[SI-LIST] Re: AC Coupled Signals
- From: lifeatthesharpend <lifeatthesharpend@xxxxxxxxx>
- To: signalintegrity@xxxxxxxxxxx, ron@xxxxxxxxxxx, Chris.Cheng@xxxxxxxx
- Date: Fri, 28 Sep 2007 14:44:02 -0700 (PDT)
What is the total loss at each point? (reciever, driver side of cap, receiv=
er side of cap) in dB =0A=0AIt seems to stand to reason that if you lose xx=
x mV when the signal is stronger at the driver and some smaller portion of =
that when the caps are placed closer to the recever is normal since loss ha=
s ocurred in the media. The total signal loss of the system could still be =
the same. =0A=0ALeonard. =0A=0A=0A=0A----- Original Message ---- =0AFrom: S=
tephen Zinck <signalintegrity@xxxxxxxxxxx> =0ATo: ron@xxxxxxxxxxx; Chris.Ch=
eng@xxxxxxxx =0ACc: si-list@xxxxxxxxxxxxx =0ASent: Friday, September 28, 20=
07 12:19:22 PM =0ASubject: [SI-LIST] Re: AC Coupled Signals =0A=0A=0AHello =
SI-LISTers, =0A=0AI thought for my part in this discussion, I should do som=
e due diligence on =0Athis AC coupling capacitor placement location questio=
n. =0A=0AScott McMorrow, Steve Weir and I had some off-line discussions tha=
t tended =0Ato suggest my position dependency results may have been caused =
by local =0Aresonances from other impedance discontinuities in the system I=
was =0Asimulating. Based on this, I set out to develop a simulation model =
that had =0Aa minimum of discontinuities (no backplane vias/connectors/trac=
e, etc.). =0A=0AI used: =0A=0A- Spice models of non-linear 3.125Gbit/s sili=
con (driver and receiver) =0A- S-parameter based package models for both dr=
iver and receiver. =0A- A 0.01uF capacitor and its associated parasitics (v=
ia, trace, pad, mount, =0Acomponent). =0A- 2D lossy W-Element transmission =
line (with di-electric and skin effect =0Alosses included). =0A=0AI made th=
e capacitor model such that I could "slide" it up and down a 15 =0Ainch tra=
ce between the driver and receiver. I iteratively simulated for the =0Afoll=
owing length combinations: =0A=0A- 500 mil trace from driver to AC coupling=
capacitor with 14500 mil trace to =0Areceiver. =0A- 5000 mil trace from dr=
iver to AC coupling capacitor with 10000 mil trace =0Ato receiver. =0A- 100=
00 mil trace from driver to AC coupling capacitor with 5000 mil trace =0Ato=
receiver. =0A- 14500 mil trace from driver to AC coupling capacitor with 5=
00 mil trace to =0Areceiver. =0A=0AThe results show around 125 mV (differen=
tial) difference between the =0Acapacitor at the source versus the capacito=
r at the destination, with the =0Abenefit going to the capacitor placed clo=
sest to the receiver. 125 mV is a =0Alot to give away... =0A=0AI am not goi=
ng to pretend to understand the physics behind these results but =0AI thoug=
ht it worth while to at least show the basis for my statements. =0A=0AI wou=
ld be happy to evolve the simulation environment if someone has a =0Asugges=
tion... =0A=0AI have put together a document that I can post to an ftp site=
or email if =0Aanyone would like a copy... =0A=0AKind regards, =0ASteve =
=0A=0AStephen P. Zinck =0AInterconnect Engineering Inc. =0AP.O. Box 577 =0A=
South Berwick, ME 03908 =0APhone - (207) 384-8280 =0AEmail - szinck@interco=
nnectengineering.com =0AWeb - www.interconnectengineering.com =0A=0A=0A=0A-=
---- Original Message ----- =0AFrom: "ronald miller" <ron@xxxxxxxxxxx> =0AT=
o: <Chris.Cheng@xxxxxxxx> =0ACc: <si-list@xxxxxxxxxxxxx> =0ASent: Tuesday, =
September 25, 2007 6:34 PM =0ASubject: [SI-LIST] Re: AC Coupled Signals =0A=
=0A=0A> Chris =0A> If your coupling cap is a problem, get a better cap and =
design the pads =0A> to have no reflection. =0A> =0A> If there are no refle=
ctions at the lowest data rate and at the highest =0A> data rate, the posit=
ion does =0A> not matter. =0A> =0A> Now, about the S11 and S22, it is much =
more intuitive and much easier to =0A> deal with TDR and =0A> reflection co=
efficients, or impedance than it is to deal with the =0A> network analyzer =
data. =0A> =0A> Although I am a microwave engineer, I have learned the hard=
way, and now =0A> I try to dtay away from =0A> the S-Parameters as models =
and for analysis because they are clumsy and =0A> non-intuitive. =0A> =0A> =
Ron =0A> =0A> Chris Cheng wrote: =0A> =0A>>Let me try my hand on why positi=
on matter. =0A>> =0A>>A normal passive channel is reciprocal. e.g. S12=3D3D=
