I want to mesure resisrtance at the resonance frequency of a resonator( 1 KHz-100 MHz) with a piezo driving the resonator to resonate and a voltage (DC/AC) applied at the source of the reonators while measuring the resitance of the resonators thorough the drain of the resonator.
You did not clearly ask a question. What part of this measurement is the problem?
I would probably consider an I/Q (in-phase and quadrature) measurement. Using complex arithmetic you should be able to separate the real part (resistive) from the imaginary part (reactive) to get what you want.