Help with Tektronix differential probes

In summary, the differential probe is picking up EMI from the test device, even though it is not connected to anything. The noise is present before the power is turned on, and the circuit diagram suggests that the noise is caused by the current waveform.
  • #1
imsmooth
152
13
I think my probes may not be working correctly and was seeking advice from those who know better than me.

I have the Tektronics p5205 differential probes with the 1103 power supply. When I power up a test device I see a voltage waveform appear on the channel connected to the probe. The probe is not connected to anything, yet it is still picking up EMI. I short the two probe leads together and I still see the noise. In fact, there is still. noise before I power anything up. I would have expected to see a flat line.

Is this not correct behavior? Any suggestions how to troubleshoot this? Could it be a ground connection in the power supply box? The power supply and oscilloscope are grounded through the ground in a standard outlet, which is properly grounded.
 
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  • #2
Please attach a photo of the noise on the 'scope screen, include gain, sweep speed, AC or DC coupling of both signal and trigger, what the nearby environment is like, and anything else you can think of that MIGHT be a clue (even what kind of lights are nearby!) Is there any high-power equipment or power lines in the area?

Thanks,
Tom
 
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  • #3
There is a circuit diagram of the P5205 here.
 
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  • #4
imsmooth10 said:
I short the two probe leads together and I still see the noise.
I have only limited experience with these (differential probes) but always gets the creep about those separated wires as inputs.
I do understand that they are required to have the 'differential' as 'symmetrical' too, but loops makes the input susceptible to magnetic disturbances at the same time.
 
  • #5
Rive said:
... but loops makes the input susceptible to magnetic disturbances at the same time.
That is why you twist the separate wires together, to cancel the loop area, and so reject the influence of the local magnetic field.
 
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  • #6
Yeah, I too try that all the time, but those old fashioned monster plugs are not really supportive :frown:

I totally understand why they are needed on a high voltage diffprobe, but still:doh:
 
  • #7
imsmooth10 said:
I think my probes may not be working correctly and was seeking advice from those who know better than me.

I have the Tektronics p5205 differential probes with the 1103 power supply. When I power up a test device I see a voltage waveform appear on the channel connected to the probe. The probe is not connected to anything, yet it is still picking up EMI. I short the two probe leads together and I still see the noise. In fact, there is still. noise before I power anything up. I would have expected to see a flat line.

Is this not correct behavior? Any suggestions how to troubleshoot this? Could it be a ground connection in the power supply box? The power supply and oscilloscope are grounded through the ground in a standard outlet, which is properly grounded.
Is the BNC connection lead grounding successfully??
 
  • #8
All setting are the same on the signal source.
I am referencing the pink waveform which is the capacitor voltage (value shown a screen bottom is peak-peak).

Wave 1 is a standard probe across the capacitor. Signal is 90 degrees lagging the blue current waveform. 104v peak-peak

Wave 2 is a standard probe across a 1:100 resistive divider. 1.04v peak to peak
Wave 3 is making same measurement across resistive divider using differential probe. Note that the voltage is close to mirroring the current waveform. The voltage (8.04v) and phase are completely wrong.

I know that the noise is my current waveform. The differential probe and/or power supply are picking it up. All my leads are twisted to reduce stray inductance. The positioning of my scope and leads are identical between waves 1&2 vs wave 3. The only difference is using a standard probe vs the differential probe.

Is my probe bad?
 

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  • #9
Any suggestions for a descent diff probe to buy?
 
  • #10
imsmooth10 said:
Any suggestions for a descent diff probe to buy?
Because a general instrument requires high specifications in all dimensions, they are often expensive, and more trouble than they are worth.
Make one, optimised for the particular job required, or work around.

Avoid building hardware. LTspice has a perfectly good differential probe, left-click on a node and drag the probe to the reference, then release. The plot will show the differential voltage to 6 digit accuracy, something impossible with hardware.
 
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  • #11
Baluncore said:
Because a general instrument requires high specifications in all dimensions, they are often expensive, and more trouble than they are worth.
Make one, optimised for the particular job required, or work around.

Avoid building hardware. LTspice has a perfectly good differential probe, left-click on a node and drag the probe to the reference, then release. The plot will show the differential voltage to 6 digit accuracy, something impossible with hardware.
Where is the link to the reference manual because on the Mac all I have is a blank menu/tool bar.
 
  • #12
Let me guess, you are referring to LTspice?
I do not know Mac, only PC.
Shortcut key; F1 will usually get you help.
Menu 'View' select 'Toolbar', might turn on the toolbar.
Menu 'Help', 'Help Topics', might give you the documentation.
Search the web for;
'LTspice Getting Started Guide'
'LTspice User Manual'
'Undocumented LTspice'
 
  • #13
It is a good HV probe with 110 dB attenuation on the front-end then gain to enable -60 or -40 dB. So calibration is critical and so is 5V low noise.
 

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