Published by Eric Bogatin on 01 Jun 2010
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If your last product passed FCC certification and shipped on time, pat yourself, your EMC engineer and your design team on the back. You accomplished something that is really hard and doesn’t usually happen by accident.
Of the various EMC certification tests, the FCC Part 15 Class B, which applies to consumer products, is one of the most stringent. In the roughly 100 MHz range, the maximum allowed radiated emissions from your fully functioning product, when measured 3 meters away, within a 120 kHz bandwidth, must be less than about 100 microVolts/m.
To put this in perspective, what do you think is the maximum power a radio station could transmit, into a 120 kHz bandwidth, and still pass this FCC test? Is it 1 watt? 1 mwatt? 1 microwatt?
The answer is shocking. A radio station would have to radiate less than 10 nanowatts of power into a 120 kHz bandwidth, in order to pass certification. This is hard.
The most common reason for products to fail this test is due to radiation from common currents on external cables. For a cable 1 meter long, it only takes a common current of 3 microAmps to radiate enough to fail a certification test.
When you consider that a 1 volt signal, driving into a 50 Ohm line, is a signal current of 20 mA, you see that the common currents must be less than 0.01% of the signal currents. This is why passing EMC tests is hard.
I have yet to encounter a single large system company that does not have a horror story to tell of a product that worked great, passed all the functional tests, but either was never able to pass FCC certification or took so long to fix an EMI problem that its release was late and it missed the market window.
You don’t pass an FCC test by accident. It is by designing radiated emissions out of the product right from the beginning, and where you can’t design them out, you add filters and shielding to minimize their impact on the certification test.
Don’t expect to learn how to design a product to pass an EMC certification test by following a list of ten habits. But, if you want a list of topics to use as a guide to begin the discussions in your design team, here are my recommendations for the Top 10 Habits to follow to increase the probability of passing an EMC certification test:
- Ground bounce drives common currents on external cables. Minimize ground bounce in all the components of your system
- Use shielded cables. The shield of the cable should be an extension of the enclosure, not connected to the ground planes of circuit boards. Cable connectors should make a 360 connection between the shield and the enclosure.
- All control wires and cables that leave the board, even if they are just routed inside the enclosure, should be routed with an adjacent return conductor. Use as long a rise time as you can afford for all signals that leave the board. Increase rise times with filters.
- Use ferrites around the outside of external cables to suppress common currents.
- Minimize mode conversion in all differential channels that leave the enclosure.
- Add common signal chokes to all differential signals that leave the enclosure.
- The largest source of noise, above 50 MHz, that gets into the power and ground network is from signals passing through the power and ground cavity. Manage this noise with return vias, differential signaling and decoupling capacitors adjacent to signal vias. Smart stack design up can enable the use of return vias.
- Design the stack up so you can have power and ground planes on adjacent layers, with as thin a dielectric as possible, and preferably close to the board surfaces.
- Plan on using a spread spectrum clock generator to smear the first harmonic of all signals into a wider bandwidth. The FCC receiver has a 120 kHz bandwidth. If you can spread the spectrum of each harmonic over 1.2 MHz, you reduce the average power detected in the FCC test by 10 dB.
- Enclosure design is not about designing enclosures, it is about designing apertures and seams.
Bring these topics up in your next design review. Have your SI engineers and EMC engineers explain what they mean. If you are still not clear on the concepts, or how to implement them, read a book, find an expert or take a class.
If you weren’t aware of these guidelines when you designed and built your last product, you may have been lucky and dodged a bullet. Don’t rely on luck for your next design.