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Re: DS1054 LF common mode aberation

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On Sun, 19 Mar 2017 22:21:20 -0400, legg wrote:

> On Sun, 19 Mar 2017 23:56:56 -0000 (UTC), Frank Miles
> <fpm@u.washington.edu> wrote:
> 
>>On Sun, 19 Mar 2017 17:04:48 -0400, 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 this unit 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 has a parallel RC connection to
>>> the frame, so a small scope probe ground current is 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
>>
>>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?
> 
> 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.
> 
>>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.
> 
> 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

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?

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DS1054 LF common mode aberation legg <legg@nospam.magma.ca> - 2017-03-19 17:04 -0400
  Re: DS1054 LF common mode aberation Frank Miles <fpm@u.washington.edu> - 2017-03-19 23:56 +0000
    Re: DS1054 LF common mode aberation legg <legg@nospam.magma.ca> - 2017-03-19 22:21 -0400
      Re: DS1054 LF common mode aberation Frank Miles <fpm@u.washington.edu> - 2017-03-20 16:30 +0000
        Re: DS1054 LF common mode aberation Jon Elson <jmelson@wustl.edu> - 2017-03-20 14:31 -0500
  Re: DS1054 LF common mode aberation legg <legg@nospam.magma.ca> - 2017-03-19 22:31 -0400
    Re: DS1054 LF common mode aberation Phil Allison <pallison49@gmail.com> - 2017-03-19 20:11 -0700
    Re: DS1054 LF common mode aberation Jon Elson <jmelson@wustl.edu> - 2017-03-20 14:33 -0500
  Re: DS1054 LF common mode aberation legg <legg@nospam.magma.ca> - 2017-03-20 11:58 -0400
    Re: DS1054 LF common mode aberation George Herold <gherold@teachspin.com> - 2017-03-20 09:12 -0700
      Re: DS1054 LF common mode aberation legg <legg@nospam.magma.ca> - 2017-03-20 21:30 -0400
        Re: DS1054 LF common mode aberation Phil Allison <pallison49@gmail.com> - 2017-03-20 18:41 -0700
          Re: DS1054 LF common mode aberation legg <legg@nospam.magma.ca> - 2017-03-21 01:43 -0400
            Re: DS1054 LF common mode aberation Jon Elson <jmelson@wustl.edu> - 2017-03-21 17:04 -0500
          Re: DS1054 LF common mode aberation Jon Elson <jmelson@wustl.edu> - 2017-03-21 17:02 -0500
  Re: DS1054 LF common mode aberation Jon Elson <jmelson@wustl.edu> - 2017-03-20 14:28 -0500
    Re: DS1054 LF common mode aberation legg <legg@nospam.magma.ca> - 2017-03-20 21:36 -0400

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