S21 It only says the =3D =0A>>off diagonal elements are symmetic. It doesn'=
t say the diagonal elements =3D =0A>>have to be equal. I believe this was t=
he basis of Jeff Loyer's =3D =0A>>discussion a while ago. =0A>> =0A>>The pr=
esence of the discontinuity affects the S11 and S22 dramatically =3D =0A>>d=
ifferent based on whether it is close to the Tx or Rx. =0A>> =0A>>In the pr=
esences of imperfect loading on the Rx side, it is the =3D =0A>>interaction=
between the S22 and loading that matters.=3D20 =0A>> =0A>>Thus position ma=
kes a difference. i.e. we are tuning the S22 with the =3D =0A>>non-ideal lo=
ading. =0A>> =0A>>QED =0A>> =0A>>-----Original Message----- =0A>>From: si-l=
ist-bounce@xxxxxxxxxxxxx =0A>>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behal=
f Of steve weir =0A>>Sent: Tuesday, September 25, 2007 1:35 PM =0A>>To: Jor=
y McKinley =0A>>Cc: Stephen Zinck; Scott McMorrow; leeritchey@xxxxxxxxxxxxx=
; =0A>>npatel@xxxxxxxxxx; si-list@xxxxxxxxxxxxx =0A>>Subject: [SI-LIST] Re:=
AC Coupled Signals =0A>> =0A>> =0A>>Jory, I think this is good example of =
where intuitively appealing=3D20 =0A>>misconceptions can seduce one into tr=
anslating correlation into=3D20 =0A>>causation. If you have more ringing in=
one case than another, it means=3D20 =0A>>that you have set up a resonance=
that is more severe in the one case. =3D20 =0A>>This can easily happen as =
a result of any number of things going on: =3D20 =0A>>suboptimal silicon to=
package launch, suboptimal IC to PCB, via stubs,=3D20 =0A>>connector trans=
itions, etc, etc. =0A>> =0A>>The very simple test is to take a VNA, a coupl=
e of sections of coax and=3D20 =0A>>a DC block. Move the DC block between t=
he transmit end, the junction of =3D =0A>> =0A>>the two cables, and the rec=
eiver and look at the behavior of that net=3D20 =0A>>channel. With good coa=
x and connectors the channel performance will=3D20 =0A>>change almost immea=
sureably. Now go and add a coax T on one side of the =3D =0A>> =0A>>DC bloc=
k. Move that whole thing around as a unit and again the channel=3D20 =0A>>p=
erformance remains the same. Add a second coax T on the other side of=3D20 =
=0A>>the DC block from the first, and again move the whole thing around. Th=
e =3D =0A>> =0A>>results will still remain uniform. Now if you go and move =
one of those =3D =0A>> =0A>>T's someplace else, then the pesky mole you're =
trying to whack moves and =3D =0A>> =0A>>the resonance will pop up somewher=
e else. The bottom line is that it's=3D20 =0A>>resonance that we need to fi=
ght and resonance doesn't know left from =3D =0A>>right. =0A>> =0A>>Regards=
, =0A>> =0A>> =0A>>Steve. =0A>>Jory McKinley wrote: =0A>> =0A>> =0A>>>To ad=
d to this, I will ask for release of lab data that I took that=3D20 =0A>>>s=
hows RX_EYE clearly improves as the AC cap/term location is moved=3D20 =0A>=
>>closer to the RX. The data indicates that even though overall channel =3D=
=0A>>> =0A>>> =0A>> =0A>> =0A>> =0A>>>loss may not be affected, the 50ps e=
dge rates we are sending through=3D20 =0A>>>the channel are affected (in te=
rms of time domain ringing) by the AC=3D20 =0A>>>cap/term placement. This k=
ind of feels right. =0A>>>-Jory =0A>>>=3D20 =0A>>> =0A>>>----- Original Mes=
sage ---- =0A>>>From: steve weir <weirsi@xxxxxxxxxx> =0A>>>To: Stephen Zinc=
k <signalintegrity@xxxxxxxxxxx> =0A>>>Cc: Scott McMorrow <scott@xxxxxxxxxxx=
om>; jory_mckinley@xxxxxxxxx;=3D20 =0A>>>leeritchey@xxxxxxxxxxxxx; npatel@m=
icron.com; si-list@xxxxxxxxxxxxx =0A>>>Sent: Tuesday, September 25, 2007 12=
:52:41 PM =0A>>>Subject: Re: [SI-LIST] Re: AC Coupled Signals =0A>>> =0A>>>=
Steve, as far as I know where we have agreement that capacitor =3D =0A>>> =
=0A>>> =0A>>location =0A>> =0A>> =0A>>>can only affect performance where th=
e combined capacitor and mount =0A>>>presents a discontinuity and that disc=
ontinuity is located such that =3D =0A>>> =0A>>> =0A>>it =0A>> =0A>> =0A>>>=
forms a resonant structure with another discontinuity in the channel. =3D =
=0A>>> =0A>>> =0A>>I =0A>> =0A>> =0A>>>fail to see where we have moved any =
closer to supporting your premise =0A>>>that locating a greater proportion =
of fixed loss before the capacitor =0A>>>changes end to end loss than placi=
ng that same fixed loss behind it. =0A>>> =0A>>>As for lab measurements, we=
