EMI and Ferrite Cores
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Does anyone have any experience with EMI on sensitive devices? We have a Biofeedback device that measures in Hz. There is a 180Hz signal appearing on the readouts of 2 different devices. I do not have much experience with this, but I have already installed a Power Cleaner (Ha!) and shielded cables, but wanted to add some Ferrite Cores to some other related cables. I ran into a roadblock though: there are different types of ferrite cores for different frequencies you want to block and I don't know the differences. That is the extent of my knowledge about utilizing ferrite cores, so any help with recommendations and suppliers would be greatly appreciated
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Before going the ferrite core route, have you looked into shielding the device itself? Might be a better way to go.
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@scottalanmiller I do not have much experience with it, but I think additional information will help.
The equipment is Biofeedback equipment. There are 2 different devices that attach probes to the surface of your skin in order to read the electrical impulses. They both output in Hz. I had been approaching it as the cables and probes picking up the interference, but I can approach it the other way as well. These are small handheld devices, approx. 10" x 6" x 4". What did you have in mind?
http://www.mayoclinic.org/tests-procedures/biofeedback/basics/definition/prc-20020004
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Something rather simple that many don't think of is your environment. Florescent lights generate RF noise that can interfere. Fans, anything with a transformer can also generate noise.
Are any of these things in the area? You stated that you have installed shielded cables and a power cleaner..
Can you give us any information on the device itself? name, model etc?
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@Brains said:
@scottalanmiller I do not have much experience with it, but I think additional information will help.
The equipment is Biofeedback equipment. There are 2 different devices that attach probes to the surface of your skin in order to read the electrical impulses. They both output in Hz. I had been approaching it as the cables and probes picking up the interference, but I can approach it the other way as well. These are small handheld devices, approx. 10" x 6" x 4". What did you have in mind?
http://www.mayoclinic.org/tests-procedures/biofeedback/basics/definition/prc-20020004
Oh I see, so not something that you can put in a Faraday bag or something of that nature then?
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@g.jacobse Unfortunately I don't have the Model# available atm, but I can get it. There is definitely Florescent lights (I do think he turns most of them off when performing tests due to the nature of the relaxation techniques) Funny thing is that this began within the last 7-8 months, but I have spoken with maintenance, and nothing has been changed. Do you think a faulty ballast could be causing this?
I have already removed the laptop power adapter (definitely was causing some EMI), and have moved the equipment from everything we can. It is still near the computer, but 2 feet above it on the desk. What about shielded PSUs? They probably make those right?
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@scottalanmiller We could try for testing, but that bag has to be 100% closed right? I assume if it wasn't, it would still block a majority of it
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@Brains said:
@scottalanmiller We could try for testing, but that bag has to be 100% closed right? I assume if it wasn't, it would still block a majority of it
Correct, EMI is like light, not like sound, so having a small hole will do very little to let EMI in. Any amount of coverage will do a lot.
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A simple Faraday Bag is a sheet of aluminum foil. You may wish to insulate any metal surfaces and connectors prior to prevent possible shorting (never know).
Just wrap the device up with some foil (tin foil hat brigade) and see if that helps.
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Thanks. I'll give it a go and see the results. Ill update in a couple of days
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Well. No dice unfortunately
Tests performed:
- Turned off all other lights. Incandescent (no florescent) attached to 4 dimmer switches
- Turned off all other surge protectors
- Cell Phones turned off
- Unplugged all other electrical devices in room
- Replaced extension cable with shielded extension cable
- Wrapped unit in aluminum foil
- Wrapped Cables in aluminum foil
- Wrapped USB Extension cable join with built in cable and majority of built in cable with aluminum foil
- Wrapped Sensors and User's Hands in aluminum foil
- Moved Device to table where printer sits <~~Cut the 180Hz signal from 3 to 1.5
- User reports similar results when walking around the building with laptop. Some places its not as bad, but it is always there
- Checked nearby offices for random items plugged in - None
Changes made:
- Moved all computer items to new surge protector, plugged into power cleaner
- Left shielded USB extension cables up there
Possible avenues
- Ask Maintenance to disconnect the power line running outside of the wall that is powering User's Cubicle Outlets and only use wall outlet
- Speak to Electrician to see if there are filters that can be placed in electrical room in case the whole building has "dirty power"
- Would explain why User can reproduce this with the laptop (Running on Battery) in various places in the building, but it does not reproduce at his home.
