[sdiy] Mains interference

[rburnett at richieburnett.co.uk]

On 2014-03-28 08:43, Needham, Alan wrote:
> 
> Could the radiation problem be line lengths? If a mains run just
> happens to be tuned to some magic RF frequency?

Every cable in a system has a number of resonant modes that make it more 
or less effective at radiating or pick-up at certain frequencies.  
Typical cable lengths of around a meter in audio systems have resonant 
frequencies in the high tens and low hundreds of MHz.  This can be a 
problem if you happen to live next to a powerful broadcast transmitter 
and the wire to one of your hi-fi speakers just happens to be a quarter 
wavelength at this frequency.  The easiest solution to this problem is 
to change the wire length, or better still slap on a ferrite sleeve to 
keep RF currents out of the wire in the first place.  Contact arcing 
tends to produce broadband RF so changing wire length is less effective, 
other than to say that shorter is usually better :-)

Most clicks/pops, taxi/CB/"mobile phone calling home" heard through 
audio systems happen like this:  It starts with common-mode pickup of RF 
signals on any one of the cables entering an amplifier.  Once RF 
currents get into the amplifier enclosure they causes voltage-drops 
across traces on the PCB due to their inductance.  These high-frequency 
signals inevitibly get rectified by a silicon junction somewhere 
resulting in low-frequency signals.  (This process is what enables a 
900MHz mobile phone signal to produce a noise you can hear from an 
amplifier.)  The gain of the audio amplifier does the rest.  The 
solution here is to firstly put the amplifier inside a decent earthed 
screened enclosure and then carefully filter all of the cables that 
enter or leave this enclosure.  If nothing gets into the box, then 
you're winning.  Sadly these days with plastic boxes becoming the norm, 
there is often less thought put into RF immunity at the design stage 
than there should be (>.<)

Appologies if some people feel this is a long-winded explanation, but 
it's something that most people have experienced.  Particularly the 
mobile phone morse-code.  EMC is also often portrayed as some sort of 
black art.  This is one area where it greatly helps to understand the 
mechanism by which something happens when trying to resolve it.

-Richie,  <--- Used to be senior EMC engineer at electronics 
manufacturer