have these as we have characterized many =0A>>>links. We also have extensi=
ve simulations. =0A>>> =0A>>>Regards, =0A>>> =0A>>> =0A>>>Steve. =0A>>> =0A=
>>>Stephen Zinck wrote: =0A>>> =0A>>> =0A>>>>Hi Steve, =0A>>>> =0A>>>>I und=
erstand your point but I actually thought Scott and I were =0A>>>>getting c=
lose. I guess I still need him to explain his statement: =0A>>>>"The only t=
ime position matters is in the face of discontinuities." =0A>>>>because thi=
s runs counter to your assertion. =0A>>>> =0A>>>>It would be good to have s=
ome concrete lab measurement results to =3D =0A>>>> =0A>>>> =0A>>back =0A>>=
=0A>> =0A>>>>either of our points up. I am sorry I don't have any. =0A>>>>=
=0A>>>>We agree on TDR/VNA characteristics... =0A>>>> =0A>>>>Steve =0A>>>>=
=0A>>>>Stephen P. Zinck =0A>>>>Interconnect Engineering Inc. =0A>>>>P.O. B=
ox 577 =0A>>>>South Berwick, ME 03908 =0A>>>>Phone - (207) 384-8280 =0A>>>>=
Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx =0A>>>>Web - www.interconnecteng=
ineering.com=3D20 =0A>>>> =0A>>>> =0A>>><http://www.interconnectengineering=
.com> =0A>>> =0A>>> =0A>>>>----- Original Message ----- From: "steve weir" =
<weirsi@xxxxxxxxxx> =0A>>>>To: <signalintegrity@xxxxxxxxxxx> =0A>>>>Cc: "Sc=
ott McMorrow" <scott@xxxxxxxxxxxxx>; =3D =0A>>>> =0A>>>> =0A>><jory_mckinle=
y@xxxxxxxxx>; =0A>> =0A>> =0A>>>><leeritchey@xxxxxxxxxxxxx>; <npatel@micron=
.com>; =3D =0A>>>> =0A>>>> =0A>><si-list@xxxxxxxxxxxxx> =0A>> =0A>> =0A>>>>=
Sent: Tuesday, September 25, 2007 12:24 PM =0A>>>>Subject: [SI-LIST] Re: AC=
Coupled Signals =0A>>>> =0A>>>> =0A>>>> =0A>>>> =0A>>>>>Stephen, OK so whe=
n you say "lossy" or "nonlinear" you mean=3D3D20 =0A>>>>>discontinuous. Dis=
continuities aggravate resonances based on =0A>>>>>specific=3D3D20 =0A>>>>>=
structure material and geometries, in other words the distance on =3D =0A>>=
>>> =0A>>>>> =0A>>a=3D3D20 =0A>> =0A>> =0A>>>>>centimeter or millimeter sca=
le between discontinuities. We =3D =0A>>>>> =0A>>>>> =0A>>have=3D3D20 =0A>>=
=0A>> =0A>>>>>essentially the same opportunities for channel discontinuiti=
es at =3D =0A>>>>> =0A>>>>> =0A>>and =0A>> =0A>> =0A>>>>>in =3D3D =0A>>>>> =
=0A>>>>>the vicinity of the transmitter as the receiver. So I still do not =
=0A>>>>>see=3D3D20 =0A>>>>>a defensible basis for the offered position: tha=
t placing a =3D =0A>>>>> =0A>>>>> =0A>>capacitor =0A>> =0A>> =0A>>>>>at =3D=
3D =0A>>>>> =0A>>>>>one end of the line versus the other changes the end to=
end loss. =3D20 =0A>>>>>What=3D3D20 =0A>>>>>matters is if wherever I place=
one discontinuity that it sets up a =0A>>>>>sharp =3D3D =0A>>>>> =0A>>>>>r=
esonance with another discontinuity. That can happen equally well =0A>>>>>a=
t=3D3D20 =0A>>>>>either end of the line. =0A>>>>> =0A>>>>>If one looks at a=
channel with only a TDR I might understand =3D =0A>>>>> =0A>>>>> =0A>>the=
=3D3D20 =0A>> =0A>> =0A>>>>>erroneous perception that placing a discontinui=
ty down the line =3D =0A>>>>> =0A>>>>> =0A>>is=3D3D20 =0A>> =0A>> =0A>>>>>b=
etter than up front. But that is an illusion. TDR resolution=3D20 =0A>>>>> =
=0A>>>>> =0A>>>falls=3D3D20 =0A>>> =0A>>> =0A>>>>>with interconnect distanc=
e. This ia a result of the inherent loss=3D20 =0A>>>>> =0A>>>>> =0A>>>of=3D=
3D20 =0A>>> =0A>>> =0A>>>>>the interconnect that shelves bandwidth and henc=
e resolution =3D =0A>>>>> =0A>>>>> =0A>>versus=3D3D20 =0A>> =0A>> =0A>>>>>d=
istance for the instrument. This is one of the big limitations of =3D =0A>>=
>>> =0A>>>>> =0A>> =0A>> =0A>> =0A>>>a=3D3D20 =0A>>> =0A>>> =0A>>>>>TDR for=
channel evaluation. A through measurement with a TDT or =3D =0A>>>>> =0A>>=
>>> =0A>>VNA=3D3D20 =0A>> =0A>> =0A>>>>>does not suffer that limitation, gi=
ve true measure of S21 and so =0A>>>>>report=3D3D20 =0A>>>>>the real channe=
l performance. Eric Bogatin spends some time on =3D =0A>>>>> =0A>>>>> =0A>>=
the=3D3D20 =0A>> =0A>> =0A>>>>>issue of bandwidth versus interconnect lengt=
h in his book. =0A>>>>> =0A>>>>>Regards, =0A>>>>> =0A>>>>> =0A>>>>>Steve. =
=0A>>>>>Stephen Zinck wrote: =0A>>>>> =0A>>>>> =0A>>>>>>Scott, =0A>>>>>>We =
may have some nomenclature issues here...=3D3D20 =0A>>>>>> =0A>>>>>>When I =
say "lossy interface to the capacitor" I mean with =3D =0A>>>>>> =0A>>>>>> =
=0A>>impedance =0A>> =0A>> =0A>>>>>>dis=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>co=
