Frequency "beading"

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devinomega
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Joined: Wed Nov 16, 2011 12:08 am
Location: San Francisco, CA

Frequency "beading"

Post by devinomega »

My group is using a 64 Channel Biosemi ActiveTwo system. We've been having some interference from our system that I can only describe as "beading."

You can see intermittent bursts in frequency. This was during a bucket test, but I've also seen it during actual cap set up. If I unplug the fiber optics cable and turn the AD box on and off - it can sometimes reset. Also, it sometimes works perfectly! I've used Biosemi and other EEG systems before, but this is the first time I've seen something like this.

Can any one help explain this? Is this a fiber optics cable problem or an AD box problems?

Thanks,

Devin
Last edited by devinomega on Wed Nov 16, 2011 12:29 am, edited 1 time in total.

devinomega
Posts: 2
Joined: Wed Nov 16, 2011 12:08 am
Location: San Francisco, CA

A picture

Post by devinomega »


Coen
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Post by Coen »

60 Hz mains interference ?

Bundle the electrode cables

Best regards, Coen

ygao1
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Joined: Thu Apr 19, 2012 8:58 pm
Location: Chicago

Similar 60Hz issue

Post by ygao1 »

[quote="Coen"]60 Hz mains interference ?

Bundle the electrode cables

Best regards, Coen[/quote]

Hi Coen,

Our lab has recently encountered the same problem with 60Hz waves (as pictured by devinomega). If I select "none" for reference in ActView the 60Hz waves are present in every electrode in every ribbon (128 electrodes). If I put reference as avg. displ., the external 8 flat electrodes show 60Hz waves but not the rest of the electrodes. This has occurred in bucket tests and with a human subject.
Interestingly, if I move the CMS and DRL electrodes to a different bucket, the 60Hz noise would disappear. Is this normal?
We have concluded that this noise was not due to environmental reasons. Please advise us on how to solve this problem.

Thank you,
Feifei

Coen
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Post by Coen »

With the ActiveTwo system, 60 Hz interference is always due to environmental sources. The ActiveTwo system is battery powered (and galvanically isolated for the mains by the optic fiber), so no 60 Hz interference source is present in the system itself.

60 Hz magnetic fields induce interference in loops formed by electrode cables.

Unreferenced: reduce interference by decreasing the loop area formed by the CMS/DRL wires and other electrode wires.
Referenced: reduce interference by reducing the loop area formed by the Reference wire and the electrode wires.

Loop area is reduced by bundling and/or twisting wires.

Summary: bundle all wires as much as possible, bundle groups of wires with the refernce of this group.

For example: bundle the headcap flatwires with the CMS/DRL, and use twisted pairs for EX bipolar electrodes than are not bundled with the other wires.

Best regards, Coen (BioSemi)

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