Those ferrite filters you see on cables

Christmas came a little earlier for me when I recently invested in a Rigol DSA815-TG spectrum analyser to do some pre-EMC testing for a project for a UK client and I ran a test this morning on a G120 based design I had for a home project just to see what it did and to get an idea how to set this up and do the tests.

Using a LISN in the DC power line I was able to capture the conductive emissions and the attached image shows on the top image that there is a peak over the 60 DBuV limit (purple line) at around 460Khz (most likely the switching supply frequency)

The lower image shows the effect of installing the 2 filters on the power line with 2 loops of each wire. This brings the design into spec and it would likely pass the emissions test for conductive testing. This was a quick fix but not idea for the end design so a proper look at the PCB design and additional filtering is needed.

I’ve ordered some Murata DC EMI filters to test with on the G120 board as this is what the design for the client will be based on and I want to see what effect they have on the spectrum analyser output.

It’s cool to see the effect even just by adding the ferrite filters to the DC power lines.

PS… The Rigol DSA815-TG (with the tracking generator for filter testing) is great value. I never though I could afford a spectrum analyser that can go from 9Khz to 1.5GHz until I discovered this on the EEVBLOG. :slight_smile:


Cool, thanks for sharing. I often wondered just how well those work. Nice to see some proof.