[SI-LIST] Re: options for reducing EMI

  • From: Ken Wyatt <ken@xxxxxxxxxxxxxxxx>
  • To: ravinder.ajmani@xxxxxxxx
  • Date: Sat, 6 Sep 2014 06:38:01 -0600

Hi Ravi,
You’re not alone in thinking of SSCG as cheating the system, and indeed, it 
does “fool” the spectrum analyzer or EMI receiver by spreading the harmonic 
energy across a wider bandwidth than the receiver/analyzer resolution 
bandwidth. Leaving the military applications out of it (as they have always 
been concerned with system compatibility), remember the original reason why we 
do these tests - to reduce the interference to commercial broadcast radio and 
TVs, mainly due to the early development of PCs starting in the late 1970s. 
Nowadays, of course, the reason has expanded to other communications and 
consumer wireless applications, as well as product to product interference. 
Having said that, I believe if you were to check with the FCC, they’d tell you 
the instance of EMI complaints has decreased substantially since those early 
days.

The article I wrote for Interference Technology Magazine last year on SSCG 
includes a discussion with the lead engineer at Lexmark who developed the first 
commercially available SSCG technology. In the article (and references), he 
discussed the testing and validation of the techniques as it applied to 
real-life broadcast radio and analog TV at that time. Later, extensive testing 
was performed on DTV signals. In all cases, they found the potential for 
interference was less by using SSCG and the evidence was sufficient to convince 
the FCC engineers.

The bottom line is that there have been no significant complaints of consumer 
EMI in years. Sounds to me like we EMC engineers are doing our jobs.

Here’s a link to that article. I believe you’ll find it informative.

http://www.interferencetechnology.com/spread-spectrum-clock-generation-theory-and-debate/

Cheers, Ken

_______________________
Ken Wyatt
Wyatt Technical Services LLC
ken@xxxxxxxxxxxxxxxx
www.emc-seminars.com
Phone: (719) 310-5418

On Sep 3, 2014, at 1:44 PM, Ravinder Ajmani <ravinder.ajmani@xxxxxxxx> wrote:

> I have considered use of Spread Spectrum as cheating.  However as data rates 
> keep on increasing, it is becoming extremely difficult to design cheap 
> hardware what will meet the FCC specs without use of Spread Spectrum.  This 
> is the reason why PCI and SAS allow use of Spread Spectrum clocking.  It is a 
> very effective way of reducing the emissions by 10 dB or more in PCI and SAS 
> applications.
> 
> Regards
> Ravinder Ajmani
> HGST, a Western Digital company
> 5601 Great Oaks Pkwy
> San Jose, CA 95119-1003
> ravinder.ajmani@xxxxxxxx
> 
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
> Behalf Of Rick Collins
> Sent: Wednesday, September 03, 2014 9:18 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: options for reducing EMI
> 
> 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
>> Teraspeed(r) 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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>>> 
>>> 
>>> --
>>> Steve Weir
>>> IPBLOX, LLC
>>> 1580 Grand Point Way
>>> MS 34689
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>>> 
>>> (775) 299-4236 Business
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