FreeLists / si-list / [SI-LIST] Re: AC Coupled Signals

["Stephen Zinck" <signalintegrity@xxxxxxxxxxx>]

Thank you for your insight Istvan and Jeff, and to all who contributed to 
this thread...

Best regards,
Steve

Stephen P. Zinck
Interconnect Engineering Inc.
P.O. Box 577
South Berwick, ME 03908
Phone - (207) 384-8280
Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx
Web - www.interconnectengineering.com


----- Original Message ----- 
From: "Istvan Novak - Board Design Technology" <Istvan.Novak@xxxxxxx>
To: <jeff.loyer@xxxxxxxxx>
Cc: <si-list@xxxxxxxxxxxxx>
Sent: Tuesday, October 02, 2007 1:38 PM
Subject: [SI-LIST] Re: AC Coupled Signals


> Jeff,
> Yes, the results might be surprising at first.  And as noted earlier,
> you dont need to assume different source and load impedances
> to get difference voltage transfer ratios: if the S22 and S11 values
> differ or change, it will also result in a change.  So going back to
> the starting scenario: when you move around an AC-coupling
> capacitor, there will be minimal or no change if the capacitor is
> 'transparent' and more change when it is not.
>
> Regards,
> istvan
>
>
>
>
> Loyer, Jeff wrote On 10/02/07 12:14,:
>
>>A late-night epiphany has finally clued me in to reconciling the
>>physics.
>>
>>I was doing my pulse response simulation with the source and load
>>impedances equal.  When I changed my source impedance to 8 ohms
>>(typical, but arbitrary) and left my load impedance at 50 ohms, the
>>pulse responses were dramatically different, depending on which side I
>>excited the system from.  I believe this is what you were alluding to,
>>though I didn't comprehend it as such.
>>
>>This makes sense, and demonstrates how 2 systems can have IDENTICAL
>>insertion loss and yet have dramatically different pulse response/eye
>>diagrams.  Very interesting...
>>
>>Thanks for sharing your insights.
>>
>>Jeff Loyer
>>
>>
>>-----Original Message-----
>>From: Istvan.Novak@xxxxxxx [mailto:Istvan.Novak@xxxxxxx]=20
>>Sent: Monday, October 01, 2007 4:15 PM
>>To: Loyer, Jeff
>>Cc: si-list@xxxxxxxxxxxxx
>>Subject: Re: [SI-LIST] Re: AC Coupled Signals
>>
>>Jeff,
>>
>>My talented friend, Gustavo Blando, with whom we analyzed this situation
>>a few years back, reminded me that the voltage-transfer function
>>formulas can be found for instance in "MicroWave Transistor Amplifiers",
>>Guillermo Gonzales, Page 185.
>>You can also look at L. S. Dutta, T. Hillmann-Ruge, "Application of Ring
>>Oscillators to Characterise Transmission Lines in VLSI Circuits," IEEE
>>Transactions on Components, Packaging, and Manufacturing Technology -
>>Part B., Vol. 18, No. 4, November 1995, pp. 651-657. Note that you will
>>find various expressions; you may want to pick the Vout/Vsource
>>formulas, as opposed to Vout/Vin. You will see that the voltage transfer
>>ratio is, in general, different from S21.
>>
>>Regards,
>>Istvan Novak
>>SUN Microsystems
>>
>>
>>Loyer, Jeff wrote:
>>
>>
>>
>>>Hello Istvan,
>>>You're keying in to what was surprising to me.  For the asymmetric=20
>>>lossy network I simulated, I noted that the pulse response remained=20
>>>constant, regardless of direction of simulation.  Of course, S11 and=20
>>>S22 changed dramatically.
>>>
>>>The pulse response was not a derivation of the s-params; it was merely=20
>>>Vout (the circuit driven with an arbitrarily chosen 2V source) of a=20
>>>simulation of the actual network (3 lossy lines).
>>>
>>>So, it appears that the pulse response and insertion loss remained=20
>>>constant, even though the return loss varied dramatically.  I wasn't=20
>>>surprised to have the insertion loss stay constant, but couldn't have=20
>>>predicted whether the pulse reponse would also remain constant, though=20
>>>I suspected it would.  This implies the eye diagram would also be the=20
>>>same, regardless of direction of excitation.
