Nothing is worse than going to the EMC lab and failing an emission test. This is especially true when you figure out that an operating frequency of the device under test (DUT), that you designed is causing excessive emissions within a frequency band which has a low limit. We see this time and time again in the test laboratory.
The best way to prevent this from happening is to evaluate the specification requirements before you start picking out clock frequencies and communication rates. If you look at the specification limits before you design, or during the design phase, you can determine frequencies that you should stay away from.
Don’t select operating frequencies that fall into frequency bands with low emissions limits. Also look at the harmonic frequencies (especially the odd harmonics of the operating frequency) to see if a harmonic will fall into a frequency band with low limits. If possible, pick operating frequencies that fall into frequency bands with higher emissions limits.
Many times communications rates can be adjusted through firmware. This is something that can be in your “bag of tricks” when designing the product and/or troubleshooting a failure. When it comes to designing for good EMC performance…review and design to the specification. If you do, this will save a lot of future headaches and re-design costs.