legg wrote:

Looking at Low-Frequency noise on a 60Hz power supply, using a Rigol
DS1054 and not getting very far.

One puzzling issue is, when ch3 or ch4 scope probe grounds are
connected to a common 0V reference point, a low frequency common mode
signal shows up in the shorted probe of CH1 or Ch2, when connected to
the same 0V reference. Disconnecting CH3/CH4 ground clips removes the
signal in shorted CH1/CH2.

Measurement using a current probe show 20mAppk of LF current passing
in the CH1 or CH2 probe lead, generating a 20mvppk voltage on the
display for that particular channel, but only when CH3 or CH4 ground
leads are connected.

The scope probes are those supplied with the retail unit.


** Sounds like the probe leads are forming a loop and a nearby transformer is injecting AC frequency into it.

What happens if you twist all the leads together, closing up the loop area to almost nothing?


..... Phil