Have you determined that it is the stick and not the host?If the problem is
just this brand of stick, then there are some layout issues that can be re-done
to reduce or eliminate the issue. As James mentions, if there is an imbalance
in the differential signal, that will generate a radiated problem. So correct
the differential signal layout and increase the decoupling on the power+return.
If you have a USB cable that you can open up so the pairs can be "sniffed"
that may indicate which is the main source, signal or power, or both.The common
mode choke with more decoupling on the power rail might help.
From: "Pawson, James" <[email protected]>
To: [email protected]
Sent: Friday, March 20, 2015 9:22 AM
Subject: Re: [PSES] USB and radiated emission issues
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div.yiv3668948668WordSection1 {}#yiv3668948668 Hi Amund, a quick brain dump for
you If the emissions are caused by imbalance in the differential signal
then a Common Mode Choke (CMC) will probably help. Pick one with a good
impedance at the frequency of interest, 480MHz in this case but you can also
expect to see emissions at 960MHz and possibly 240MHz as well. The two main
performance graphs are common mode impedance (higher is better) and
differential mode impedance / insertion loss (lower is better) against
frequency. There are wirewound and multilayer ceramic CMCs made by Murata,
Laird, Wurth, TDK and probably others. The datasheet generally states if the
part is suitable for USB 2.0. Ceramic parts usually give better high frequency
performance from the datasheets I’ve looked at but are slightly more expensive.
I’ve seen some manufacturers with combination ESD protection and CMC and
some with CMC built into the USB connector. You’ll obviously need to measure
signal integrity before and after adding the chokes to ensure your eye diagram
looks OK. Hope this helps James
From: Amund Westin [mailto:[email protected]]
Sent: 20 March 2015 12:10
To: [email protected]
Subject: [PSES] USB and radiated emission issues Got some serious radiated
emission issues from a USB 2.0 stick (high-speed 480Mb/sec). Spectrum shows the
480MHz way over the EN55022 limit line. We’ve been told to implement a
common mode choke between the USB IC and the input/output port. That means on
the D+ and D- transmission lines. http://www.coilcraft.com/0805usb.cfm
Anybody who have experience with such design? #Amund -
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