Hi,
You are (scope) probing some signals on a board with an offline SMPS on it. Signals are on the primary side. You feed the kit via an isolation transformer, so you can use your DIY coaxial probe to probe one of the signals without it being caked in common mode noise. However, the other signal is referenced to a different node, so you cant use another coaxial probe into the scope.
You’ve already added extra Y caps across the transformer, and from downstream side of common mode choke to earth (the board’s earth connection is capacitive). You’ve even added an extra common mode choke to the boards input…you’ve also wrapped the diff probe wires through a torroid…but that second signal is still very noisy as youre looking at it with the diff probe.
To get the second signal less noisy, would you say its best to then make your own diff probe out of an opamp in diff connection with 4 MEG input resistors, and use coaxial cable to port the signal to and from it? (ie not the noisy diff probe connection leads)?
Or is there a cheaper way to get the second signal less noisily scoped?