Automatic gain control loop (AGC/VGA) design help

[Ghost Tweaker]

agc loop design

Hi everyone,

I'm designing an AGC/VGA for a wireless receiver. The signal is already downconverted fron 2GHz to 420kHz and its level is around -70dBm at the VGA input. I know the theory of loop design and VGA/AGC you learn at university. I have realized the AGC/VGA in discrete components and many problems happen. Indeed it is supposed to provide from 0dB to 96dB gain. However I can't have a claen signal for inputs of -80dBm! The gain control is very dirty to me also... I also do not have filtering in the previous stages yet...
Does someone have a good article about this stuff or already experienced with this? My AGC is an opamp/diode rectifier followed by an integrator.

Thanks for your help
Ghost

 

[hwswboy]

automatic gain control theory

Read This paper.

 

[Ghost Tweaker]

vga design

Thanks for the paper. This theory I thonk I know. What I need now is a kind of troubleshooter....
There are 2 problems:
-first when I enter in the AGC with a signal generator, set the ref point for an output power of -10dBm and it works for inputs from -40 to -70dBm which is higher than expected. I would like to be able to reach level down to -90dBm.
-second, when I put the LNA+mixer+IQ demodulator before the AGC, the output is -28dBm for a 70dBm input which is 18dB less then expected. Does it have something to do with the noise beeing at some point higher than the signal and lowering the required gain?

Thanx for your help
Ghost

 

[guopeng]

agc theory

Thanx for your help