ntinuities. So I think we are on a similar page given your =3D =0A>>>>> =0A=
>>>>> =0A>>statement: =0A>> =0A>> =0A>>>>>>"The only time position matters =
is in the face of =3D =0A>>>>>> =0A>>>>>> =0A>>discontinuities." =0A>> =0A>=
> =0A>>>>>>Again, most often, my role is to simulate the customers system a=
t =0A>>>>>>the 1=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>1th hour. I don't recomme=
nd this, I just work within the customer's =0A>>>>>needs=3D3D =0A>>>>>/requ=
irements. I make real world recommendations from simulation =0A>>>>>results=
=3D3D =0A>>>>>for designs where these discontinuities you mention are a fa=
ct of =0A>>>>>life. G=3D3D =0A>>>>>ranted my customers are not doing 5+ Gbi=
t/s designs (right now ;-). =0A>>>>> =0A>>>>> =0A>>>>>>Above these data-rat=
es, all you mention, capacitor transition =3D =0A>>>>>> =0A>>>>>> =0A>>(pad=
, =0A>> =0A>> =0A>>>>>>via=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>s, etc) are of =
the utmost importance. And I would absolutely agree =0A>>>>>that t=3D3D =0A=
>>>>>he more perfect you make these transitions, the less it matters =3D =
=0A>>>>> =0A>>>>> =0A>>where =0A>> =0A>> =0A>>>>>the=3D3D =0A>>>>>y are pla=
ced... =0A>>>>> =0A>>>>> =0A>>>>>>So I do believe AC coupling capacitor pos=
ition does matter, as you =0A>>>>>>stat=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>e,=
for the bulk of the designs occurring these days where component =0A>>>>>f=
ootp=3D3D =0A>>>>>rint and via optimization, etc. is NOT occurring... =0A>>=
>>> =0A>>>>> =0A>>>>>>Steve =0A>>>>>> =0A>>>>>>Stephen P. Zinck =0A>>>>>>In=
terconnect Engineering Inc. =0A>>>>>>P.O. Box 577 =0A>>>>>>South Berwick, M=
E 03908 =0A>>>>>>Phone - (207) 384-8280 =0A>>>>>>Email - szinck@interconnec=
tengineering.com =0A>>>>>>Web - www.interconnectengineering.com=3D20 =0A>>>=
>>> =0A>>>>>> =0A>>><http://www.interconnectengineering.com> =0A>>> =0A>>> =
=0A>>>>>> ----- Original Message -----=3D3D20 =0A>>>>>> From: Scott McMorro=
w=3D3D20 =0A>>>>>> To: Stephen Zinck=3D3D20 =0A>>>>>> Cc: jory_mckinley@yah=
oo.com ; leeritchey@xxxxxxxxxxxxx ; =0A>>>>>>npatel@micro=3D3D =0A>>>>>> =
=0A>>>>>> =0A>>>>>n.com ; si-list@xxxxxxxxxxxxx=3D3D20 =0A>>>>> =0A>>>>> =
=0A>>>>>> Sent: Tuesday, September 25, 2007 11:08 AM =0A>>>>>> Subject: Re:=
[SI-LIST] Re: AC Coupled Signals =0A>>>>>> =0A>>>>>> =0A>>>>>> Steven, =0A=
>>>>>> =0A>>>>>> I would not agree with your following statements. =0A>>>>>=
> =0A>>>>>> "I agree in theory with all you state. Assuming a lossless =0A>=
>>>>>interface =3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>to the capacitor, it shoul=
dn't matter where you place it, given a =0A>>>>>purely =3D3D =0A>>>>>linear=
system. But the real world is lossy, even when one makes =3D =0A>>>>> =0A>=
>>>> =0A>>great =0A>> =0A>> =0A>>>>>3D =3D3D =0A>>>>>solved structures. Man=
ufacturing and other tolerances tend to take =0A>>>>>the tr=3D3D =0A>>>>>ek=
towards perfection to task." =0A>>>>> =0A>>>>> =0A>>>>>> "Would either of =
you agree that AC coupling capacitor location =3D =0A>>>>>> =0A>>>>>> =0A>>=
may =0A>> =0A>> =0A>>>>>>ma=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>tter with a lo=
ssy interface to the capacitor?" =0A>>>>> =0A>>>>> =0A>>>>>> Insertion loss=
in a flat impedance linear lossy system will be =0A>>>>>>indepe=3D3D =0A>>=
>>>> =0A>>>>>> =0A>>>>>ndent of capacitor location. Run the math and see. T=
he only time =0A>>>>>positi=3D3D =0A>>>>>on matters is in the face of disco=
ntinuities. In fact, given a =3D =0A>>>>> =0A>>>>> =0A>>low =0A>> =0A>> =0A=
>>>>>loss=3D3D =0A>>>>>interconnect with discontinuities and a high loss in=
terconnect with =0A>>>>>disc=3D3D =0A>>>>>ontinuities, the low loss system,=
with it's higher Q, will often =3D =0A>>>>> =0A>>>>> =0A>>have =0A>> =0A>>=
=0A>>>>>wor=3D3D =0A>>>>>se behavior. =0A>>>>> =0A>>>>> =0A>>>>>> An impro=
perly designed 0402 capacitor transition for a 50 ohm =3D =0A>>>>>> =0A>>>>=
>> =0A>>line =0A>> =0A>> =0A>>>>>>ca=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>n eas=
ily exhibit a discontinuity of 35 ohms for 50 ps. If attached =0A>>>>>to po=
=3D3D =0A>>>>>orly designed via transitions, the discontinuity will be even=
=3D =0A>>>>> =0A>>>>> =0A>>worse. =3D20 =0A>> =0A>> =0A>>>>>Whe=3D3D =0A>>=
