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Artarka , 06-24-2024, 02:37 PM

Hi All, not sure where or how to put this.
I've finally manage to pass my product through EMI emissions, by having a ferrite on the mains harness cable and earthing.
However, it has been bothering me whether if pcb layout have anyway affect these kind of emissions.
My personal thought is since putting the whole setup in faraday cage didn't affect emissions one bit, I feel like theres nothing I could've improve layout-wise?

Side note, how do I actually troubleshoot noisy cables emissions?
Any recommeded courses?

Robert Feranec , 06-24-2024, 04:40 PM

Did you watch my EMC videos? They may help: https://youtu.be/Lf51sx6sC0I https://youtu.be/fkNa-FejWsQ https://youtu.be/J4UUGSIP770

André Araújo , 06-24-2024, 06:06 PM

1st: congratulations!!!
2nd: can you share more details about the design?

QDrives , 06-24-2024, 08:43 PM

What kind of product?
Earthing? As in connecting to earth or did you mean shielding?

Making an earth connection and the ferrite core on the mains cable seems to me as common mode noise.
This is (partially) shown in the last video Robert mentioned (https://youtu.be/J4UUGSIP770?t=5139 with time stamp).
Depending on the application it can be difficult or easy to solve.

Artarka , 06-25-2024, 10:16 AM

Artarka , 06-25-2024, 10:18 AM

hope noone within company see this lmao, very small board 45x65mm, 4 layer, P/S-G-G-S

Artarka , 06-25-2024, 10:18 AM

and hope you all trust me that I watch most videos and stick to best practices

Artarka , 06-25-2024, 10:19 AM

fail frequency at 67.53mhz 107.3mhz 167.5mhz seems too far off from 50mhz to be a problem of RMII, (they all have 33R series resistor to curb rise time)
all critical lines are <30mm in length which is absolutely no where near frequency and could have been routed like dead normal net.

Robert Feranec , 06-25-2024, 04:11 PM

this may be also grounding problem ... for example voltage difference between these two cables .. but I am not expert for EMC 🙂

QDrives , 06-25-2024, 08:09 PM

It is a bit wideband and seems common mode.
I assume you ferrite core needed to be on the 24V and RS485 cable?
Thing is, those RS485 lines couple to earth. A common mode choke on the signal lines could prevent the ferrite core. Alternatively, a lower EMI generating driver may be a beter solution.

User , 06-26-2024, 06:47 PM

Maybe you had ground bounce that generated the common mode current and they flew through the external cables.Did you have solid ground planes? Did you have 2 signal layers sharing 1 ground plane? What was your stackup ? Can you indicate which interface was routed on which layers?

Artarka , 06-27-2024, 02:44 PM

yea ferrite core is on 24v and 485 lines. would be grateful if you can explain rs485 line coupling to earth.
common mode chokes / smd ferrite is on my next test to see if they work. sucks to not have a bench myself.
by lower EMI driver, do you mean lower rise-time ones (and hence throughput)? or are there other factors on datasheet that affect drive EMI?

QDrives , 06-27-2024, 08:42 PM

RS485 communication means that the voltage in the lines changes. That creates (changing) electric fields.
Every cable, wire, board, etc. has a parasitic capacitance to other objects, including earth. There are calculators that give you how much capacitance it is.
With ferrite and the earth connection, you added a filter (choke) on one connection and a path (earth) for the noise back to you board.

Ferrite beads will not do much here I assume, as they are differential mode chokes.

"*by lower EMI driver, do you mean lower rise-time ones (and hence throughput)? or are there other factors on datasheet that affect drive EMI?*" -- Yes and no. I do not know you throughput, but seeing it is wide band and I cannot see much narrow band, I go for low baud rates (<= 1Mb/s).
But, yes, it is mainly a reduction of the rise-time.

"*sucks to not have a bench myself.*" -- Do you have an oscilloscope? What if you use the clip lead as a probe?
https://www.youtube.com/watch?v=C9qw_NwJWK4
https://www.youtube.com/watch?v=l2YS1s6HxHU
https://www.youtube.com/watch?v=S27zk_Gj9nU