I have been performing some EMC testing on a product (which I didnt design)
Its been a right barrel of laughs and it was quite challenging decreasing the radiated emissions below the 50dBuV/m limit
I had an issue with a 100MHz radiated emission which was decreased a lot with a proper earth and screen termination
The other problem area was around 33MHz and I only managed to just scrape it, took the emissions from 57dbuv to approximately 49dBuV/m which is just too close for comfort
After some investigation we determined the radiation is transmitted by the input power supply cable
I am designing an add on PCB fitted directly to the input to handle surge and my thinking is why I have the chance is there anything I can do to filter these troublesome 33MHz radiated emissions
Filters are normally designed for conducted emissions and theres little I can find to help me, the radiated emissions (to my knowledge) are a function of common mode current probably drawn from the SMPS that is fitted directly to the input cable that is radiating
So my question
If you had a radiated emissions issue at approximately 30MHz what would/could you do to filter them to reduce them
I have laid out a simple PCB, its basically a standard EMI filter but with surge and transient protection with some RC snubbers included to have a play around with
Is an LC snubber a bad idea?, my thoughts are I can include an L that is larger than the parasitic inductance from the supply so I can (in my mind) tune my components to shunt whatever frequency I am interested in as the parasitics I have no control over become negligible but am I just including a resonance that will make things worse?
Is it even possible to filter signals that are radiating? nobody seems to talk about this when it comes to EMC its always shield a cable or earth things better with ground planes etc no mention of filtering which makes me think its just another case of I havent got a clue what I am doing!