diy IF transformers: first attempt.

[ppppenguin]

Basic ×1 probe = 103pF
Switchable ×1-10 probe =  15pF × 10  1nanofarad ×1 

The first 2 results are within normal range. The 3rd result of 1nF looks like a measurement problem of some kind. I cannot see how a probe plus cable plus scope input could come anywhere near that much capacitance.

I've never tried measuing the C at a probe tip. I've just assumed an approximate number and never had a problem. Where I suspect that the probe is affecting the measurements I've used the old trick of hooking on a 2nd probe to the same point to see what happens.

The way you connect the earth lead of a probe matters a lot as you look at higher frequencies. Sub 1MHz isn't very high in this context. I've often done rough probing without an earth lead, relaying on fortuitous connection from the scope's earth to the 0V rail on the DUT. When working with fast logic it all becomes very critical indeed. Rather more than I can set out here. There are some excellent articles on the subject of probing available online. I think there's at least one by Jim Williams, one of the all time great analogue designers. This paper certainly covers a lot of it: http://www.mit.edu/~6.331/an47fa.pdf