Switching power supplies can generate some nasty RFI spurs. Now, some designers are turning toward "dithering" the power supply switching frequency courtesy of a built-in random number generator. With this approach, instead of having a lot of RFI on one frequency, the RFI will be thinned out and spread across a wide spectrum like cheese on a cracker. See: http://powerelectronics.com/mag/603PET07.pdf Apparently this is cheaper than building in filter components, hence improving the profit margin for the manufacturers