>>>
>>>Having the pulse response remain constant appears contrary to your=20
>>>experience.  Any explanation?
>>>
>>>Thanks for your thoughts,
>>>Jeff Loyer
>>>
>>>
>>>-----Original Message-----
>>>From: Istvan.Novak@xxxxxxx [mailto:Istvan.Novak@xxxxxxx]=3D20
>>>Sent: Monday, October 01, 2007 4:46 AM
>>>To: Loyer, Jeff
>>>Cc: istvan.novak@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
>>>Subject: Re: [SI-LIST] Re: AC Coupled Signals
>>>
>>>Hi Jeff,
>>>
>>>Let me see if I can summarize this in simple physical terms. The=20
>>>scattering matrix deals with power values.  In a reciprocal  network,=20
>>>the 'through' power is the same regardless of the direction we go.  The
>>>
>>>
>>
>>
>>
>>>node voltages, on the other hand, depend on the local impedance levels,
>>>
>>>
>>
>>
>>
>>>which are related to S11, S22 and source and load reflection=20
>>>coefficients.=3D20
>>>
>>>So probably a more generalized condition is that we can shuffle around=20
>>>reciprocal building blocks inside a cascaded network, and the resulting
>>>
>>>
>>
>>
>>
>>>eye diagram will stay the same as long as we do not change S11 and S22.
>>>
>>>Going back to your simulations: if you calculate the pulse response of=20
>>>your example circuits strictly from S21 of the network, I agree, it=20
>>>will not change, since S21 will not change as you move the reciprocal=20
>>>building blocks around.=3D20 Your eye diagram, however, should be =
>>>
>>>
>>usually
>>
>>
>>
>>>calculated as Vout/Vsource (and for sake of simplicity, we usually=20
>>>assume linear driver and receiver impedances, where their linearity=20
>>>does not change this argument). This voltage transfer ratio can be=20
>>>analytically calculated if you wish, from the S-parameter flow graph. =20
>>>The resulting formula is symmetrical in S12 and S21, but asymmetrical=20
>>>in S11, S22 as well as source and load reflection coefficients.=3D20 If =
>>>
>>>
>>
>>
>>
>>>you simulate the transfer response or the pulse response of=20
>>>Vout/Vsource in HSPICE, you should see the change as soon as the=20
>>>electrical symmetry is changed.
>>>
>>>Regards,
>>>
>>>Istvan Novak
>>>SUN Microsystems
>>>
>>>
>>>
>>>
>>>
>>>
>>>Loyer, Jeff wrote:
>>>
>>>=20
>>>
>>>
>>>
>>>>Hi Istvan,
>>>>Could you point me towards more information on the "voltage=20
>>>>transfer=3D20 ratio", and the difference between it and s21/s12?  When =
>>>>
>>>>
>>I
>>
>>
>>
>>>>did the=3D20 pulse response of a system where S21 =3D3D3D S12 (though =
>>>>
>>>>
>>S11=20
>>
>>
>>>>and S22 were =3D
>>>>  =20
>>>>
>>>>
>>>>
>>>=20
>>>
>>>
>>>
>>>>very different, and the reflections were significant), it came =
>>>>
>>>>
>>out=3D20=20
>>
>>
>>>>equal (p21 =3D3D3D p12, see my posting of 9/15).  Thus, I think the=20
>>>>eye=3D20 diagram will =3D3D come out equal if S21 =3D3D3D S12.  =
>>>>
>>>>
>>=3D3D20
>>
>>
>>>>Thanks,
>>>>Jeff Loyer
>>>>
>>>>
>>>>-----Original Message-----
>>>>From: si-list-bounce@xxxxxxxxxxxxx=3D20
>>>>[mailto:si-list-bounce@xxxxxxxxxxxxx]
>>>>On Behalf Of Istvan Novak
>>>>Sent: Saturday, September 29, 2007 12:38 AM
>>>>To: Chris.Cheng@xxxxxxxx
>>>>Cc: lifeatthesharpend; signalintegrity@xxxxxxxxxxx;=20
