I wanted to see if a low load crystal oscillator in one of my designs was actually oscillating (with the conclusion being that my load caps are more or less sized correctly). Just probing with my cheap stock Rigol probes didn’t sound like the best idea because they have fairly high capacitance, and two bad things can happen:
Either the probe stops the oscillator from running, by overloading it, or the probe’s extra capacitance increases the load enough to make it oscillate properly at all. Both are not good. So I went to look for something with low capacitance.
What I found was a FET probe design by Wolfgang described here: A Homebrew FET Probe up to 1.2GHz – Electronic Projects for Fun
It’s based on a bachelor’s thesis by Thomas Hirschbüchler (TU Wien): https://www.instructables.com/DIY-1GHz-Active-Probe-for-Under-20/
I first recreated Wolfgang’s layout in Kicad, and then compacted it a bit and added an EMI shield on top. He tested it with his equipment and is satisfied, results here: Solder-In Probes – Electronic Projects for Fun
BOM is short and cheap. The only thing that is a bit troublesome is the shield, which requires manual work to make it fit (one wall has to be removed). I’ll have to make a proper project page, but that shouldn’t take long and I’ll link to it here once it’s done.