I recently had one of my new rechargeable Lithium Ion products tested (RE pre-compliance scan) and I had a few frequencies (around 150 MHz) pop (7-10 dBuV/m) above the FCC and CISPR limits.
I'm not an expert at dealing with EMC/EMI problems by any means, but I am familiar with proper PCB layout/routing practices to minimize the chances of problems. I don't think the PCB is the problem. But lets see what you all think. I can provide images of the layout if needed.
I have done some probing using H and E near field probes and a StiCo 18" VHF antenna to try to pick up some of these harmonics. The fields do not seem very strong in the problem frequencies (+17-20 dBuV/m with the inductor inside the loop of my BeeHive 100C H field probe between 50-150 Mhz). This doesn't seem bad to me (but again, not an EMC expert). The field does have a 500 kHz harmonic, which confirms it's coming from the switcher which runs at that frequency. There's nothing else on the board that generates noise. The measurement drops to zero when moving more than about an inch from the emissions source. I don't pickup anything with the smaller loop probes. I see a (+5 dBuV) E field when probing the switcher and inductor directly.
I know it's very difficult to causally link near field measurements into far field results, but would it be safe to deduce from all of this, that i'm probably not radiating out from the PCB? Is there some other way to confirm this? It seems to me like the input wire is my problem. I do not have any other EMI filtering on the board other than ceramic bypass caps.
My next step is going to be to do a few more scans using a clip-on ferrite. If that works and gives me good margin, then i'll probably spec out a new AC/DC converter with the ferrite built in. I'm also looking at adding a proper EMI filter to the board, but I just don't think I have enough room on the PCB for those components, so the ferrite seems like my only option.
Some notes about the PCB design that I think may be pertinent;
- PCB is small, 25mmx28mm, 6 layers with very short traces and power/ground planes
- PCB is contained within a grounded metal (aluminum) housing that is open on only one end
- 12V input, 3V output, 2.5A output
- Maxim integrated 500kHz switcher capable of 4A continuous output current
- The regulator is very stable and running at 90% efficiency with clean/no ringing switch node
- Using a shielded 5x5mm Coilcraft inductor
- The system uses a charging cradle base which has a long (1m) wire delivering input power
Any input or suggestions would be appreciated. Thanks