Verdin IMX8MM radiated and conducted emissions

Hi all,

We have seen a noise source at 700 MHz when performing our modem compliance tests. Is there any information on the radiated and conducted emissions testing you have done on the Verdin boards? Any data you can share from that? Or on what clocks are present on the board?

Thanks

Ed

@walter.tx @matthias.tx

Hi @edwaugh ,

thanks for posting this on the community. We will have a look at this and get back to you with the best delays.

Walter

Hi @walter.tx and @matthias.tx,

One of our contractors has taken a look at the board and has identified a potential problem as the 27th harmonic of the 25 MHz crystal on the Verdin board.

It looks like this is the second ethernet controller that we are not using:
https://ww1.microchip.com/downloads/en/DeviceDoc/00002841B.pdf
How is it connected to the IMX8MM? I can see that the RESET_N pin is high indicating the device is active. How can we disable it if it is not in use?

Thanks

Ed

Hello. This is the contracting company Ed referred to above. We have used near field probes, a low noise preamp powered from a battery, feeding into a spectrum analyser and we can clearly see a 700.000MHz spurious radiated transmission from the Ethernet 25MHz crystal and from top of the Microchip 9131. Also there is a second 700.000MHz spurious on the bottom of the board coming from the 20MHz crystal and the CAN controller chip. There are no other 700.000MHz spurious emissions from the rest of the CloudCycle PCB.

Andy Rawling

Hello,

the second Ethernet is only MAC interface.
So there is only PHY on board.
O am checking on how to software disable it.

Best Matthias

Hello Andy and Ed,

The same Ethernet circut we also use on other modules like the Apalis IMX6, 8 and 8X and we never had this kind of Problem. It does not look like that the 700 MHz being the 27th harmonic of 25MHZ that would be 675MHz. what could be possible that it is an emission from the SD card.
Are you using an SD card? can you test without ?
I is always a good Idea to prepare the software for the test that you can switch of different part to see where it comes from.

Best Regards,

Matthias Gohlke

Hi @matthias.tx,

We have SD card but only for programming it is not present in normal operation or during these tests.

Thanks

Ed

Hi @matthias.tx,
Any news on how to disable the second ethernet MAC?
Thanks
Ed

Hello Ed,

I like to take a look at your PCB design.
can you sen me a mail with some pictures on the PCB layout?

Best Regards,

Matthias Gohlke

Hello Ed,

is there anything new on the EMC issue? Is this still persistent?

Best Regards,

Matthias Gohlke

Hi Matthias, we will retest in a few weeks and are making some modifications to the PCB to improve performance. I can let you know how we get on.

Hello Ed,
ok please check our Layout design guides and also take a look how we do things on the GND and Chasis GND.

Best Regards,

Matthias