- Bring in Professional (No idea what he would be a professional in)
Important Notes
- Problem is only existent when using sensors far apart from each other (more opportunity for EMI to get in the middle)
- Doesn't happen if sensors are 2 inches from each other on the same arm.
- Happens when 1 sensor on each hand and hands are shoulder-width apart
- Parking Garage Below Him - Maybe something to look into to? Wonder if there is any equipment below him?
- This is happening on both devices
- J&J SEMG sensors
- Sampling Rate = 1024 per second
- Thought Technology SEMG
- Sampling Rate = 2048 per second
- J&J SEMG sensors
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Also when I worked for the cable company we had "Signal Leakage" meters. Is there a similar meter that can assist me in pinpointing the source or direction the interference is coming from?
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@Brains said:
Also when I worked for the cable company we had "Signal Leakage" meters. Is there a similar meter that can assist me in pinpointing the source or direction the interference is coming from?
I see that you have dimmers - these themselves can be noisy. if they are off, they should not generate noise.
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@g.jacobse said:
@Brains said:
Also when I worked for the cable company we had "Signal Leakage" meters. Is there a similar meter that can assist me in pinpointing the source or direction the interference is coming from?
I see that you have dimmers - these themselves can be noisy. if they are off, they should not generate noise.
So much that you can actually HEAR them!
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any other ideas or suggestions guys? This is pretty far out of scope, and I have limited experience this deep in.
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@Brains said:
any other ideas or suggestions guys? This is pretty far out of scope, and I have limited experience this deep in.
Unfortunately this is an electrical engineering thing and not an IT thing. While we know of the issues, I don't think that anyone here is trained or qualified to help. Have you looked into bringing in an EE for this?
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Im talking to some of my EE friends from college, but they havent been of any significant help yet. I would like to pass this off to a professional qualified to deal with it, but I dont even know where to start. Any ideas?
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Did some EE, still do for a hobby.
Having trouble figuring out what the actual problem is in your post - 180hz signal shouldn't be there? What should? How do you know it's 180hz?
Could you link to the products you're using? From below you didn't really give what model etc. I found these, but not sure.
http://www.jjengineering.com/C6.htm -
@MattSpeller said:
didn't really give what m
Thanks Matt. I appreciate the offer of help.
I have attached a screenshot from the monitoring software. Yes the 180Hz signal should not be present, or at least prevalent in the pictures. He is reading very low frequency electrical impulses and this signal is throwing off the tests.
He is sending me the model numbers of the equipment in the morning. In the meantime, could you help me pick out some ferrite cores that may be able to help filter this 180Hz (3rd Harmonic?) interference? I was about to order some, but then I noticed that there were different types for different frequencies, and I cant make heads or tails of which one I need.
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@Brains Alright, well from what I remember I don't think chokes work that way - at least how you want it to in this instance. I will have to read up on them again to make sure I'm correct there!
Onwards; 180hz is super low and an odd one. 3rd harmonic of the 60hz powerline, no doubt that could be the source. Unusual to only see the harmonic though, do you also see 120 & 60? (60 should be a huge spike if it's AXT (alien cross talk) from a transformer or something nasty). You've already tried isolating it from outside noise and it sounds like you did a pretty good job of it.
Next up I'd look at the device it's self and read it's manual. I'm willing to bet it has internal filters for this kind of thing - if not I'm almost certain that it'll mention how to deal with AXT as it's such a sensitive device you won't be alone in having issues with interference.
