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Use of ferrite core against RF noise

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2007-2009 » Archive through 31 July, 2007 » Use of ferrite core against RF noise « Previous Next »

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nielsejner
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Username: nielsejner

Post Number: 19
Registered: 06-2005

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Posted on Monday, 23 July, 2007 - 05:57 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I have been advised to use a ferrite core to remove electrical radiated noise from a wireless router into a surveilance camera system.

I forgot to ask all the right questions at the time so now I need some advise.

Should all the camera wires go trough the (same) core, audio, video, supply, ground ?

Is two or more turns better than just one ?

Thanks in advance.
Sejner
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obiwan
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Username: obiwan

Post Number: 1926
Registered: 12-2005

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Posted on Monday, 23 July, 2007 - 07:54 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

You can slip, or clamp the core right onto the cable. Adding more loops of wire will increase it's inductance, and lower the frequency that it responds to.

So, if you have a very HF noise, just place the core on the cable. If it's not so high, add a turn, and so on.

You should probably have some sort of method for measuring the noise so you'll know if you have done right.

But generally, the noise will be on the signal lines, the composite video. You shouldn't have to worry about the power supply lines.

The audio system shouldn't need it either since the whole circuit probably wouldn't respond to that high of a frequency anyway.

But if it's all in one cable, you can just place the core on that bundle.
Do Not Hit The Fly That Lands On The Tigers Head.
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dave_g
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Post Number: 296
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Posted on Monday, 23 July, 2007 - 07:57 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I am no expert in this field, but if you think of a PC monitor, the video cable has a ferrite which encapsulates all conductors.

Noise can travel through any of them, even ground (no conductor is perfect).

Reading your post again, how close is the router to the cabling? You might be better off using more shielding across the entire cable, or putting it in metal conduit, trunking, or even just some metal capping over it. If it's a surveillance camera, presumably it could do with mechanical protection anyway.

Others may or may not agree, but this sort of thing is often "try and see": go for it!
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obiwan
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Username: obiwan

Post Number: 1927
Registered: 12-2005

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Posted on Monday, 23 July, 2007 - 08:42 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Yeah, without knowing what the noise is, what's causing the problem etc.... it's hard to give any real advice.

But I would assume that the video cable is already shielded, coax cable normally.

Can you just move the router??
Do Not Hit The Fly That Lands On The Tigers Head.
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nielsejner
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Username: nielsejner

Post Number: 21
Registered: 06-2005

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Posted on Tuesday, 24 July, 2007 - 10:33 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks for all advices.

Yesterday evening I added a ferrite core inside the camerahouse, all vires one winding through, and this morning I clamped anoter core on the outside of the cable in the other end, near the monitor, this has reduced, but not removed the noise.

The source seems to be the wireless PCMCIA of this notebook. Whenever doing anything that requires transmision to the router, placed in another room, the noise on the cameramonitor is present.

The card is an old netgear and I will look for a replacement I can borrow, to see if this reduces the noise, otherwise I will just have to live with it.

Sejner

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