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.

It's perfectly possible that the DUT being tested has ground leakage
issues, but the safety ground lead on both DUT and scope are secure
and share a common return point. The 0V reference in the DUT has a
parallel RC connection to the frame, so a small scope probe ground
current could be anticipated.

The 0V reference effect is only present when the largish 60Hz power
unit is on. The shape of the voltages and currents look ~capacitive,
with displacement coincident with line zero-crossing, where most
low-frequency noise in the DC regulator outputs is anticipated, and so
cannot be isolated or ignored.

Is there some issue with DS1054 ground impedance imbalance between
channels that can be corrected?

Is there some issue with basic LF CMRR of the DS1054 inputs that needs
looking into?

I have other scopes to use for this work, but expected the DS1054 to
be useful in simple low frequency measurement.

RL