>>>n this is coupled closely to a high capacitance receiver input, a =0A>>>=
>>high ca=3D3D =0A>>>>>pacitance transmitter output, a low impedance via st=
ub =3D =0A>>>>> =0A>>>>> =0A>>discontinuity, =0A>> =0A>> =0A>>>>>or =3D3D =
=0A>>>>>a low impedance connector discontinuity, it can form a 1/2 wave =0A=
>>>>>resonant =3D3D =0A>>>>>circuit. This is most likely the problem you ar=
e seeing. =3D3D20 =0A>>>>> =0A>>>>> =0A>>>>>> If the interconnect has essen=
tially flat impedance, position =3D =0A>>>>>> =0A>>>>>> =0A>>does =0A>> =0A=
>> =0A>>>>>>not=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>matter. If the capacitor t=
ransition is properly designed, position =0A>>>>>does=3D3D =0A>>>>>not matt=
er. All of the data we have on this is proprietary at this =0A>>>>>time=3D3=
D =0A>>>>>=3D3D2E Our understanding of the physics has been verified by ful=
l =3D =0A>>>>> =0A>>>>> =0A>>wave =0A>> =0A>> =0A>>>>>mode=3D3D =0A>>>>>lin=
g, simulation and measurement. =0A>>>>> =0A>>>>> =0A>>>>>> =0A>>>>>> regard=
s, =0A>>>>>> =0A>>>>>> Scott =0A>>>>>> =0A>>>>>> =0A>>>>>> =0A>>>>>>Scott M=
cMorrow =0A>>>>>>Teraspeed Consulting Group LLC =0A>>>>>>121 North River Dr=
ive =0A>>>>>>Narragansett, RI 02882 =0A>>>>>>(401) 284-1827 Business =0A>>>=
>>>(401) 284-1840 Fax =0A>>>>>> =0A>>>>>>http://www.teraspeed.com =0A>>>>>>=
=0A>>>>>>Teraspeed=3D3DAE is the registered service mark of =0A>>>>>>Teras=
peed Consulting Group LLC =0A>>>>>> =0A>>>>>> =0A>>>>>> Stephen Zinck wrote=
:=3D3D20 =0A>>>>>> Hi Scott and Steve, =0A>>>>>> =0A>>>>>> To answer both o=
f your questions, it is the resulting Hspice =0A>>>>>>(with =3D3D =0A>>>>>>=
=0A>>>>>> =0A>>>>>S-parameters) differential eye patterns, as viewed at th=
e receiver =0A>>>>>die, t=3D3D =0A>>>>>hat were used to make a comparison o=
f source versus destination AC =0A>>>>>coupli=3D3D =0A>>>>>ng capacitor loc=
ations. The system was excited with a string of =3D =0A>>>>> =0A>>>>> =0A>>=
ones, =0A>> =0A>> =0A>>>>>fol=3D3D =0A>>>>>lowed by a single zero, followed=
by a string of ones.=3D3D20 =0A>>>>> =0A>>>>> =0A>>>>>> I have not specifi=
cally designed a test board that varies the =3D =0A>>>>>> =0A>>>>>> =0A>>AC=
=0A>> =0A>> =0A>>>>>>co=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>upling capacitor =
location along a trace. =0A>>>>> =0A>>>>> =0A>>>>>> I understand the "shade=
s of gray" here and agree that one =3D =0A>>>>>> =0A>>>>>> =0A>>can't =0A>>=
=0A>> =0A>>>>>>mak=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>e a "rule of thumb" ge=
neralization in our line of work these =3D =0A>>>>> =0A>>>>> =0A>>days.=3D3=
D20 =0A>> =0A>> =0A>>>>>> I agree in theory with all you state. Assuming a =
lossless =0A>>>>>>interface=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>to the capacit=
or, it shouldn't matter where you place it, given a =0A>>>>>purely=3D3D =0A=
>>>>>linear system. But the real world is lossy, even when one makes =3D =
=0A>>>>> =0A>>>>> =0A>>great =0A>> =0A>> =0A>>>>>3D=3D3D =0A>>>>>solved str=
uctures. Manufacturing and other tolerances tend to take =0A>>>>>the t=3D3D=
=0A>>>>>rek towards perfection to task.=3D3D20 =0A>>>>> =0A>>>>> =0A>>>>>>=
Do either of you have real world measured results, that you =0A>>>>>>could=
s=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>hare, that show no marked difference in=
received signal =0A>>>>>characteristics w=3D3D =0A>>>>>hen the AC coupling=
capacitor position is varied through a 30 inch =0A>>>>>backpl=3D3D =0A>>>>=
>ane system (or similar)? =0A>>>>> =0A>>>>> =0A>>>>>> I believe my experien=
ce with capacitor location may prove true =0A>>>>>>if t=3D3D =0A>>>>>> =0A>=
>>>>> =0A>>>>>he capacitor interface is lossy (which is the case). A lot of=
my =0A>>>>>customer=3D3D =0A>>>>>s are just looking for quick ways to maxi=
mize performance using =0A>>>>>standard =3D3D =0A>>>>>component packages an=
d standard layout practices (in the end, I =3D =0A>>>>> =0A>>>>> =0A>>don't=
=0A>> =0A>> =0A>>>>>lik=3D3D =0A>>>>>e to give anything away that is low l=
ying fruit). Most of the time =3D =0A>>>>> =0A>>>>> =0A>>I =0A>> =0A>> =0A>=
>>>>am d=3D3D =0A>>>>>oing my analysis after the board is in layout, where =
I have limited =0A>>>>>abili=3D3D =0A>>>>>ty to change the design (unless i=
t is really broken). In a perfect =0A>>>>>world,=3D3D =0A>>>>>where I am in=
volved early, the package optimization and layout =0A>>>>>structure=3D3D =
