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grommeteer 31st July 2012 06:15 PM

EMI problem with dLan, PowerLan
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I would like to share an experience I made with my dLan. Mod, if this is not the right place, please move this thread.
I was trying to repair an amplifier that produced a strange clattering noise. I used my new DSO to track the unwanted signal. The scope triggered to a burst of oscillation, about 400Ás long and repeating every few tens of milliseconds. It cost me a few hours to find that this signal had nothing to do with my amplifier. It was everywhere, it even continued when I switched the amp off. At last I even measured it by touching the ground of my scope (outer rim of the BNC) with the tip of the probe. Again after a lot of running around, testing, switching off every electric device in the house, testing in the garden shed, emails to Hameg (maker of the DSO, very helpful), even bringing a second DSO (Agilent this time) from work, I found the signal was generated by a dLan connection I used in the house.
- My probe, with the tip short circuited to the BNC connector is a maganetic antenna.
- The dLan adds its signal to the power lines in my house, in the tens of MHz.
- The mains wires transmit the signal around the house, the loop antenna picks it up.
- What I found on both DSOs was not negligible, around 40mVpp.
- It can not be seen on an analog(ue) scope, because it happens too fast and too rarely to be displayed/seen on the crt.

Every loop of wire will pick up this signal. Shielding does not help, this must be magnetic.
You probably do not want anything like that on your phono, line or even amplifier feedback signals. To be sure nothing of that kind irritates you circuits I would recommend to banish dLan or PowerLan adapters from your house. That is what I did.
What is your experience?

DF96 31st July 2012 08:51 PM

Yes, mains signalling creates interference. Everybody seems to know this apart from the European regulatory authorities who seem to deliberately turn a blind eye to it. They seem to think that allowing people to spray electronic graffiti all around them will create an e-paradise of creativity and wealth which will more than compensate for all the problems created for radio listeners, music lovers and (in time of local crisis) potentially even the military (whose HF radio might have problems too). There has even been concern expressed by the air industry that planes flying over cities might not be able to use HF radio because of the interference.

marce 1st August 2012 12:45 PM

Why does Keith Armstrong want PLC banned, why has PLC got an exemption from CE EMC testing, because it is deliberately adding EMC to your mains. I had to stop using my RF TV transmitter when next door installed PLC for their homenetwork.
It should be BANNED, it goes against everything I have learned working on numerous electronic and electrical systems. The joke is that military radios and communication equipement has to be CE and FCC compatable.

wwenze 2nd August 2012 04:37 AM

Would IEEE 802.11n be worse I wonder?

marce 2nd August 2012 07:19 AM

Its airborne RF wich we already live with, hense shielding etc, but mains borne has to be handled by PSU etc.
Here is a link to the UK EMC club (Keith Armstrong etc) with numerous articles on the problems it causes.

grommeteer 2nd August 2012 07:47 AM

What annoyed me is that I thought MLC was bound to stay on the mains lines.
It does not!!
I understand it like this:
A HF signal is superimposed to the mains lines. In many of my household devices there are X and Y capacitors that try to short out this HF signal. So there will be some HF current flowing, well, somewhere, everywhere, in the house. A HF magnetic field is created.
The effect that picked up the signal was clearly a magnetic loop antenna. A straight piece of wire did show the same effect.
I my struggle to get rid of the interference, I used mains filters and/or isolation transformers for the supply of the DSO. Did not change a thing! It did not creep into the DSO through the power supply.
I have WiFi, DECT, GSM, 3G devices in the house and none of them show up in my measurements. Their frequencies are way too high for my equipment. So, I can not be sure of what is going on there..........

DF96 2nd August 2012 11:19 AM

You would still get some radiation in the absence of X and Y capacitors. Basically, mains wiring acts like a leeky feeder antenna. The only solution is not to use the mains for HF signalling. Apart from that, filtering will reduce the problem for some equipment but HF radio will still suffer. There are supposed to be notches for amateur radio bands, but these are not always properly implemented.

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