In our Car infotainment Unit, a HDMI Sink port is designed using an ADV7611W HDMI Receiver.
The EDID is configured to receive only 480p (720x480p or 640x480p) resolution.
Hence the HDMI TMDS clock is received at 27MHz and the data per TMDS lane is received at 270Mbps.
Whenever the HDMI Cable is conencted, an strong Emission is observed at 81MHz (in both Radiated and Conducted Emission measured in the Rear panel Cables) of about -65 to -85dBm depending on the Source device (HDMI Transmitter) connected (Phones, Laptop, etc).
This 81MHz is the 3rd Harmonic of the TMDS Clock 27MHz.
The Similar behaviour is observed in the ADV7611W Evalualtion Board also.
In our deisgn, the HDMI TMDS Traces are routed from a HDMI Connecter Board --> HDMI FPC (Flex PCB) --> CPU Board with the HDMI Receiver. Total length of the TMDS Traces is about 6.5inches.
However, even though the Eval board has only traces of about 1inch length, the emission levels are similar.
We performed some tests to confirm that the emission is predominantly through Radiation only. (part of the same can be though conduction which is unable to be verified due to the presence of Radiated field in and around the board/unit).
The Possible Sources are the HDMI Cable (mostly), HDMI Source, the HDMI Receiver and its TMDS Traces.
Since it is an Automotive product, the limits are very stringent at this 81MHz band.
Please let us know if there is any possibility of reducing this Emission with any SW/HW configuration changes related to the HDMI Receiver/circuit.
Please let us know if any similar phenomenon is observed in any of the other designs with ADV7611W in low resolution 480p mode.
Thanks & Regards,