measure a small AC voltage from a large DC baseline

I need to measure a small envelope change on a 8GHz carrier with a diode detector. When there is no envelope on the carrier, the diode detector output voltage is about 50mV. Once the carrier is modulated, the peak change in output voltage is estimated to be 1e-4 of the DC baseline (a.k.a, the change is about 5uV). The signal bandwidth is about 100MHz. I thought I could use a DC block to get rid of the DC baseline, and then a voltage amplifier to measure such a small change. But a quick search makes me wonder if it is really possible. An op amp like LT6202 (with BW=100MHz) has a voltage noise density of 2nV/sqrt(Hz), meaning there will be 20uV noise. My signal will be lost in the noise. Is there another way of doing this? Or am I missing something here? Thanks.

Hi eigenroot
I've another solution for you ! using a filter ( fixed in frequency of your message signal ! ) and before that a rectifier section ( for detection ! ) thus when your carrier has been modulated , you'll be able toy know !
Could i understand your question correctly ?
Best Wishes
Goldsmith

Use a standard LNA instead of Opamp OR High quality Instrumentation amplifiers from well known suppliers
Opamps are general purpose amplifiers and they can not be used for very small level signals because of high noise densities.

In my case the signal is not just 100MHz. It extends all down to very low frequency. So a bandpass filter may not be an ideal solution. But thanks for your reply.

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Thanks. Can you please recommend some LNAs that are applicable in this case?

Hi again
When i told band pass ? my mean was , LPF , with cut off frequency nearby , frequency of maximum message signal .
Good luck
Goldsmith

You could use a log amp detector, like analog devices AD8317. that would give you a bigger output voltage. If you still have noise issues, you either have to lowpass filter, or repetitively sample and average the video output to improve Signal to Noise Ratio.

LNA make sense. What's your signal freq bandwidth?