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EMI problems...

Sniper2 , 10-14-2025, 08:46 PM
So i just got from testing , barely passed but man i have no idea what went wrong. ( passed calss A but not B or it is the other way around*)
From the frequencies in the report i know it is USB and a serial cable but how ? and why? i have continuous GND plane and i used the right impedance profile. Do i always need ferites on my cables?
Or Am i picking up from the USB in my other cables? for the serial link i can put some series resistance and a extra cap to slow the edges and i should be ok since i ham just barely crossing the lower threshold .
USB seems to be the problem ut i even have a CMC on it what more can i do to it?
PS it is in a box so that is shielding too but i have very long USB cables so i suspect i am radiating from them.
Any ideas?
Robert Feranec , 10-14-2025, 10:34 PM
what power supply did you use to power your board? sometimes that can be a problem.
QDrives , 10-14-2025, 11:03 PM
What is connected to the other end of the USB cable? I have seen the PC attached to it.
Sniper2 , 10-15-2025, 04:00 AM
Usb and my mcu is 5v , there is a pi filter in between and there is also a buck to 1.2v and 3v3 for some other stuff but it is like 200ma
Sniper2 , 10-15-2025, 04:25 AM
A PC but through a self powered USB isolator this did pass emi in the past ( isolator usb)
QDrives , 10-15-2025, 11:11 AM
Do you have series terminating resistors on the USB data lines?
Sniper2 , 10-15-2025, 04:13 PM
thould be inside the chip on the other side
Sniper2 , 10-15-2025, 04:14 PM
atm there is no SMD placed across the D+- since both ends are terminated in chips
QDrives , 10-15-2025, 04:25 PM
Is it the USB communication (ie. 12MHz) or from the power?
What IC do you have for the USB communication?
Sniper2 , 10-15-2025, 05:19 PM
iti s for both and it runs usb 2.0 480MHz
Sniper2 , 10-15-2025, 05:19 PM
1 end is from PC via USB isolator other end is some TI HUB
QDrives , 10-15-2025, 06:53 PM
https://e2e.ti.com/support/interface-group/interface/f/interface-forum/893313/faq-tusb4041i-how-to-configure-usb-2-driver-strength-of-ti-usb-2-hubs
https://e2e.ti.com/support/interface-group/interface/f/interface-forum/664662/tusb2046b-tusb2046-hub-design-review
Sniper2 , 10-16-2025, 11:03 AM
Thx
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