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Rigol DS1054 LF common-mode aberation

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

Reply to
legg
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** 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

Reply to
Phil Allison

The scope probe pairs are normally used as bundled pairs to cut down on desktop clutter.

It appears that the current is being induced in the physical loop, created by the separation of the bundled pairs, by a strong local magnetic flux field being generated by the DUT power supply.

The power supply employes two large low frequency ferroresonant power transformers inside a skeleton frame.

The current amplitude can be modulated/inverted by manipulation of the loop's area and orientation.

Methods of reducing these effects (the transformers stay) in measurement and basic circuit function are being examined.

RL

Reply to
legg

** Say "thankyou Phil" for spotting this so quickly from the other side of the planet.

And despite a convolted and misleading description.

..... Phil

Reply to
Phil Allison

Sorry Phil, it's only a fluke that I copied this to sci.electronics.repair in the first place, after little response on S.E.D. I did see Trevor's note.

Only placed the last note here for consistency.

The bundled pairs of scope probes were originally adopted as standard practice in order to avoid this issue, so long ago that I was unprepared to consider the effect in scopes with a larger channel count.

Sensing and power lead wiring in or around that beast is already in twisted pairs for similar reasons, but I'm going to have to re-examine the voltage vs current pair issue and a number of rotary switch routings for the same, if reduced, influence.

Thanks for your input.

RL

Reply to
legg

I thought they were all commoned via the chassis? I usually only connect the ground clip of a single probe.

Reply to
Andy Burns

Read on.....

RL

Reply to
legg

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