[stuarth]

I think the hand capacity effect could be due to the long ground lead.

There is a loop, a one turn coil, formed by the circuit from the signal source to the probe input, then through the probe and its ground lead and the connection back to the signal source. This loop will have resonances which will cause peaks and troughs in the frequency response. The size and area enclosed by the loop should be minimised - see GMB’s post 21.

For high impedance scope probes, Tektronix (and others) supply a range of clips and leads. On fast digital signals, you can see the effect of a long earth lead as aberrations, overshoot, undershoot, ringing etc, on the fast edges, and if it is affects what you’re trying to measure, you can see the problem and take steps to fix it, ie to get a clean edge.

My default is a 3” lead, with a little probe tip ground pin used when looking for problems on fast edges. A long 12” ground lead is only really for audio.

For RF measurements, you will just have a wrong amplitude, or an amplitude which varies as you move the ground lead, but it’s hard to see what is “right”, you don’t have the equivalent of the “clean edge”.

The fact they you are seeing hand capacity effects shows that there is a problem. It’s worth trying the shortest ground wire possible, connected to ground as close to the signal source as possible.

Stuart