[SI-LIST] Re: Ferrite Beads on Differential Signal Lines?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 11 Sep 2014 19:22:13 -0700
On 9/11/2014 6:53 PM, Chen, Sherman wrote:
> Assuming the two ferrite beads are perfectly matching (anyone has even seen
> such kind of products?) there will be no mode conversion.
There are no such products. You can only get close matching with a
common core, and that's a CM choke. Beads are just not appropriate for
diff pairs except in narrow circumstances where one has a low data rate
and a fast edge rate. But for such situations an X2Y cap works much better.
> Then the implications will be
> 1. reflections.
That's the whole point: mismatch the line to the outside world. The
price is that now memories of energy past can go sailing back towards
the signal source.
> 2. emissions. Theoretically ferrite bead will convert the energy into heat so
> no much left to be emitted. But from the typical freq. curve of a bead, it
> would be likely to emit EM waves at freq. where its impd. is not high.
> So as long as the system can tolerate the attenuation brought by the
> reflection (the src better has a good matching!)
You bet, or bad things can happen.
> and the EMI will not cause problem, using ferrite beads as the means of
> lowering tr should be worth trying? The attraction of doing so is some chip
> vendor only provide fixed tr in their products...
If we are talking about inside a product, the vendors of iron have
suggested such practices for decades and decades using either beads or
more complex integrated filters.
>
> Best Regards,
>
> Sherman Chen
> Signal Integrity
> EMC Global Hardware Engineering
> Tel: +86 21 60951100-3329
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of Tesla
> Sent: Friday, September 12, 2014 9:21 AM
> To: craigleefrancis@xxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Ferrite Beads on Differential Signal Lines?
>
> The Ferrite beads indeed will attenuate some differential energy, increasing
> rising time.
> Tesla Lee.
>
>
>
>
>
>
>
>
>
> At 2014-09-12 07:13:27, "Craig Francis" <craigleefrancis@xxxxxxxxx> wrote:
>> SI-List,
>> I haven't seen any literature concerning using ferrite beads on each
>> signal line of a differential pair.
>> Usually only a common-mode choke is used to reduce common-mode EMI.
>>
>> Can ferrite beads be used on a differential pair to increase the the
>> rise time of the signal and reduce unnecessary high-frequency content
>> that may radiate?
>>
>> The two ferrite beads would be sized to allow the bandwidth necessary
>> for the differential signal's highest data rate.
>>
>>
>> Thanks,
>> Craig Francis
>>
>>
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