Trumpy,

For "transformer", read "utility transformer" i.e. the xfos used to supply the site with power. Harmonic content that adds on the Neutral creates circulating currents in the utility delta-star (delta-wye) xfos. PDH spoke of supplying the hi-tech load phase-phase so I assumed he had a transformer giving him a 3-phase supply (and also assumed this to be a delta-star). Sorry for the confusion!

The EMC laws pertain to any equipment actually but specifically SMPSs as SMPSs transformers are driven by high frequency oscillators with very sharp switching edges (and the sharper the edge, the more efficient the SMPS). These sharp edges, using Fourier's idea, are full of high frequency content and it is this content that must be kept in check (hence the EMC filters that exist on both the input and output of the power supply). And...

[Linked Image from marcspages.co.uk]

This is the typical input to an SMPS and, as can be seen, is asymmetrical i.e. Live and Neutral have exactly the same components. Their purpose is to keep the noise inside the power supply (shown in red arrows) – but some will leak out, however, the noise of the oscillator will emit on both the Live and Neutral, not just the Neutral alone.

I'm going to throw a little curve-ball here; I actually believe the noise you are experiencing from your PC is not the SMPS (or only a small percentage is the SMPS), but rather the motherboard and other associated peripherals such as keyboard, monitor, etc. My PC is a Dell and they are constructed in shielded cases (the edging uses a finger type material that ensures, once the case is closed, it is like a solid impenetrable block). I can operate all bands without an issue. There are plenty articles on the 'Net about shielding a PC for radio use.

Hope I've cleared up any confusion I created?

M.



[This message has been edited by marcspages (edited 02-22-2005).]