i want to built a AGC with a rf detector and a VGA working at frequency 7-8GHz.But i see from the detector (AD8318 ADL5902 etc)datasheet that the output voltage is different for different frequencies with the same power.The difference is large especially at higher frequency (7-8GHz)So the output power of the AGC will vary with frequency.
What should I do to solve this problem?
Have you looked at the SDLVA products? They are much flatter over frequency than the traditional log amps. Try looking at: HMC613, HMC813, HMC913 or HMC1013.