DS1054 LF common mode aberation

If the signal of interest is at low frequencies, then by all means get rid of the extra ground connections. They're only making matters worse.

Is the origin of the problem in the 'scope? You should be able to check that out without your power supply, testing for proper gain, frequency response, input impedance, and leakage currents. If you're using say, Ch3 and Ch4 as cheapo differential input (via subtraction) you should test its CMRR too. Once you're sure your test equipment are working properly you can measure your power supply properties knowing its limitations. Until then us outside non-observers can only make guesses.

-F

This is the question. None of the other scopes here show the problem, singly or in varying combinations with the scope in question (all others are 2ch scopes).

I'm looking for a correctible fault in construction.

I'm not using them for anything complicated - not even their simple vertical deflection, while their probes are shorted.

The interaction seems to be between either of CH1/CH2 and either of CH3/CH4 in a manner suggesting that the channel groups do not share the same ground return, or safety earth connection.

The measured current flow in the scope probe cables indicates that the current flow is out of ch3/ch4 and into ch1/ch2, when the DUT power supply is on - AND GET THIS - connection to the 0V reference by the scope's probes or ground leads is not required.

The shorted scope probes do not have to have any galvanic connection to the DUT, except their input power L and N - use of a cheater cord to lift the scopes ground wire has no effect.

All units are plugged into the same input line phase, without obvious L/N substitution.

On second inspection and paying attention to the current probe amplifier scales, the current flowing between the scope input channel pairs is actually 140mApkpk, easily modulated by the quality of short existing between the scope probe ground lead contacts.

RL

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

FYI:

legg crossposted the same query to " sci.electronics.repair".

.... Phil

Flux banding the transformers, though it looks a bit credulous with parts this size, reduced current in the unaltered probes by an order of magnitude.

RL

Flux banding?

You put a layer of conductor around the transformer? Does it need to be grounded? At 60 Hz isn't that a thick piece of copper? Or are you doing something else?

I've sometimes had issues with 60 Hz getting into ground/power lines..

George H.

Tektronix scopes (most of them, specifically excluding those with isolated channels) have their input BNC outer shells solidly connected to common/ sheet metal. Is this not true with your Rigol? If you measure the resistance between these channel grounds, what do you get, both at 'scope inputs and scope probe ground clips?

A lot of commodity scopes these days have pretty crummy tolerance to ground current. The way to see theis is to connect the probe's ground clip ONLY to some other equipment, such that it might be getting 10's of mA of ground loop current. Turn the attenuator down to some 10's of mV/div and see how much signal you get. Ought to be WAYYY down there around 1 mV, but no, on a lot of recent gear, you will see 10's of mV at least. Instead of the 1/8" thick aluminum housings of the old Tek gear, now the BNC shells are grounded through PC boards to some other circuitry inside the scope.

Your results above sound EXACTLY like what I'm talking about.

The only hope is to tie big bonding straps from the unit under test to the scope, so the ground difference is reduced as much as possible.

Jon

Oh, that USED to be true, in the "good old days". Now, they use PC boards, and it leads to lots of ground currents flowing in one BNC shell and out on all the others. You will see this on pretty much all the plastic shell Tek scopes, DPO, etc.

Jon

OK, this is a totally different problem, and ferroresonant transforers are known to have a LOT of fluxc leakage.

Just keep your probe pairs bundled and as far away from the source of the magnetic field as you can.

Jon

I used 20mil thick copper strap, cut to the width of the winding breadth.

RL

Making the external connection between scope ground lug and DUT frame was an iteration that produced no changes in the readings, at the original levels of induced probe screen current..

In later measurements, it does result in differences that are still overshadowed by other influences, in other measurements. Scope probe lead location is still something that is intentionally misarranged to separate or identify sources of scope readings of noise levels.

RL

Do not do what someone I know did and put the copper strap around the winding.

The tranny got rather hot after that.

.... Phil

What do you mean by 'around the winding'. A flux band circles both core and winding, on the same axis as the windings.

RL

Oh, yeah, a shorted turn. if anything but a tiny transformer, there will be a blown fuse! You only want to short out the leakage flux.

Jon

Yes, but it is only linked to the LEAKAGE flux, which you want to get rid of (especially in audio and instrument applications), rather than the main flux from the primary.

The situation Phil mentions, the person wrapped the copper strip THROUGH the window of the core! That makes a single, low resistance, shorted turn. BZZZZT!

Jon