Your math is right, but it may be even worse than this.
I don't have a reference handy, but I have read or been told that many
10Mohm, x10 probes also have a significant resistive loss or shunt factor
(perhaps due to dielectric losses?) at high frequencies. Thus the input
impedance is something much less than 10Mohm in parallel with 10pF.
I haven't used a x10 probe in a very long time.
Get good probes, then pay utmost attention to grounding and probe leads. 1"
is too much.
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