Hi
I have a problem with a Ethernet PHY that makes too much noise. We are digging into the problem, iterating on the very poor layout done, but the client is looking for a fix right now with now HW changes.
The problem comes from the RMII communication to the PHY. Seems a 25MHz clock is feeding the PHY (LAN8742), which then in turn creates a 50MHz clock feeding back to the STM32 MAC.
If I remove the clock, the problem is gone. Problem is a radiated emissions +100MHz, so quasipeak detection is used in final measurements.
So have been trying to slow it down, and other tricks but thought about this solution:
Initiate the PHY as normal and let the PHY/MAC get an IP address.
Then disconnect the PHY for let's say 500ms, by removing the 25MHz clock. After 500ms, activate the clock again.
That would possibly reduce the emissions by 3-4dB due to the quasipeak detector.
But, can an ethernet connection work like this?
How does the line react, will it do preamble communication again, or will it just work again (some packets might be lost, but we can re-transmit those)?
I know from earlier measurements on an ethernet line that it is active all the time even though no data is send.
Cheers
Klaus