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?


1 Answer 1


On RFID resonant circuits the Q is not usually so high that you can't place 100kohm in parallel with it. That's the first point. The second point is that the 100k could be 99k and 1k in series with the 1k connected to ground. Connect your scope across the 1kohm to do measurements.

The 1kohm and 100pF of your probe won't start to attenuate frequencies till about the 3dB point and that is: -

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

OK there will be attenuation of 100:1 but it may be visible. If it isn't I'd consider using a decent fast op-amp stage as a gain of ten to help out your viewing of the signal.


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