Radiative Emissions
Radiative Emissions
(OP)
I'm working on a book scanner product which contains several PCBs and a variety of cables and connectors. We recently submitted our product for ETL testing and failed the radiative emissions portion. I am a Mechanical Designer so my electrical knowledge is limited. We have incorporated shielded wires, enclosures for the boards, and verified proper grounding of all components. We're about 4.5dB over limits at 840Mhz and 2.0dB over limits at 660Mhz. Any suggestions to troubleshooting the EMI?





RE: Radiative Emissions
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Radiative Emissions
Just a random question, I have been learning as much as I can about radiative emissions and found some information regarding component shielding which discusses "redundant shielding for noisy areas". Is this feasable?
Thanks for the feedback!
RE: Radiative Emissions
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Radiative Emissions
One should consider the cost of each test run (thousands?), plus the cost of delays (tens of thousands?), and balance that against the price of bringing in an expert to sort it out.
On the other hand, 4.5 and 2.0 dB are so close (on the log scale)! By way of comparison: on a different thread on a related forum, we had one poor fellow with a product that was as much as 26dB over the limit.
Are your enclosures shielded, or plastic? Do the shielded cables have correct 360° shield termination at the enclosure? Did the product pass any Conducted Emission testing?
RE: Radiative Emissions
The machine was originally designed by a contracted freelance designer, I've been trying to figure out some of his logic and correct problematic areas. Originally the machine had little shielding and improper grounding of all components. A few areas that raised flags to me were the main PCB layout, which had no shield, and the camera CCD (PCB), where the I/O power connection is right next to a data cable connection on the board. I added a sheet metal enclosure (grounded) to the PCB layout which contained a CPU board, servo controller board, and a camera board. We also added wrap around self adhesive wire shielding (grounded) to the power wires that run to the camera CCD (PCB) and a grounded sheet metal shield over the board. This all helped (we were about 15dB over @ 75Mhz on the first test) but we're still a little off at higher frequencies.
What bothers me is how the geometry of the machine may affect the far field test results (how do we analyze/manage this) and how much consideration for EMI/EMC testing was taken into account during the original design. I'm concerned we may have to redesign some of the electronic layout. Like I said, I'm not an EE but I'm not sure how much experience the original designer has had with EMI/EMC testing.
Any additional feedback would be great! Thanks again!
RE: Radiative Emissions
The company I work for would consider hiring an EE that has experience with this type of work, meaning EMI/EMC product testing. I haven't had any luck finding someone that fits these qualifications at this point.
Also, we did pass the conductive emissions test (7.6dB below limit) after an inline filter was installed between the power cord connection on the back of the machine and the power supply.
I would really like to find an EE to work with that can analyze our test results along with the overall design and electrical component layout of the machine and provide some sound advice on how we should go about correcting these noisy areas so the next time we have it tested we will be confident in success.
RE: Radiative Emissions
I highly suggest getting that EE in sooner rather than later as you guys are throwing darts at a board and you can't be sure where the board is even located. A guy worth his salt would probably be able to calm most EMI by many dB just by looking at how the thing is currently designed and suggesting changes, no test equipment necessary.
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Radiative Emissions
Again, any help pointing me in the right direction to find a consulting EE with this kind of expertise would be extremely helpful.
Thanks!
RE: Radiative Emissions
David Zimmerman
EMC Consultant at Spectrum EMC Consulting
Location Greater Minneapolis-St. Paul Area
Industry Aviation & Aerospace
(I know nothing about him.)
RE: Radiative Emissions
HOOLIHAN EMC CONSULTING
Address: 32515 NOTTINGHAM CT, LINDSTROM, MN 55045-9102
Telephone: (651) 213-0966
RE: Radiative Emissions
RE: Radiative Emissions
Quick Example... those darn little mag jacks (combined magnetic + RJ45 connectors) drive me nuts! There area some pretty good ones, but there area ton of junk ones (they are a few cents cheaper for a reason). If you have the room using a seperate magnetic and jack does help the EMI issues.
Other one I ran into was, when reviewing a schematic, was no ferrite beads were installed. When asked about it, the Engineer said they told him to save 0.02c per board as they aren't needed. Hmm, small volume product (5K boards), so I was curious if the $100 savings offset the board spin to add them back on.
Other favorite one, when I was designing a dashboard controller, was to use a 21.8Mhz XTAL. Later someone that that was stupid, as 20Mhz was a cheaper XTAL. I about died laughing about a purchasing agent saving a few cents.... Sigh.
RE: Radiative Emissions
Also look at the metal pieces with either wires or components mounted to them. These metal pieces should be bonded together with the entire chassis. If painted parts are screwed together the metal may be isolated and forming a good antenna.
Wire routing. Wires should be routed near well grounded metal surfaces not floating in the air.
When contracting electronic design for consumer sales always include an EMC clause in there so the original designer has to keep this in mind throughout the process.
Good luck.
RE: Radiative Emissions
A re-layout of the board sometimes may not make economic sense. Once I helped a large computer manufacturer solve an EMI problem. Within 1 hour of troubleshooting it became apparent that the circuit layout in the power supply was causing the problem. My client however, liked that PS manufacturer and didn't want to force them to deliver a product which meets FCC specs. The solution was huge ferrite beads on the power line. The new "small" product became bigger.
RE: Radiative Emissions
Thanks for all the feedback!
RE: Radiative Emissions
Dan - Owner

http://www.Hi-TecDesigns.com