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?
We have a couple of applications notes available on our website, CN-0050 (ADL5330, AD8318) and CN-0082 (ADL5331 and AD8319), that deal with this problem. I'm just using these for reference, as they operate in the 1GHz to 2GHz range, lower than what you need.
These circuits use a VGA and the detector in a control loop for AGC. A directional coupler is used on the RF amplifier to take a small amount of the output power back to the detector input. The result is that variations in the output amplitude are greatly reduced (no circuit is perfect) given variations in input amplitude and also accounting for variation of detector and VGA gain over frequency. As long as the loop gain remains high enough, variations in the output amplitude should be suppressed.
I've tested an AGC circuit using the ADL6010. The goal was for 20GHz to 40GHz applications, and we did not have an available VGA, so I used the HMC985LP4KE VVA and the HMC635LC4 RF amplifier to create the VGA function. As Eamon said, the ADL6010 has no internal controller (which the AD8318 and AD8319 have) so I used the ADA4077 op-amp as the controller.
I've attached the schematic and results I have at 20GHz. The HMC635 and HMC985 won't run down to your desired band, so it's possible you could substitute the HMC694LP4 as a VGA, or another option would be the HMC712LP3CE as a VVA used with an RF amplifier to form the VGA function. The HMC712 is currently not recommended for new designs but is being replaced by the HMC712A, should be available in Q3 2016.
There are more details I can share with you if this looks interesting.
Let me know if there is more I can do to help,