First order low pass filter - Calculate capacitance change

Question

I'm trying to calculate the relative change in permittivity of a capacitor. I have created a circuit that feeds a 80MHz square wave signal into a first order low pass filter.

I don't know the capacitance of said low pass filter, but I don't think I care as I just want the relative change in permittivity.

The output of the low pass filter is then feed into a envelope detector (ADL5910). I then measure the voltage on the VCALC pin and converts that voltage to dBm.

This is where I'm currently stuck...

My question is; Is it possible to calculate the change in capacitance/relative-permittivity of the capacitor in the low pass filter from the change in dBm and if so, how would one calculate this?

VCALC measurement 1 in medium A: 302mV - 2.0dBm

VCALC measurement 2 in medium B: 233mV - -0.5dBm

Answer 1

Seems that before "measuring", ADL5910 must be "3 points" calibration. Seems also that it need a CW (sin ?). Calculation possible, but on a known "test bench" ...

Here is a "device test" (first draft) for measuring "capacitance change", for different permittivities. A bridge could be also used if changes are "small".

Peaks are "proportional" to capacitance.