=0A>>>>>s can be optimized as you state, but only if the margins warrant it=
=0A>>>>>(syst=3D3D =0A>>>>>em performance issues are expected after initia=
l "what-if" =0A>>>>>simulations ha=3D3D =0A>>>>>ve occurred). The right too=
l for the right job rules the day... =0A>>>>> =0A>>>>> =0A>>>>>> Would eith=
er of you agree that AC coupling capacitor location =0A>>>>>>may m=3D3D =0A=
>>>>>> =0A>>>>>> =0A>>>>>atter with a lossy interface to the capacitor? =0A=
>>>>> =0A>>>>> =0A>>>>>> All the best, =0A>>>>>> Steve =0A>>>>>> =0A>>>>>> =
Stephen P. Zinck =0A>>>>>> Interconnect Engineering Inc. =0A>>>>>> P.O. Box=
577 =0A>>>>>> South Berwick, ME 03908 =0A>>>>>> Phone - (207) 384-8280 =0A=
>>>>>> Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx =0A>>>>>> Web - www.inter=
connectengineering.com=3D20 =0A>>>>>> =0A>>>>>> =0A>>><http://www.interconn=
ectengineering.com> =0A>>> =0A>>> =0A>>>>>> ----- Original Message -----=3D=
3D20 =0A>>>>>> From: Scott McMorrow=3D3D20 =0A>>>>>> To: Stephen Zinck=3D3D=
20 =0A>>>>>> Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ; =0A>>=
>>>>npatel@m=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>icron.com ; si-list@freelists=
.org=3D3D20 =0A>>>>> =0A>>>>> =0A>>>>>> Sent: Tuesday, September 25, 2007 9=
:44 AM =0A>>>>>> Subject: Re: [SI-LIST] Re: AC Coupled Signals =0A>>>>>> =
=0A>>>>>> =0A>>>>>> Stephen =0A>>>>>> =0A>>>>>> Define "better" and then re=
late your simulations and =0A>>>>>>conclusions =3D3D =0A>>>>>> =0A>>>>>> =
=0A>>>>>to linear system theory and measurements. =3D3D20 =0A>>>>> =0A>>>>>=
=0A>>>>>> I contend that the only difference an AC coupling capacitor =0A>=
>>>>>can p=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>ossibly have due to position in=
a linear interconnect is a result =3D =0A>>>>> =0A>>>>> =0A>>of =0A>> =0A>=
> =0A>>>>>impe=3D3D =0A>>>>>dance mismatch. I contend that the capacitor wi=
ll form a 1/2 wave =0A>>>>>resona=3D3D =0A>>>>>nt circuit with other interc=
onnect discontinuities (connectors, =3D =0A>>>>> =0A>>>>> =0A>>vias =0A>> =
=0A>> =0A>>>>>stub=3D3D =0A>>>>>s, packages, Tx die, Rx die ... etc) and th=
at this interaction is =0A>>>>>system,=3D3D =0A>>>>>chip, connector and pac=
kage design dependent. I contend that it is =0A>>>>>this=3D3D =0A>>>>>1/2 r=
esonance that can cause differences that can be measured, but =0A>>>>>that =
=3D3D =0A>>>>>there is no "rule of thumb", since the position and magnitude=
of =0A>>>>>disconti=3D3D =0A>>>>>nuities are different in every system. In=
some systems the =3D =0A>>>>> =0A>>>>> =0A>>receiver =0A>> =0A>> =0A>>>>>c=
ons=3D3D =0A>>>>>titutes a larger discontinuity than the transmitter. In ot=
her =0A>>>>>systems th=3D3D =0A>>>>>is is reversed. In yet other systems, c=
onnectors and vias =3D =0A>>>>> =0A>>>>> =0A>>represent =0A>> =0A>> =0A>>>>=
>larg=3D3D =0A>>>>>er discontinuites than do either the transmitters or rec=
eivers. It =0A>>>>>all "j=3D3D =0A>>>>>ust depends". To state a specific ru=
le is just plain incorrect. =0A>>>>> =0A>>>>> =0A>>>>>> I contend that once=
you remove the magic and myths =3D =0A>>>>>> =0A>>>>>> =0A>>surrounding =
=0A>> =0A>> =0A>>>>>>AC=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>coupling capacitor=
s, analysis of the 3D structure shows that by =0A>>>>>reducing=3D3D =0A>>>>=
>the signal path discontinuity through the capacitor, you will =0A>>>>>nece=
ssaril=3D3D =0A>>>>>y improve performance. An AC coupling capacitor, with i=
t's =0A>>>>>associated vi=3D3D =0A>>>>>a and pad transition design, can be =
viewed as a black box which has =0A>>>>>inser=3D3D =0A>>>>>tion loss and re=
turn loss, and can be modeled quite well using =3D =0A>>>>> =0A>>>>> =0A>>e=
ither =0A>> =0A>> =0A>>>>>lum=3D3D =0A>>>>>ped element approximations or (m=
y favorite) S-parameters. As such =3D =0A>>>>> =0A>>>>> =0A>>it =0A>> =0A>>=
=0A>>>>>wil=3D3D =0A>>>>>l cascade in a simulation model just like any oth=
er linear element. =3D =0A>>>>> =0A>>>>> =0A>>=3D20 =0A>> =0A>> =0A>>>>>If =
w=3D3D =0A>>>>>e start with a system with flat 50 ohm impedance from end to=
end, =3D =0A>>>>> =0A>>>>> =0A>>it =0A>> =0A>> =0A>>>>>can =3D3D =0A>>>>>b=
e easily shown that no matter what the position of the capacitor =0A>>>>>al=
ong t=3D3D =0A>>>>>he interconnect is, the insertion loss of the system is =
