We’ve been doing pre-compliance scans on a customer’s product and helping them overcome some interesting radiated emissions problems (spoiler, 3rd party display module. Again.)
Now that it is back in the lab for it’s final EMC measurements we suddenly found that we were measuring an extra 10dB of noise at 170MHz.
Hang on… it was passing during the pre-compliance measurements last month… what is going on?
During the pre-compliance measurements, we only saw this peak when we were powering the EUT from the provided open frame mains power supply. So we removed it for some investigations.
Using our Tekbox TBCP2-750 current probe and Signal Hound BB60C spectrum analyser, we measured all of the cables connected to the power supply.
On the AC mains supply:
On the DC power output cables:
and on the 12V auxiliary fan power cable
What we measured was a bit of a surprise:
Of all the cables we expected to have a problem with, the low power 12V fan cable was not our first candidate. It looked to be carrying the most noise at 170MHz so we did what every good EMC engineer does – put a ferrite on it!
Now it meets Class A with a 5dB margin at that frequency.
Upon further investigation, a second fan had appeared inside the equipment since our pre-compliance measurements. The engineer had mentioned improving the temperatures within the product but we hadn’t opened it up to verify if any changes had been made.
The cable routing for the new fan was undefined, allowing it to lie across the power supply, or next to other components depending on how it was assembled. This appeared to be the cause of variability that we had observed in our testing.
One of the key rules of EMC troubleshooting is to change only one thing at once, and be careful that you are only changing one thing. Reassembling the unit with different fan cable position accounted for some of the variability in emissions performance.
Don’t assume that just because you are using a pre-approved component that it will automatically pass when integrated into your system. Having worked with many Meanwell power supplies of all different flavours over the years this is only the second time we’ve had any significant issues with one.
Bye for now,