# 125 kHz RFID Antenna Measurement using CRO - Passive Probe

I have a coil inductor and a capacitor in parallel set to the resonant frequency of 125 kHz for receiving RFID. There is no issue with the operation of my circuit but I would like to measure the induced voltage across the inductor.

I only have a digital CRO with a passive probe. The capacitance of the probe is somewhere between about 80 pF and 150 pF. Naturally when connecting the probe to the LC combination the resonant frequency changes and I can't measure even a small fraction of the signal.

Can anyone suggest a way of converting my passive probe to an active probe to reduce the input capacitance so as I can make the measurements?

$\dfrac{1}{2\pi\cdot RC} =$ 1.6 MHz.