In that case, I document well in the test report the process that determined it
is the commercial off the shelf stick as an accessory that has a problem and
not my product.And continue on. One cannot fix some other suppliers
devices.But you can shop around to find some sticks that pass, so the test
report looks cleaner.ps. If you put a clamp on ferrite on a USB cable to pass,
you'll be required to ship that USB cable with ferrite as part of your product.
The memory stick is not your product?Just a load for your ports?Cabled or
directly plugged into port?If your product has a noisy port, the CMC can help,
so while putting that in, fix the layout of the diff pair, then save the cost
of the CMC.A good clean memory stick cannot fix a noisy port.ps. and the bad
news is that at that frequency, and higher, discrete parts don't work well as
the high frequency stuff can couple around the parts. Layout can make the
biggest difference. Match exactly the D+ and D- in length and symmetry. Note
that the tolerance on exactly will make the layout designers about as noisy as
that memory stick, but a few minutes of thought and trying a couple of things
and they will find out how to make exactly work. And hopefully there are not
some other issues with the layout causing the noise.
The above assumes the USB IC is not the problem.
From: Ravinder Ajmani <[email protected]>
To: [email protected]
Sent: Friday, March 20, 2015 2:11 PM
Subject: Re: [PSES] USB and radiated emission issues
#yiv2164002629 #yiv2164002629 -- _filtered #yiv2164002629
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{}#yiv2164002629 Hi Bill, What you say may be true. I often use memory
sticks to terminate unused USB ports during testing, and on several occasions I
have come across high emissions caused by the sticks. Regards
Ravinder Ajmani
HGST, a Western Digital company
5601 Great Oaks Pkwy
San Jose, CA 95119-1003
[email protected]
From: Bill Owsley [mailto:[email protected]]
Sent: Friday, March 20, 2015 10:52 AM
To: [email protected]
Subject: Re: [PSES] USB and radiated emission issues 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 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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