We will be submitting an Apalis SOM based product (custom carrier) for regulatory testing shortly and have opportunity to update the design in preparation for manufacturing introduction. In none of the eval designs nor the Ixora carrier design, do the HDMI signals have any common-mode chokes in the differential pairs to the HDMI receptacle. As there is no mention of EMC measures for either the DVI or HDMI reference within the Apalis Carrier Board Design Guide (V1.5), our carrier design reflects this approach.
Can you confirm, either through your own testing and/or through feedback from customers, that EMC compliance is achievable without common-mode filtering, initially for the TK1 based SOM but possibly also for the IMX8 based SOM as well? Our experience with HDMI 2.0 designs has made us wary of EMC difficulties with certain TMDS transmitters, and we were forced to implement chokes for regulatory compliance. Although the TK1 supports HDMI 1.4b, possible future Apalis SOM variants may support HDMI 2.0. Would the inclusion of filter chokes on the carrier not be a form of future-proofing, or will EMC compliance measures always be incorporated in the SOM itself?
As very often, the answer is: it depends. First of all, it is always required that the whole system (module, carrier board, and housing) is measured for EMC compliance. If the system passes with one module, this does not mean that it will pas with another SOM.
So far, we did not add the common mode chokes, since we did not have the need. We have been able to pass the tests with the system we have been in the lab. However, in the end, it depends much on your own carrier board design. If you feel more comfortable adding chokes, please do it. If you can afford the chokes in your BOM cost, I probably would add them. If you are planning to upgrade to a SOM which features HDMI 2.0 (e.g. Apalis iMX8QM) and you need higher resolutions and frame rates, make sure your chokes and ESD protection diodes are appropriate for the higher frequencies.
The application will be fixed resolution and likely won’t ever need 4K, but could require an updated CPU core that forces us to go with more advanced SOMs. Saving the cost of a few tiny choke packages may not be worth the risk of failing approvals.