>>>>ron@xxxxxxxxxxx;=3D20 si-list@xxxxxxxxxxxxx
>>>>Subject: [SI-LIST] Re: AC Coupled Signals
>>>>
>>>>Chris and All,
>>>>
>>>>I think the source of misunderstandigs might stem from the fact that=20
>>>>in
>>>>  =20
>>>>
>>>>
>>>>
>>>=20
>>>
>>>
>>>
>>>>linear, time-invariant, reciprocal networks S21=3D3D3DS12 regardless =
>>>>
>>>>
>>of=20
>>
>>
>>>>=3D
>>>>  =20
>>>>
>>>>
>>>>
>>>the
>>>
>>>=20
>>>
>>>
>>>
>>>>sequence of the smaller building blocks within the network, BUT,=20
>>>>the=3D20 eye diagram shows us voltage transfer ratio between source =
>>>>
>>>>
>>and=20
>>
>>
>>>>load,=3D20 and it is NOT S21; the voltage transfer ratio DOES depend =
>>>>
>>>>
>>on=20
>>
>>
>>>>the=3D20 sequence of contributing blocks.  This happens also with =
>>>>
>>>>
>>linear
>>
>>
>>
>>>>source=3D20 and load and even if the source and load are perfectly=20
>>>>matched.=3D3D20
>>>>
>>>>The only case when the voltage transfer ratio remains the same in=20
>>>>spite
>>>>  =20
>>>>
>>>>
>>>>
>>>=20
>>>
>>>
>>>
>>>>of moving building blocks around, if/when reflections at the=20
>>>>boundaries
>>>>  =20
>>>>
>>>>
>>>>
>>>=20
>>>
>>>
>>>
>>>>of the particular building block are negligible.
>>>>
>>>>Thanks
>>>>
>>>>Istvan Novak
>>>>SUN Microsystems
>>>>
>>>>
>>>>
>>>>Chris Cheng wrote:
>>>>=3D20
>>>>
>>>>  =20
>>>>
>>>>
>>>>
>>>>>I think the point is tuning S22 for the non-ideal load. S12 or S21 =
>>>>>
>>>>>
>>=3D
>>
>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>=3D3D3D
>>>
>>>=20
>>>
>>>
>>>
>>>>>=3D3D
>>>>> =3D20
>>>>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>>=3D20
>>>>
>>>>  =20
>>>>
>>>>
>>>>
>>>>>remains being equal (symmetric).
>>>>>
>>>>>-----Original Message-----
>>>>>From: lifeatthesharpend [mailto:lifeatthesharpend@xxxxxxxxx]
>>>>>Sent: Friday, September 28, 2007 2:44 PM
>>>>>To: signalintegrity@xxxxxxxxxxx; ron@xxxxxxxxxxx; Chris Cheng
>>>>>Cc: si-list@xxxxxxxxxxxxx
>>>>>Subject: Re: [SI-LIST] Re: AC Coupled Signals
>>>>>
>>>>>
>>>>>What is the total loss at each point? (reciever, driver side of cap,=20
>>>>>=3D
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>=3D3D
>>>=20
>>>
>>>
>>>
>>>>> =3D20
>>>>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>>=3D3D3D
>>>>
>>>>=3D20
>>>>
>>>>  =20
>>>>
>>>>
>>>>
>>>>>receiver side of cap) in dB=3D3D3D20
>>>>>
>>>>>It seems to stand to reason that if you lose xxx mV when the signal=20
>>>>>is
>>>>> =3D20
>>>>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>>=3D20
>>>>
>>>>  =20
>>>>
>>>>
>>>>
>>>>>=3D3D3D stronger at the driver and some smaller portion of that =
>>>>>
>>>>>
>>when=3D20
>>
>>
>>
>>>>>the=3D3D20 caps =3D3D3D are placed closer to the recever is normal =
>>>>>
>>>>>
>>since=20
>>
>>
>>>>>=3D
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>loss=3D20
>>>=20
>>>
>>>
>>>
>>>>>has=3D3D20 ocurred in the =3D3D3D media. The total signal loss of the =
>>>>>
>>>>>
>>=3D
>>
>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>system=3D20
>>>=20
>>>
>>>
>>>
>>>>>could=3D3D20 still be the same.=3D3D3D20
>>>>>
>>>>>Leonard.=3D3D3D20
>>>>>
>>>>>
>>>>>
>>>>>----- Original Message ----=3D3D3D20
>>>>>From: Stephen Zinck <signalintegrity@xxxxxxxxxxx>=3D3D3D20
