Hi all,

If i have an AGC which uses a piecewise log-linear VGA, how do i prevent the vga from going back and forth between two states? i.e. signal is higher than Vref so decrease gain => signal is lower than Vref so increase gain etc...

thanks,
Aaron

aaron_do wrote on Apr 9^th, 2007, 6:28am:


you use hysteresis....

when i say hysteresis, i mean like how any household thermostat works, you need to offset gain up and gain down decision making...

i'm not sure about references... i've always designed my own AGC strategies.

Actually i've never designed AGC before and I was just having some doubts. I'm not worried about steady-state error for the moment...I intend to implement two AGC loops. A coarse one and a fine one. Right now i'm looking at the coarse one.

What i want to do is have a peak detector and filter, then have a comparator bank to get about 6-8 control signals. They can select the gain level. So you end up with 9 gain levels (one is always set). So i'm worried that if i have say 10 dB gain differences, the output amplitude will osciallate by about 10 dB. I've read some papers but none of them seem to talk about that.

ACWWong, i'm starting to understand what you mean now by hysteresis. The idea had occured to me but i thought it would be too complex (i'm going for low power) to implement. Is there a simple way to do this?

thanks,
Aaron

my opinion is to use exponential detector every time... i think a good google/IEEE search for limitter/RSSI designs will give you some ideas....

oh course i meant log as well  ;)