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

On 20/03/2017 10:32 AM, 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.

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


**I can't speak for that precise problem with the 1054Z, but I can say
that the supplied probes are, arguably, the worst probes I have ever
encountered. I used mine for about a month, before I threw them out and
replaced them with decent probes. Perhaps you should start there. They
really are horrible probes. It's weird, because the probes supplied with
my earlier, 1052, were not as bad. Not Tek quality, but passable.

--
Trevor Wilson
www.rageaudio.com.au

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

On Sun, 19 Mar 2017 19:32:30 -0400, legg wrote:

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

legg wrote:


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.



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

And despite a convolted and misleading description.



...... Phil

On Sun, 19 Mar 2017 20:00:52 -0700 (PDT), Phil Allison
wrote:

legg wrote:


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.



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

And despite a convolted and misleading description.



..... Phil


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

legg wrote:

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

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

On Mon, 20 Mar 2017 08:56:41 +0000, Andy Burns
wrote:

legg wrote:

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

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

Read on.....

RL