>>>>>To: ron@xxxxxxxxxxx; Chris.Cheng@xxxxxxxx=3D3D3D20
>>>>>Cc: si-list@xxxxxxxxxxxxx=3D3D3D20
>>>>>Sent: Friday, September 28, 2007 12:19:22 PM=3D3D3D20
>>>>>Subject: [SI-LIST] Re: AC Coupled Signals=3D3D3D20
>>>>>
>>>>>
>>>>>Hello SI-LISTers,=3D3D3D20
>>>>>
>>>>>I thought for my part in this discussion, I should do some=20
>>>>>due=3D3D20=3D20 diligence =3D3D3D on=3D3D3D20 this AC coupling =
>>>>>
>>>>>
>>capacitor=20
>>
>>
>>>>>placement =3D
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>location=3D3D20
>>>
>>>=20
>>>
>>>
>>>
>>>>>question.=3D3D3D20
>>>>>
>>>>>Scott McMorrow, Steve Weir and I had some off-line discussions that =
>>>>>
>>>>>
>>=3D
>>
>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>=3D3D
>>>=20
>>>
>>>
>>>
>>>>> =3D20
>>>>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>>=3D3D3D=3D3D20
>>>>=3D20
>>>>
>>>>  =20
>>>>
>>>>
>>>>
>>>>>tended=3D3D3D20 to suggest my position dependency results may have =
>>>>>
>>>>>
>>=3D
>>
>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>been=3D3D20
>>>
>>>=20
>>>
>>>
>>>
>>>>>caused by local=3D3D3D20 resonances from other impedance=20
>>>>>discontinuities =3D
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>=3D3D
>>>=20
>>>
>>>
>>>
>>>>> =3D20
>>>>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>>in=3D3D20
>>>>=3D20
>>>>
>>>>  =20
>>>>
>>>>
>>>>
>>>>>the system I was=3D3D3D20 simulating. Based on this, I set out to =3D
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>develop=3D20
>>>=20
>>>
>>>
>>>
>>>>>=3D3D
>>>>> =3D20
>>>>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>>a=3D3D20
>>>>=3D20
>>>>
>>>>  =20
>>>>
>>>>
>>>>
>>>>>simulation model that =3D3D3D had=3D3D3D20 a minimum of =
>>>>>
>>>>>
>>discontinuities =3D
>>
>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>(no=3D3D20
>>>
>>>=20
>>>
>>>
>>>
>>>>>backplane vias/connectors/trace, etc.). =3D3D3D
>>>>>
>>>>>
>>>>>I used:=3D3D3D20
>>>>>
>>>>>- Spice models of non-linear 3.125Gbit/s silicon (driver =
>>>>>
>>>>>
>>and=3D3D20=3D20=20
>>
>>
>>>>>receiver)=3D3D3D20
>>>>>- S-parameter based package models for both driver and =3D
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>receiver.=3D3D3D20
>>>=20
>>>
>>>
>>>
>>>>>- A 0.01uF capacitor and its associated parasitics (via, trace, pad,=20
>>>>>=3D
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>=3D3D
>>>=20
>>>
>>>
>>>
>>>>> =3D20
>>>>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>>=3D3D3D
>>>>
>>>>=3D20
>>>>
>>>>  =20
>>>>
>>>>
>>>>
>>>>>mount,=3D3D3D20 component).=3D3D3D20
>>>>>- 2D lossy W-Element transmission line (with di-electric and=20
>>>>>skin=3D3D20 =3D
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>=20
>>>
>>>
>>>
>>>>>effect =3D3D3D
>>>>>
>>>>>losses included).=3D3D3D20
>>>>>
>>>>>I made the capacitor model such that I could "slide" it up and=20
>>>>>down=3D20 a=3D3D20
>>>>>15 =3D3D3D
>>>>>
>>>>>inch trace between the driver and receiver. I iteratively =3D
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>simulated=3D3D20
>>>
>>>=20
>>>
>>>
>>>
>>>>>for =3D3D3D the=3D3D3D20 following length combinations:=3D3D3D20
>>>>>
>>>>>- 500 mil trace from driver to AC coupling capacitor with 14500 mil =
