We're seeing spikes of energy at ~200 MHz in our EMI reports with the CP220x. What could be causing this?
EMI issues with the CP220x itself would be in multiples of 20 MHz, since this is the frequency of the internal oscillator. We have seen some issues with EMI in the ~200 MHz range related to the magnetics.
The latest schematic and typical connection diagram in the CP220x datasheet (revision 1.0) has component and configuration updates to reduce the effects of EMI related to the magnetics. The latest layout splits two capacitors on the TX pin with a ground connection in between. This allows a place for energy to discharge and generally reduces EMI.
If using the CP220x with a C8051F34x, another source of EMI could be the 'F34x device. The C8051F38x family will have generally better EMI performance and is a drop-in replacement for the C8051F34x in most cases. See AN789: Porting Considerations from C8051F34x to C8051F38x for more information. Additional application notes can be found on www.silabs.com/8bit-appnotes.
For other general EMI recommendations, see the Knowledge Base Article "Electromagnetic Interference (EMI) and Electromagnetic Compatibility (EMC)"