[G0HZU_JMR]

Thanks. That P6202 probe circuit has a classic shunt RC snubber circuit in it. This is R13 (570Ω) and C13 (1.8pF). There is also a series 50Ω resistor R10 that helps to offset any remaining negative resistance.

In the case of the poor man's probe, the value of the shunt RC snubber will depend on the components fitted at the source. If the 47Ω resistor was changed to a higher value, then the snubber might be something like 39Ω in series with 1.3pF. This assumes there is a terminated 50Ω cable connected at the output of the probe.

If the BF998 produces a broadband -39Ω in series with 1.3pF at the g1 pin, then the shunt RC snubber would need to be 39Ω in series with 1.3pF to cancel out the negative resistance. In reality, there will need to be a compromise here if the aim is to get rid of all negative resistance at the input with some margin. The snubber will need to have some additional damping in order to give adequate margin at all frequencies.

The penalty of the RC snubber will be increased input capacitance, but this can be offset by having a tiny series capacitance in the input divider.