[SI-LIST] Re: High speed USB Query

Hi Mani,

I am curious about your question. 480Mbps is the data transfer rate, what is 
the clock frequency that is used to modulate the signal on the line? If you 
remove the FTB generator, and measure the CM currents on the USB line, will 
you see noise harmonics at the frequencies which are integral times of the 
clock frequency? I have ever used CM chokes for both the USB line and our 
device when they are connected for BCI test, the enumeration problem is 
solved. I am not sure the case for one with, and one without under FTB test. 
  I think an alternative EMI suppresser to the CM chokes is clip-on ferrite 
cores(big dia or multi-core, with turns) that suits the problem frequencies. 
Low pass filters might also help if your crystal frequency is not too high. 
I have ever seen the EMI filter design with bypass capacitor for USB systems 
provided by ST,  for your reference: 
http://www.st.com/stonline/products/literature/ds/7004.pdf

Regards,
Celine

>From: "Subramanian R" <subramanian1683@xxxxxxxxx>
>Reply-To: subramanian1683@xxxxxxxxx
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] High speed USB Query
>Date: Thu, 27 Jul 2006 15:25:58 +0530
>
>Hello All,
>We are testing Fast Transients (IEC 61000 4-4) through capacitive coupling,
>on USB High speed lines (480Mbps).Requirements like differential impedance
>(90), transient protection have been maintained.
>Results:
>1. Our device re-enumerates during testing. But there is no data 
>corruption.
>But the main processor is not reset. Re-enumeration is not accepted in our
>application.
>2. Using a common mode choke (Recommended in INTEL guidelines for High 
>speed
>USB Design) on the data lines, test was successfull.
>
>But many are saying that it is not advisable to use chokes. Using chokes 
>may
>cause even enumeration problems (i.e when 2 devices are connected, one with
>choke another without, the devices may not enumerate)
>
>Any experience on EMI compliance with USB lines? Anybody has faced
>enumeration problems when multiple devices (with chokes) are connected?
>Please share.
>
>Regards
>Mani
>
>
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