Electronic – What could be generating this waveform in an EMC Compliance test


We are having trouble with a CISPR-11 (EN55011) Radiated Emissions Class B compliance test in one of our solutions. The troubling part of it is basically an industrial motherboard (with a commercial SSD and commercial RAM) within a well grounded metal enclousure, sharing space with an industrial switch, a switching 12V industrial power supply and a industrial touch panel and monitor. Every single one of these industrial components have its corresponding compliance certificate for CISPR-11. The antenna picks up the following waveform (Vertical orientation, 3mts distance, 30 to 200 MHz bandwidth). Crosses above the threshold are non-compliant peaks.
non-compliant peaks

The main offender is a 60MHz peak that looks like a strong square wave in frequency domain. That is new to me in these tests; I am more used to specific harmonics in the spectrum.
If we turn off the motherboard (and thus also the RAM and SSD), the 60MHz and also those crests at ~80MHz dissapear. Turning of the rest of the components (except the 12V PSU) has no effect.

I have the motherboard's compliance test and procedures, and their peaks are nowhere near in power or frequency. They also used commercial RAM and a mechanical commercial hard drive.

Has anyone seen this waveform before for radiated emissions and/or has any idea, given the description of our system, what could be generating it?

EDIT: Based on comments and answers, I will clarify some things. First, I have 1 Ethernet cable (unshielded) inside the case, connected to the switch, also inside, but then 3 Ethernet SSTP cables going out from the switch to 3 ethernet Basler cameras. These 3 do have a ferrite clamp on each, but I haven't fiddled with the one inside. I'll note that down.

The SMPS was our main target for the whole ordeal but I still need to try ferrite beads on its outputs and maybe enhance the shielding around it.

Unfortunately we do not have our own spectrum analyser and our scopes are up to 60MHz. We do not develop high frequency custom electronics, so they were not neccesary. Given the circumstances, we may adquire one and do our own pre-compliance testings, as many of you suggested.

I will keep this updated.

Best Answer

It looks like roughly 5MHz impulses with a resonance at 60MHz to me judging by the sidebands (20 per 10 MHz) but could be a 60MHz clock with roughly 5MHz recursive sidebands.

The fastest way to find the offending signal is to use a small loop antenna probe to DSO or scope (Coil or wire in a small loop) to sniff out the near-field noise. If you have 2 sized loops you can get a quick look then a smaller loop to get near the chip, trace etc. 60 MHz is about the size of a small room resonant frequency.

If indeed it is the data on the external cable then a Balun on the cable is a valid fix for that until you put a CM choke or SMT PHY on the data stream on the MOBO.

This is also the best practice for pre-test scanning the signature emissions before paying for the unintended radiation tests at 3m at an outside lab. I would use a current loop probe for near-field inside the box and an E-field probe or just a wire on a coax end into the Spectrum analyzer for a broadband spectrum. Antenna efficiency drops for longer than 1/4 wavelengths but it is still broad spectrum.

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