EMI reduction by spread spectrum clocking PLL

[jihrenee]

I know with spread spectrum technique,
we could spread the power of a signal in a wider bandwidth
reducing the peak power of a signal which is narrowband originally.
But i don't know why this action can reduce EMI?

 

[Tunelabguy]

It does not reduce the total EMI energy. It just spreads the effect across a wider spectrum so that less energy is present in any one narrow band, which will cause less objectionable interference to devices that use narrow band communication. It turns out that the governmental regulations for measuring EMI are written that way so that's why people do it. But as I said the total EMI energy does not change by using spread spectrum clocking.

 

[jihrenee]

It does not reduce the total EMI energy. It just spreads the effect across a wider spectrum so that less energy is present in any one narrow band, which will cause less objectionable interference to devices that use narrow band communication. It turns out that the governmental regulations for measuring EMI are written that way so that's why people do it. But as I said the total EMI energy does not change by using spread spectrum clocking.

Thanks for the reply.
By the way, what is the meaning of "will cause less objectionable interference" when spread spectrum clocking is utilized?
Since, the energy of EMI doesn't change by SSC.
It's weird by measuring EMI level in this way while the EMI energy doesn't change at all.

 

[Tunelabguy]

Narrow band communication filters out all the interference that is not inside the band of frequencies they are using. So they would only be sensitive to the small amount of EMI that falls within that narrow band. But non-spread spectrum EMI concentrates all its interference on a few narrow frequencies, and if someone happens to be using one of those narrow frequencies for their communication, they will be especially affected. So instead of affecting a few channels a lot, spread spectrum affects many more channels, but to a much lower degree. The total amount of energy is the same. But spreading it around makes it so that no one service is overly affected.