the NF and the cascaded noise figure overall you can get the SNR at any point in your receiver chain. AGC whether digital or analog provides a constant total power as determined by the power detector that is used in the AGC. There are also implementations where the power is measured digitally controlling the analog gain so we can unify it to just call it AGC regardless of digital or analog: it provides a constant power out and that power is whatever your way of measuring it (detector) gives you. $\endgroup$ – Dan Boschen Jun 11 at 16:20

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