identical. =3D =0A>>>>> =0A>>>>> =0A>> =0A>> =0A>> =0A>>>>>It is=3D3D =0A>>=
>>>only the return loss, as seen from each end that changes. =0A>>>>> =0A>>=
>>> =0A>>>>>> I've been designing AC coupling capacitor mounting =3D =0A>>>=
>>> =0A>>>>>> =0A>>transitions =0A>> =0A>> =0A>>>>>>pr=3D3D =0A>>>>>> =0A>>=
>>>> =0A>>>>>operly for quite a few years now and have some 0402 designs th=
at =3D =0A>>>>> =0A>>>>> =0A>>keep =0A>> =0A>> =0A>>>>>S12=3D3D =0A>>>>>abo=
ve -0.2 dB up to 7.5 GHz, S12 below -20 dB @ 5 GHz, and below =3D =0A>>>>> =
=0A>>>>> =0A>>-15 =0A>> =0A>> =0A>>>>>dB =3D3D =0A>>>>>@ 10 GHz. For all pr=
actical purposes, these designs are =3D =0A>>>>> =0A>>>>> =0A>>transparent =
=0A>> =0A>> =0A>>>>>and =3D3D =0A>>>>>may be placed anywhere in an intercon=
nect design where there is =0A>>>>>space, si=3D3D =0A>>>>>nce there is litt=
le resonance interaction with other devices and =0A>>>>>structur=3D3D =0A>>=
>>>es. =0A>>>>> =0A>>>>> =0A>>>>>> Scott =0A>>>>>> =0A>>>>>> =0A>>>>>>Scott=
McMorrow =0A>>>>>>Teraspeed Consulting Group LLC =0A>>>>>>121 North River =
Drive =0A>>>>>>Narragansett, RI 02882 =0A>>>>>>(401) 284-1827 Business =0A>=
>>>>>(401) 284-1840 Fax =0A>>>>>> =0A>>>>>>http://www.teraspeed.com =0A>>>>=
>> =0A>>>>>>Teraspeed=3D3DAE is the registered service mark of =0A>>>>>>Ter=
aspeed Consulting Group LLC =0A>>>>>> =3D3D20 =0A>>>>>> =0A>>>>>> Stephen Z=
inck wrote:=3D3D20 =0A>>>>>> Hi Scott, =0A>>>>>> =0A>>>>>> My simulations s=
how that the capacitor is best placed at =3D =0A>>>>>> =0A>>>>>> =0A>>the =
=0A>> =0A>> =0A>>>>>>re=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>ceiver end of the =
transmission-line. Do you disagree? If so, why? =0A>>>>> =0A>>>>> =0A>>>>>>=
Steve =0A>>>>>> =0A>>>>>> Stephen P. Zinck =0A>>>>>> Interconnect Engineer=
ing Inc. =0A>>>>>> P.O. Box 577 =0A>>>>>> South Berwick, ME 03908 =0A>>>>>>=
Phone - (207) 384-8280 =0A>>>>>> Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxx=
m =0A>>>>>> Web - www.interconnectengineering.com=3D20 =0A>>>>>> =0A>>>>>> =
=0A>>><http://www.interconnectengineering.com> =0A>>> =0A>>> =0A>>>>>> ----=
- Original Message -----=3D3D20 =0A>>>>>> From: Scott McMorrow=3D3D20 =0A>>=
>>>> To: signalintegrity@xxxxxxxxxxx=3D3D20 =0A>>>>>> Cc: jory_mckinley@yah=
oo.com ; leeritchey@xxxxxxxxxxxxx ; =0A>>>>>>npat=3D3D =0A>>>>>> =0A>>>>>> =
=0A>>>>>el@xxxxxxxxxx ; si-list@xxxxxxxxxxxxx=3D3D20 =0A>>>>> =0A>>>>> =0A>=
>>>>> Sent: Tuesday, September 25, 2007 8:30 AM =0A>>>>>> Subject: Re: [SI-=
LIST] Re: AC Coupled Signals =0A>>>>>> =0A>>>>>> =0A>>>>>> Stephen, =0A>>>>=
>> =0A>>>>>> I'm sorry, this is a linear system. Except for possible =0A>>>=
>>>reso=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>nances that are created by discont=
inuities and modal conversion =0A>>>>>(which ha=3D3D =0A>>>>>ve absolutely =
zero to do with signal rise time), there is no =0A>>>>>difference i=3D3D =
=0A>>>>>n the attenuation of a capacitor placed at the Tx as opposed at =3D=
=0A>>>>> =0A>>>>> =0A>>the =0A>> =0A>> =0A>>>>>Rx. =3D3D =0A>>>>>W.R.T. th=
e receiver, if it is "lost in the rise-time degradation of =0A>>>>>the =3D3=
D =0A>>>>>system", it will be lost wherever it is placed. =0A>>>>> =0A>>>>>=
=0A>>>>>> =0A>>>>>>Scott McMorrow =0A>>>>>>Teraspeed Consulting Group LLC =
=0A>>>>>>121 North River Drive =0A>>>>>>Narragansett, RI 02882 =0A>>>>>>(40=
1) 284-1827 Business =0A>>>>>>(401) 284-1840 Fax =0A>>>>>> =0A>>>>>>http://=
www.teraspeed.com =0A>>>>>> =0A>>>>>>Teraspeed=3D3DAE is the registered ser=
vice mark of =0A>>>>>>Teraspeed Consulting Group LLC =0A>>>>>> =3D3D20 =0A>=
>>>>> =0A>>>>>> Stephen Zinck wrote:=3D3D20 =0A>>>>>>Hi Jory, =0A>>>>>> =0A=
>>>>>>I have simulated this at length and concur with your experience =3D =
=0A>>>>>> =0A>>>>>> =0A>>that =0A>> =0A>> =0A>>>>>>th=3D3D =0A>>>>>> =0A>>>=
>>> =0A>>>>>e=3D3D20 =0A>>>>> =0A>>>>> =0A>>>>>>capacitor is best placed at=
the receiver... =0A>>>>>> =0A>>>>>>In effect, the attenuation associated w=
ith the capacitor placement =0A>>>>>>at t=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>=
he=3D3D20 =0A>>>>> =0A>>>>> =0A>>>>>>receiver (parasitics/pads/vias) is los=
t in the rise-time =3D =0A>>>>>> =0A>>>>>> =0A>>degradation =0A>> =0A>> =0A=
>>>>>>of=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>the=3D3D20 =0A>>>>> =0A>>>>> =0A>=
