Periodic bursts of noise with no BNC cables attached (2204A)

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TimNJ
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Periodic bursts of noise with no BNC cables attached (2204A)

Post by TimNJ »

Hi all,

Purchased our first Picoscope this week. We bought a 2204A to do some control loop measurements.

I am seeing periodic bursts of noise on both channels, even with nothing connected to the inputs. The bursts appears at 1.0-1.2 second intervals, are about 50-100ms long, and the content within the bursts is about 400Hz, from what I can tell.

Please see the attached images.

For the first 1.5-2 minutes after the Picoscope is plugged in, and detected by Picoscope 6, there is no noise! If I quickly un-plug and re-plug in the scope, the noise magically vanishes, and then shows up again after 1.5-2 minutes. Very peculiar to me.

I've tried the following:

1. Tested on desktop PC plugged into AC mains, then tried on battery powered laptop
2. Moved the laptop setup to the opposite side of the building
3. Turned off WiFi on the laptop
4. Enabled/disabled the waveform generator
5. Disconnected all USB devices from both computers
6. Disconnected all scope probes and BNC cables from Picoscope

Thank you,
Tim
Attachments
~1second burst period.png
~400Hz content within burst.png
~50-100ms burst length.png

bennog
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Re: Periodic bursts of noise with no BNC cables attached (2204A)

Post by bennog »

2 more things you can test come to mind.

1. short the inputs of the scope.
2. power down your phone. Or put it at least 10-20 meters away.
3. If your laptop has mobile connect turn that off.

Benno

TimNJ
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Re: Periodic bursts of noise with no BNC cables attached (2204A)

Post by TimNJ »

Hi Benno,

Thanks for replying. I've already tried with my phone powered down. The laptop does not have any sort of GSM/CDMA card. I've also already tried with WiFi switched off. I will try to short the inputs...I imagine the noise will go away.

I put a benchtop scope across both inputs. The noise is really there, at the BNC connectors.

The reason I don't think it's from anything near-field is because *the bursts only start after a 1.5-2 minute delay after plugging in the scope to PC USB*. This happens after every time I plug it in. Once the noise is present, if I quickly unplug and re-plug in the USB connector, once the scope re-initializes in Picoscope 6, the noise goes away.If the issue was related to radiated pickup, the noise would still be there once the scope was re-initialized, in my opinion.

Thanks,
Tim

Martyn
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Re: Periodic bursts of noise with no BNC cables attached (2204A)

Post by Martyn »

With the scope set at 1s/div and the vertical range set to +/-200mV, not Auto, does it still take over a minute for the noise to appear ?

If yes then please email support@picotech.com and the team can investigate this in more detail.
Martyn
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TimNJ
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Re: Periodic bursts of noise with no BNC cables attached (2204A)

Post by TimNJ »

Hello Martyn,

Please see the attached with 200mV vertical scale on both channels.

I will contact the support email.

Thanks,
Tim
Attachments
200mV fixed vertical scale.png

Martyn
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Re: Periodic bursts of noise with no BNC cables attached (2204A)

Post by Martyn »

From that trace it looks like the noise was there from the start, if it wasn't the trace to the left of the trigger would be flat, is that correct?

That suggests it is external noise being picked up by the scope.
Martyn
Technical Support Manager

TimNJ
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Re: Periodic bursts of noise with no BNC cables attached (2204A)

Post by TimNJ »

Hi Martyn,

I'm not exactly sure what you mean. I only took a screenshot once the noise bursts started to appear. For about 2 minutes, the trace was flat at 0V. For slow time-base like 1sec/div, the trigger is just rolling.

We have 5 other bench top scopes without any noise sensitivity issues, all in the same workspace. Granted, they are not connected to PCs. In my experience, with no probes attached, pickup should not be that severe.

I took the laptop and Picoscope into our fully shielded EMI test chamber. The noise is still there. The laptop has all wireless communications turned off. I left my phone outside the chamber. As I noted, the issue is instantly reset once I disconnect the Picoscope USB cable and plug it back in. If it was external noise, I do not see how reconnecting the USB cable would be related at all.

Thanks for your help.

Tim

Glubb
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Re: Periodic bursts of noise with no BNC cables attached (2204A)

Post by Glubb »

Hello Tim,

Have you tried the Picoscope on another computer ?

Glubb

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Re: Periodic bursts of noise with no BNC cables attached (2204A)

Post by TimNJ »

Hi Glubb,

I opened a support case. Xujie recommended to try a different USB cable. (Seems like an "obvious" thing to do, but I had not done it.)

The issue is gone when using any other USB A-to-B cable, other than the blue one shipped with the unit. Very strange as I have no idea what kind of internal fault might be responsible for that. Something intermittent? But then why would it be periodic? I can't worry about it too much, but so far, everything is okay with a new cable.

Thanks.
Tim

Glubb
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Re: Periodic bursts of noise with no BNC cables attached (2204A)

Post by Glubb »

Hi Tim,

Maybe a interference capacitor : cable not well done, or contact problem. This could explane the time delay and the periodic aspect...
Connections could be very tricky... and USB is not only a numeric bus, you also have CC in it.

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