Weird noise

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mayaank
Posts: 3
Joined: Sun Sep 12, 2021 12:14 pm

Weird noise

Post by mayaank »

Hello,
We have encountered a problem while using the Biosemi system in our lab. We have 2 systems in 2 separate rooms (Faraday cages). In one of the rooms (room #1 from now on) we started receiving a very noisy signal (see picture 1 below) which didn't improve after cleaning and adding more gel to the CMS-DRL electrodes.
Picture 1- noise
Picture 1- noise
picture1.jpg (82.68 KiB) Viewed 179 times
We tried checking several things:
• We tried switching to a different set of electrodes. in room#1 we received the noise we described. We used the exact set of electrodes in room#2, and the noise disappeared (see picture 2). We did this with another set of electrodes- with the same results.
Conclusion: It's not a problem with a specific electrodes set.
Picture 2- no noise
Picture 2- no noise
picture2.jpg (69.35 KiB) Viewed 179 times
• We tried switching the AD-boxes (including the batteries) between the rooms. We used the AD-box from room#2 and put it in room#1- the signal was still noisy. Next, we used the AD-box from room #1 in room #2- the signal was clear, there was no unusual noise.
Conclusion: it's not the AD-box
• We tried switching the receiver between the rooms. We used the receiver from room#2 and put it in room#1- the signal was still noisy. Next, we used the receiver from room #1 in room #2- the signal was clear, there was no unusual noise.
Conclusion: it's not the receiver
• We did the same thing with the optical fiber, the presentation cable and the USB cable connecting the receiver to the computer.
Conclusion: it's not a cable problem

Importantly, there was no new equipment in the rooms or in the lab area, there wasn't any external change that we know of. We tried disconnecting all unnecessary electrical equipment in the lab, there was no change.

We ran out of ideas of what the cause of the problem might be. We will appreciate any help\ideas. Thank you!

Coen
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Joined: Fri Mar 26, 2004 7:00 pm
Location: Amsterdam, Netherlands
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Re: Weird noise

Post by Coen »

Gel bridge between CMS and DRL, see viewtopic.php?f=7&t=810&p=3048

Best regards, Coen (BioSemi)

mayaank
Posts: 3
Joined: Sun Sep 12, 2021 12:14 pm

Re: Weird noise

Post by mayaank »

Dear Coen,
As we said, when we transferred the same participant without touching the electrodes set or changing anything from room 1 to room 2, the problem disappeared. When we brought him back to room 1, again, without changing anything- it appeared again. This is not a pattern you would expect if it was a bridge.
In addition, it now happened on 5 different occasions, with different participants and different people doing the setup. Personally, I do EEG setup for more than 6 years now, and I have never caused a bridge unintentionally. I did it on purpose to show students how it looks. This is definitely not what is happening now.
Any other ideas?

Coen
Site Admin
Posts: 1024
Joined: Fri Mar 26, 2004 7:00 pm
Location: Amsterdam, Netherlands
Contact:

Re: Weird noise

Post by Coen »

Determine the frequency of the interference (zoom in, make spectrum), it looks like 50 Hz from the mains supply.

Test with electrodes in water. Increase/decrease the area of the included loop between cables of CMS/DRL and other electrodes to determine whether an electric or magnetic field causes the interference (must be magnetic when in a Faraday cage without electric interference sources). Change the location and orientation of the loop to find the source of magnetic interference. Change the location of the water boil and AD-box to find the source of electric interference.

Best regards, Coen (BioSemi)

mayaank
Posts: 3
Joined: Sun Sep 12, 2021 12:14 pm

Re: Weird noise

Post by mayaank »

Thank you, we found out that the noise was because of a problem with the screen monitor power supply.

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