Power supply manufacturers often break the EMI rules, especially in low-cost PCs.
I’ve been following the excellent series on EMC by the gurus at Kimmel-Gerke Associates. Every engineer should have their materials close to hand when designing any kind of electronic equipment.
And the engineer should obey their advice and – here’s the hard part – convince management to obey their advice.
A couple of years ago, I began to experience significant interference to my ham equipment, mostly on the 80- and 160-meter bands. Signals on these bands from long distances are generally pretty weak, and it doesn’t take much interference to wipe them out. And this interference was huge…obviously originating close by, and with the typical signature of a noisy switching power supply: wideband crud with peaks every 60 kHz or so. After some trial-and-error, I tracked it down to the new PC my wife was using, and specifically to the USB-powered speakers. Unplug speakers, no interference; plug back in, lots of noise. I figured that unplugging the speakers every time I wanted to operate on the low bands was inconvenient, so I decided to take the system approach and ask my wife to turn off her PC when I wanted to operate.
Lo and behold, shutting the PC off with the speakers plugged in did NOT make the interference go away! It seems that this PC (and maybe others do the same) keeps the USB supply alive when everything else is shut down…with the PC “off”, I could make the interference go away by unplugging the speakers OR unplugging the PC power cord.
When my wife wasn’t looking, I took the machine apart, and examined the power supply. Right behind the IEC connector was a big gaping hole, about the same size as a packaged Corcom or similar line filter or a couple of chokes and caps. I installed a filter I bought for a buck at a local surplus place in a small external box with a short power cord and the interference problem went away completely.
I figure that the manufacturer of the power supply submitted one unit to a testing lab with a filter installed, then after getting certified as meeting the specs for conducted and radiated noise, modified the BOM to remove the filter. Yeah, it saved a buck or two of cost, and most customers would never know the difference. Does your company do this?
This power supply manufacturer broke the rules, cheated, and caused the kind of interference to a licensed service (in this case, the Amateur Service) that Class B computing devices are not supposed to cause.
I can still hear lots of noise sources in my receiver, but some of them are likely in neighbors’ houses and I figure it’s easier to live with the noises (as long as they are small) than approach neighbors who have no idea how illegal their PC/TV/modem/phone-charger/electric blanket really is.