[SI-LIST] Re: Remove Ground underneath Differential signal isdeservedor not?
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: yun <yunnan@xxxxxxxxxx>, silist <si-list@xxxxxxxxxxxxx>
- Date: Fri, 09 Aug 2002 09:17:12 -0700
yun,
The traces which we extracted and tested where 30" segments. Remember
that -56 dB is a magnitude of .00158. So, for a 30" line at 10 MHz with
a 1 Volt signal you would see 1.58 mV of noise. This is not a large
amount of crosstalk, except for analog signals. It is doubtful that
there would be crosstalk issues for any digital signal through a plane.
The high frequency crosstalk is significantly lower than this, and
therefore, negligible.
As for the configurations there were several:
5mil traces above and below a ground plane
edge-coupled stripline above a ground plane without a ground shield
edge-coupled stripline above a ground plane with a ground shield.
In all cases, Ansoft Maxwell 2D extractions were an extremely close
match to measurements in system.
regards,
Scott
yun wrote:
> Scott,
>
> Thank you very much for your excellent explain!
>
> I completely agree with you that plane separation makes high frequency
> signal crossalk quite small. However, for high frequency digital
> signal, it¡¯s spectrum occupy from Dc to nearly 1/(3*Tr), within this
> frequency range, the amplitudes are the same for different frequency,
> that is what I concern. From your description, the crosstalk from
> traces driven underneath the ground plane is about -56 dB with 1/2 oz
> copper at 10MHz, then for lower frequency, the crosstalk will be
> larger. At last, the crosstalk will be large enough and can not be
> neglected, and it will influence the integrity of high frequency
> digital signal. Is this thought correct?
>
> Another thing, you have said that¡±Spice simulations showed that the
> crosstalk matched the VNA measurement results within -1 dB across
> several different trace configurations.¡± What are the different
> configurations? And is the horizon distance effect also within -1 dB?
>
> Best regards,
>
> Yun Nan
>
> Senior Engineer
>
> Wuhan Fiberhome Networks
>
> yunnan@xxxxxxxxxx
>
> ------------------------------------------------------------------------
> Yun,
> Thepenetrationofasignalthroughameterialisdescribedbyan
> exponentialfunctionandis:
> e(-alpha*z)theexponentialfunctionof(-alpha*z)
> zisthedistancebelowthesurface.
> wecall1/alphatheskindepth
> where
> delta=1/(sqrt(pi*f*mu*sigma))
> Thisisthepointwherethefieldstrengthhasdropped37percent.But
> fieldstillpenetratesthecopperandisdescribedbytheexponential
> function:
> e**(sqrt(pi*f*mu*sigma))*z
> Asthedistancezincreases,thefieldstrengthfallsoffexponentially,
> anddependsonfrequency.Thefieldthatpenetratestheplaneisquite
> small.Andathighfrequenciesisneglibible.Butforlowfrequency
> analogwork,thepenetrationmaybeenoughtocauseisolationproblems.
> Inmycase,Iwastryingtoisolate10MHzVideosignalsinavideo
> switchfromoneanotherbybetterthan-66dB.
> (-60dBforthecrosstalkfromtwoneighbortraces.)Weusedguard
> tracetoreduceadjacentneighborcrosstalktobelow-70dB,butfound
> thattherewasstillexcessivecrosstalkfromtracesdrivenunderneath
> thegroundplaneofabout-56dBwith1/2ozcopper.)
> Inanalyzingtheproblem,IusedAnsoft2D,whichisafiniteelement
> meshfieldsolver,wherethemeshsizeanderrorenergyforthe
> extractioncanbeusercontrolled.(Atmuchpain,Imightadd).
> Byreducingthemeshsize,usingafinemeshontheplaneandreducing
> theerrorenergyto-70dB,Iwasabletogetanextractionofan
> equivalentcircuitwhichpredictedcrosstalkbetweeentracesseparated
> byaplane.SpicesimulationsshowedthatthecrosstalkmatchedtheVNA
> measurementresultswithin-1dBacrossseveraldifferenttrace
> configurations.
> Sinceourplanethicknesswas1/2oz,wewereabletoincreaseitto1
> ozandsawnearlya-10dBimprovementincrosstalkisolation.
> Thisisnotanimportanteffectforhighfrequencydigitalsignalling.
> However,itmightbeforlowerfrequencyanalogsignals,suchasaudio
> andvideo,andisthereasonwhyaudioplanesareoftenwillisolated
> fromallothers.Inmycase,thetotalvideosignaltracelength
> throughtheswitchwasabout70inches.Itdidn'ttakemuchtodevelop
> -66dBofcrosstalk.
> Youcanfindatreatmentofskindepthandfieldpenetrationinmostany
> undergraduateelectromagneticfieldstextbook.
> regards,
> scott
> --
> ScottMcMorrow
> TeraspeedConsultingGroupLLC
> 2926SEYamhillSt.
> Portland,OR97214
> (503)239-5536
> http://www.teraspeed.com
> yunnanwrote:
> >> Scott,
> >>
> Forwhatyousay:¡°InthesameprojectIalsowassuccessfullyabletoextractwitha2Dfieldsolver,simulatewithHspiceandcorrelatetomeasurements,thecrosstalkisolationbetweentwotracesseperatedbya1/2ozcopperplane.Throughsimulationwewereabletodeterminethatbyusing1ozcopperwecouldincreasethecrosstalkisolationby10dB.¡±youhavementionedthecrosstalkbetweentwotracesseperatedbyaplane,canyoumakeamoredetaildescription?Iamveryinterestedinit.
> >> Bestregards,
> >> YunNan
> >> SeniorEngineer
> >> WuhanFiberhomeNetworks
> >> yunnan@xxxxxxxxxx
> >>
> -------------------------------------------------------------------------------
> >> I'vebeenabletosuccessifullyachievebetterthan-70dBofcrosstalk
> >> isolationbetweenmultipleanalogvideosignalsusingguardtracesfor
> >> over60inchesoftotaltracelength.Theydoworkquitenicelyifvias
> >> areusedtogroundthetraceoverregularintervals.
> >>
> >> NotonlydidIhavetheseresultsinSpicesimulations.Butalsothey
> >> weresubsequentlyconfirmedbyVNAmeasurementsonthetraces,within1
> >> dBofthesimulatedresults.Withouttheguardtraces,thecrosstalk
> >> wasgreaterthan6dBworse.InthesameprojectIalsowas
> >> successfullyabletoextractwitha2Dfieldsolver,simulatewith
> >> Hspiceandcorrelatetomeasurements,thecrosstalkisolationbetween
> >> twotracesseperatedbya1/2ozcopperplane.Throughsimulationwe
> >> wereabletodeterminethatbyusing1ozcopperwecouldincreasethe
> >> crosstalkisolationby10dB.
> >>
> >> Weutilizedguardtracesforseveralhundredsingle-endedandseveral
> >> hundreddifferentialtracesonananalogvideocrosspointswitch.
> >> Withouttheguardtracesandstitchvias,wewouldnothavebeenable
> >> tomeetourend-to-endcrosstalkisolationrequirementof-60dB.
> >>
> >> Bestregards,
> >>
> >> Scott
> >>
> >>
> >>
> ------------------------------------------------------------------------
--
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
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