On Thu, Apr 11, 2024 at 02:31 PM, Steve Hendrix wrote:
I've been following this discussion with interest, no particular expertise to contribute. Is your statement above based on the leads and other parasitics adding some inductance, which starts to cancel the capacitance at higher frequencies?
I think only a small part of the input capacitance is going to be fairly consistent with frequency. This will be the shunt capacitance to the ground ring right at the tip where the tip will be a bit like a really short coaxial section ahead of the divider resistance. Much of the rest of the capacitance will be the series compensation cap (across the divider resistance in the tip) and this is then in series with a run of lossy coax. Up at UHF the loss will tend to make the net (parallel) capacitance Cp go down slightly with increasing frequency because the coax section will look like a loss resistance in series with the tip capacitance. In other words, the parallel equivalent of (say) 10pF in series with a 50R loss resistance will gradually go down with increasing frequency.
Ideally, there should be no reflections in the coax section of the probe and this is usually achieved by making the coax really lossy and this makes it look more like a resistance.
This can probably explain the way the capacitance changes up to 100MHz or so, but above this it will be much harder to understand without taking the probe apart.