>>>>>system. =0A>>>>>>The classic "don't break it until you have to" rule i=
s =3D =0A>>>>>> =0A>>>>>> =0A>>applicable... =0A>> =0A>> =0A>>>>>>OK=3D3D =
=0A>>>>>> =0A>>>>>> =0A>>>>>this=3D3D20 =0A>>>>> =0A>>>>> =0A>>>>>>is my ru=
le... :-) =0A>>>>>> =0A>>>>>>All the best, =0A>>>>>>Steve =0A>>>>>> =0A>>>>=
>>Stephen P. Zinck =0A>>>>>>Interconnect Engineering Inc. =0A>>>>>>P.O. Box=
577 =0A>>>>>>South Berwick, ME 03908 =0A>>>>>>Phone - (207) 384-8280 =0A>>=
>>>>Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx =0A>>>>>>Web - www.interconn=
ectengineering.com=3D20 =0A>>>>>> =0A>>>>>> =0A>>><http://www.interconnecte=
ngineering.com> =0A>>> =0A>>> =0A>>>>>>----- Original Message -----=3D3D20 =
=0A>>>>>>From: "Jory McKinley" <jory_mckinley@xxxxxxxxx> =0A>>>>>>To: <leer=
itchey@xxxxxxxxxxxxx>; <npatel@xxxxxxxxxx>; =0A>>>>>><si-list@freelists=3D3=
D =0A>>>>>> =0A>>>>>> =0A>>>>>=3D3D2Eorg> =0A>>>>> =0A>>>>> =0A>>>>>>Sent: =
Monday, September 24, 2007 5:31 PM =0A>>>>>>Subject: [SI-LIST] Re: AC Coupl=
ed Signals =0A>>>>>> =0A>>>>>> =0A>>>>>> I will elaborate a bit on what I h=
ave seen. I have measured =3D =0A>>>>>> =0A>>>>>> =0A>>(time =0A>> =0A>> =
=0A>>>>>>dom=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>ain)=3D3D20 =0A>>>>> =0A>>>>>=
=0A>>>>>>in the lab some effects that appears to be location specific in =
=3D =0A>>>>>> =0A>>>>>> =0A>>the=3D3D20 =0A>> =0A>> =0A>>>>>>placement of t=
he AC coupling caps at the rcvr. Now this may be =3D =0A>>>>>> =0A>>>>>> =
=0A>>due =0A>> =0A>> =0A>>>>>>in =3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>part=3D3=
D20 =0A>>>>> =0A>>>>> =0A>>>>>>to the fact that I am using 50-ohm resistor =
termination in each =3D =0A>>>>>> =0A>>>>>> =0A>>lead =0A>> =0A>> =0A>>>>>>=
as=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>=3D3D20 =0A>>>>> =0A>>>>> =0A>>>>>>well=
and the combination (cap plus rcvr reflection) is giving =3D =0A>>>>>> =0A=
>>>>>> =0A>>some=3D3D20 =0A>> =0A>> =0A>>>>>>imbalance depending on distanc=
e. The best rcvr eye that I am =3D =0A>>>>>> =0A>>>>>> =0A>>seeing =0A>> =
=0A>> =0A>>>>>>is=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>=3D3D20 =0A>>>>> =0A>>>>=
> =0A>>>>>>when I can move the AC/term as close to the rcvr as I can. By th=
e =0A>>>>>>way =3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>>these are 5Gb/s signals. =
=0A>>>>>>If I have time I will try and isolate what I am seeing and even =
=0A>>>>>>simulat=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>e=3D3D20 =0A>>>>> =0A>>>>=
> =0A>>>>>>it, has anyone else seen or simulated this? =0A>>>>>>-Jory =0A>>=
>>>> =0A>>>>>>----- Original Message ---- =0A>>>>>>From: Lee Ritchey <leeri=
tchey@xxxxxxxxxxxxx> =0A>>>>>>To: "npatel@xxxxxxxxxx" <npatel@xxxxxxxxxx>; =
si-list@xxxxxxxxxxxxx =0A>>>>>>Sent: Monday, September 24, 2007 1:06:06 PM =
=0A>>>>>>Subject: [SI-LIST] Re: AC Coupled Signals =0A>>>>>> =0A>>>>>>Nikil=
, =0A>>>>>> =0A>>>>>>I have made measurements on test PCBs and the location=
is not all =3D =0A>>>>>> =0A>>>>>> =0A>>that =0A>> =0A>> =0A>>>>>>importan=
t. In identical pairs, one with AC coupling capacitors =3D =0A>>>>>> =0A>>>=
>>> =0A>>and =0A>> =0A>> =0A>>>>>>the=3D3D =0A>>>>>> =0A>>>>>> =0A>>>>>>oth=
er without, the loss vs. frequency is virtually identical at =3D =0A>>>>>> =
=0A>>>>>> =0A>>leas =0A>> =0A>> =0A>>>>>>ou=3D3D =0A>>>>>> =0A>>>>>> =0A>>>=
>>t=3D3D20 =0A>>>>> =0A>>>>> =0A>>>>>>to =0A>>>>>>6 GHz. That would be 12 M=
b/S. =0A>>>>>> =0A>>>>>>Lee Ritchey =0A>>>>>> =0A>>>>>> =0A>>>>>> [Original=
Message] =0A>>>>>>From: <npatel@xxxxxxxxxx> =0A>>>>>>To: <si-list@freelist=
s.org> =0A>>>>>>Date: 9/24/2007 10:21:37 AM =0A>>>>>>Subject: [SI-LIST] AC =
Coupled Signals =0A>>>>>> =0A>>>>>>Hi all, =0A>>>>>>In case of AC coupled s=
ignals does anyone know of an optimum =3D =0A>>>>>> =0A>>>>>> =0A>>placemen=
t =0A>> =0A>> =0A>>>>>>for the caps? I mean should they be placed near the =
source, =3D =0A>>>>>> =0A>>>>>> =0A>>receiver, =0A>> =0A>> =0A>>>>>>middle =
of the transmission line? =0A>>>>>>How much difference does it make in the =
opening of the eye? =0A>>>>>>The signals are differential CML running at 3.=
0Gbps =0A>>>>>> =0A>>>>>>Thanks, =0A>>>>>>Nikhil =0A>>>>>> =0A>>>>>> =0A>>>=
>>> =0A>>>>>> =0A>> =0A>> =0A>>--------------------------------------------=
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