>>>>>
>>>>>
>>=3D
>>
>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>=3D3D
>>>=20
>>>
>>>
>>>
>>>>> =3D20
>>>>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>>=3D3D3D=3D3D20
>>>>=3D20
>>>>
>>>>  =20
>>>>
>>>>
>>>>
>>>>>trace to=3D3D3D20 receiver.=3D3D3D20
>>>>>- 5000 mil trace from driver to AC coupling capacitor with 10000 mil=20
>>>>>=3D
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>=3D3D
>>>=20
>>>
>>>
>>>
>>>>> =3D20
>>>>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>>=3D3D3D
>>>>
>>>>=3D20
>>>>
>>>>  =20
>>>>
>>>>
>>>>
>>>>>trace=3D3D3D20 to receiver.=3D3D3D20
>>>>>- 10000 mil trace from driver to AC coupling capacitor with 5000 mil=20
>>>>>=3D
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>=3D3D
>>>=20
>>>
>>>
>>>
>>>>> =3D20
>>>>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>>=3D3D3D
>>>>
>>>>=3D20
>>>>
>>>>  =20
>>>>
>>>>
>>>>
>>>>>trace=3D3D3D20 to receiver.=3D3D3D20
>>>>>- 14500 mil trace from driver to AC coupling capacitor with 500 mil =
>>>>>
>>>>>
>>=3D
>>
>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>=3D3D
>>>=20
>>>
>>>
>>>
>>>>> =3D20
>>>>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>>=3D3D3D=3D3D20
>>>>=3D20
>>>>
>>>>  =20
>>>>
>>>>
>>>>
>>>>>trace to=3D3D3D20 receiver.=3D3D3D20
>>>>>
>>>>>The results show around 125 mV (differential) difference=20
>>>>>between=3D3D20=3D20 the=3D3D3D20 capacitor at the source versus the=20
>>>>>capacitor at the=3D3D20=3D20 destination, with =3D3D3D the=3D3D3D20 =
>>>>>
>>>>>
>>benefit=20
>>
>>
>>>>>going to the capacitor=3D20 placed=3D3D20 closest to the receiver. =
>>>>>
>>>>>
>>125 mV
>>
>>
>>
>>>>>is =3D3D3D a=3D3D3D20 lot to =3D
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>give=3D20
>>>=20
>>>
>>>
>>>
>>>>>away...=3D3D3D20
>>>>>
>>>>>I am not going to pretend to understand the physics behind=20
>>>>>these=3D3D20=3D20 results =3D3D3D but=3D3D3D20 I thought it worth =
>>>>>
>>>>>
>>while to at
>>
>>
>>
>>>>>least show the =3D
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>=3D3D
>>>=20
>>>
>>>
>>>
>>>>> =3D20
>>>>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>>basis=3D3D20
>>>>=3D20
>>>>
>>>>  =20
>>>>
>>>>
>>>>
>>>>>for my statements.=3D3D3D20
>>>>>
>>>>>I would be happy to evolve the simulation environment if someone=3D20 =
>>>>>
>>>>>
>>
>>
>>
>>>>>has=3D3D20 a=3D3D3D20 suggestion...=3D3D3D20
>>>>>
>>>>>I have put together a document that I can post to an ftp site or=20
>>>>>email
>>>>> =3D20
>>>>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>>=3D20
>>>>
>>>>  =20
>>>>
>>>>
>>>>
>>>>>=3D3D3D if=3D3D3D20 anyone would like a copy...=3D3D3D20
>>>>>
>>>>>Kind regards,=3D3D3D20
>>>>>Steve=3D3D3D20
>>>>>
>>>>>Stephen P. Zinck=3D3D3D20
>>>>>Interconnect Engineering Inc.=3D3D3D20
>>>>>P.O. Box 577=3D3D3D20
>>>>>South Berwick, ME 03908=3D3D3D20
>>>>>Phone - (207) 384-8280=3D3D3D20
>>>>>Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx=3D3D3D20
>>>>>Web - www.interconnectengineering.com=3D3D3D20
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> =3D20
>>>>>
>>>>>    =20
>>>>>
>>>>>
>>>>>
>>>>=3D20
>>>>
>>>>  =20
>>>>
>>>>
>>>>
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