Unusual EMI frequencies

What else is in those systems?

You should be able to narrow down exactly where that comes from, using EMCO near field probes or the like. A possibility is that something intermodulates. For example one strong TV signal with another, inside your unit. But no matter what, you've got to figure out where that happens or they won't give you that coveted compliance certificate.

Joerg,

Thanks for the reply. We have a variety of different peice parts in each system. For instance, one uses a linear power supply (AC/DC) and the other uses a switching. There aren't and RF communication hardware or monitors (except in one system, which was turned off and we still saw the spike albeit lower than before. I am heading up to the lab tomorrow so I will try using a near field probe on the system. I'll report my findings.

Thanks, Matt

It seems Rich had used my computer while I stepped away. The message above was posted by me. Sorry for any confusion.

Matt

That's what I thought :-)

Anyhow, what I meant with other stuff: Is there some kind of micro controller or other circuit that needs and has a clock source? Or any OEM parts that might have something like that inside? If so, then I'd first look for anything that is common between the linear and the switcher system.

Switch mode supplies rarely generate noise above a few MHz although I have seen a serious blunder by a major power supply manufacturer where it emitted 250MHz at full bore. Probably an oscillating opamp or comparator that nobody had noticed until I did the EMI, for years abd years. In other words one cannot necessarily trust OEM components when it comes to EMI no matter what their claims on the datasheets are.

Sounds to me like a room resonance. Are these measurements taking place inside a shield room?

John,

Sorry for such a late delay, I was up at the lab testing quite a bit. The room was a shielded room and the input power was running threw line filters before it reached the EUT. We believe at this moment that it has something to do with an unshielded power cable because we were able to run the system using internal battery power and we didn't see the same noise level at the 22 MHz frequency range. Unfortunately, we got booted from the EMI chamber so the company we were using could perform an audit. I will have to try a few more tests next week.

Thanks, Matt