[SI-LIST] Re: options for reducing EMI

  • From: Rick Collins <gnuarm.2007@xxxxxxxxx>
  • To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 03 Sep 2014 12:17:33 -0400

I'm not even sure you can call frequency 
spreading to be a band-aid.  It is more like wearing dark glasses.

I've never been convinced that frequency 
spreading is anything more than a trick to fool 
the measuring instruments.  At any given moment 
the radiator is emitting the same strength signal 
it was before the spreading.  The fact that you 
move the frequency around slow enough to still 
work but fast enough to smear out in the 
frequency measurement does not mean it will 
interfere any less with the equipment sensitive to that frequency.

Rick


At 09:30 AM 9/3/2014, Scott McMorrow wrote:
>Chen
>You can certainly do that, but it's a band aid for whatever the actual
>problem is.  You should just not be radiating at 8 GHz.
>
>Regards,
>
>Scott
>
>
>Scott McMorrow
>TeraspeedĀ® Consulting - A Division of Samtec
>16 Stormy Brook Rd
>Falmouth, ME 04105
>(401) 284-1827 Business
>http://www.teraspeed.com
>
>
>On Wed, Sep 3, 2014 at 9:20 AM, Chen, Sherman <sherman.chen@xxxxxxx> wrote:
>
> > Thanks, Steve.
> > The freq. exceeding the EMI margin is 8GHz. Enabling SSC should help on
> > that I think.
> > Any impact & precaution for turning SSC on? Looks there would be no big
> > issue since all our chip vendors claims their products work fairly well
> > with SSC on...
> >
> > 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 steve weir
> > Sent: Tuesday, September 02, 2014 7:04 AM
> > To: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: options for reducing EMI
> >
> > This is another infamous:  "It depends".
> >
> > First:  Will any combination of anticipated measures yield a solution that
> > meets both your SI and EMC requirements?
> > Second: Do you prioritize margin to SI or EMC, or split them?
> >
> > It sounds like you have a design that has failed EMC.  Before twiddling
> > anything that you can look at where you are failing and how badly.
> >
> > TX swing can only reduce noise in the best case dB for dB.  So, if you
> > just need a couple of dB to squeak by and have the link budget, it is an
> > option.
> > SSC can buy up to 10dB on the clock and harmonics.
> > Edge rate could help if you have resonance issues.  Otherwise, it is
> > similar to fiddling with the Tx swing.
> >
> > Steve
> > On 9/1/2014 1:43 PM, Chen, Sherman wrote:
> > > Hi SI Gurus,
> > > We are considering the following options in order to reduce the EMI of
> > one of our boards, which is mainly caused by the PCIe Gen3 link:
> > >
> > > 1.       Reducing TX swing.
> > >
> > > 2.       Turn on SSC.
> > >
> > > 3.       Slowing down edge rate.
> > > While we are running test to verify which option will bring us the best
> > result & least impact, any comments or suggestions from those who had
> > experiences on this matter?
> > >
> > > Best Regards,
> > >
> > > Sherman Chen
> > > Signal Integrity
> > > EMC Global Hardware Engineering
> > > Tel: +86 21 60951100-3329
> > >
> > >
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