[SI-LIST] Re: AC Coupled Signals
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <Istvan.Novak@xxxxxxx>
- Date: Mon, 1 Oct 2007 07:34:59 -0700
Hello Istvan,
You're keying in to what was surprising to me. For the asymmetric lossy
network I simulated, I noted that the pulse response remained constant,
regardless of direction of simulation. Of course, S11 and S22 changed
dramatically.
The pulse response was not a derivation of the s-params; it was merely
Vout (the circuit driven with an arbitrarily chosen 2V source) of a
simulation of the actual network (3 lossy lines).
So, it appears that the pulse response and insertion loss remained
constant, even though the return loss varied dramatically. I wasn't
surprised to have the insertion loss stay constant, but couldn't have
predicted whether the pulse reponse would also remain constant, though I
suspected it would. This implies the eye diagram would also be the
same, regardless of direction of excitation.
Having the pulse response remain constant appears contrary to your
experience. Any explanation?
Thanks for your thoughts,
Jeff Loyer
-----Original Message-----
From: Istvan.Novak@xxxxxxx [mailto:Istvan.Novak@xxxxxxx]=20
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
scattering matrix deals with power values. In a reciprocal network,
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
coefficients.=20
So probably a more generalized condition is that we can shuffle around
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
your example circuits strictly from S21 of the network, I agree, it will
not change, since S21 will not change as you move the reciprocal
building blocks around.=20
Your eye diagram, however, should be usually calculated as Vout/Vsource
(and for sake of simplicity, we usually assume linear driver and
receiver impedances, where their linearity does not change this
argument). This voltage transfer ratio can be analytically calculated if
you wish, from the S-parameter flow graph. The resulting formula is
symmetrical in S12 and S21, but asymmetrical in S11, S22 as well as
source and load reflection coefficients.=20
If you simulate the transfer response or the pulse response of
Vout/Vsource in HSPICE, you should see the change as soon as the
electrical symmetry is changed.
Regards,
Istvan Novak
SUN Microsystems
Loyer, Jeff wrote:
>Hi Istvan,
>Could you point me towards more information on the "voltage transfer=20
>ratio", and the difference between it and s21/s12? When I did the=20
>pulse response of a system where S21 =3D3D S12 (though S11 and S22 were =
>very different, and the reflections were significant), it came out=20
>equal (p21 =3D3D p12, see my posting of 9/15). Thus, I think the eye=20
>diagram will =3D come out equal if S21 =3D3D S12. =3D20
>
>Thanks,
>Jeff Loyer
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx=20
>[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; ron@xxxxxxxxxxx;=20
>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 in
>linear, time-invariant, reciprocal networks S21=3D3DS12 regardless of =
the
>sequence of the smaller building blocks within the network, BUT, the=20
>eye diagram shows us voltage transfer ratio between source and load,=20
>and it is NOT S21; the voltage transfer ratio DOES depend on the=20
>sequence of contributing blocks. This happens also with linear source=20
>and load and even if the source and load are perfectly matched.=3D20
>
>The only case when the voltage transfer ratio remains the same in spite
>of moving building blocks around, if/when reflections at the boundaries
>of the particular building block are negligible.
>
>Thanks
>
>Istvan Novak
>SUN Microsystems
>
>
>
>Chris Cheng wrote:
> =20
>
>>I think the point is tuning S22 for the non-ideal load. S12 or S21 =
=3D3D
>>=3D
>> =20
>>
>
> =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, =
=3D
>> =20
>>
>=3D3D
>
> =20
>
>>receiver side of cap) in dB=3D3D20
>>
>>It seems to stand to reason that if you lose xxx mV when the signal is
>> =20
>>
>
> =20
>
>>=3D3D stronger at the driver and some smaller portion of that when=20
>>the=3D20 caps =3D3D are placed closer to the recever is normal since =
loss=20
>>has=3D20 ocurred in the =3D3D media. The total signal loss of the =
system=20
>>could=3D20 still be the same.=3D3D20
>>
>>Leonard.=3D3D20
>>
>>
>>
>>----- Original Message ----=3D3D20
>>From: Stephen Zinck <signalintegrity@xxxxxxxxxxx>=3D3D20
>>To: ron@xxxxxxxxxxx; Chris.Cheng@xxxxxxxx=3D3D20
>>Cc: si-list@xxxxxxxxxxxxx=3D3D20
>>Sent: Friday, September 28, 2007 12:19:22 PM=3D3D20
>>Subject: [SI-LIST] Re: AC Coupled Signals=3D3D20
>>
>>
>>Hello SI-LISTers,=3D3D20
>>
>>I thought for my part in this discussion, I should do some due=3D20=20
>>diligence =3D3D on=3D3D20 this AC coupling capacitor placement =
location=3D20
>>question.=3D3D20
>>
>>Scott McMorrow, Steve Weir and I had some off-line discussions that =
=3D
>> =20
>>
>=3D3D=3D20
> =20
>
>>tended=3D3D20 to suggest my position dependency results may have =
been=3D20
>>caused by local=3D3D20 resonances from other impedance discontinuities =
=3D
>> =20
>>
>in=3D20
> =20
>
>>the system I was=3D3D20 simulating. Based on this, I set out to =
develop=20
>>=3D
>> =20
>>
>a=3D20
> =20
>
>>simulation model that =3D3D had=3D3D20 a minimum of discontinuities =
(no=3D20
>>backplane vias/connectors/trace, etc.). =3D3D
>>
>>
>>I used:=3D3D20
>>
>>- Spice models of non-linear 3.125Gbit/s silicon (driver and=3D20=20
>>receiver)=3D3D20
>>- S-parameter based package models for both driver and =
receiver.=3D3D20
>>- A 0.01uF capacitor and its associated parasitics (via, trace, pad, =
=3D
>> =20
>>
>=3D3D
>
> =20
>
>>mount,=3D3D20 component).=3D3D20
>>- 2D lossy W-Element transmission line (with di-electric and skin=3D20 =
>>effect =3D3D
>>
>>losses included).=3D3D20
>>
>>I made the capacitor model such that I could "slide" it up and down=20
>>a=3D20
>>15 =3D3D
>>
>>inch trace between the driver and receiver. I iteratively =
simulated=3D20
>>for =3D3D the=3D3D20 following length combinations:=3D3D20
>>
>>- 500 mil trace from driver to AC coupling capacitor with 14500 mil =
=3D
>> =20
>>
>=3D3D=3D20
> =20
>
>>trace to=3D3D20 receiver.=3D3D20
>>- 5000 mil trace from driver to AC coupling capacitor with 10000 mil =
=3D
>> =20
>>
>=3D3D
>
> =20
>
>>trace=3D3D20 to receiver.=3D3D20
>>- 10000 mil trace from driver to AC coupling capacitor with 5000 mil =
=3D
>> =20
>>
>=3D3D
>
> =20
>
>>trace=3D3D20 to receiver.=3D3D20
>>- 14500 mil trace from driver to AC coupling capacitor with 500 mil =
=3D
>> =20
>>
>=3D3D=3D20
> =20
>
>>trace to=3D3D20 receiver.=3D3D20
>>
>>The results show around 125 mV (differential) difference between=3D20=20
>>the=3D3D20 capacitor at the source versus the capacitor at the=3D20=20
>>destination, with =3D3D the=3D3D20 benefit going to the capacitor=20
>>placed=3D20 closest to the receiver. 125 mV is =3D3D a=3D3D20 lot to =
give=20
>>away...=3D3D20
>>
>>I am not going to pretend to understand the physics behind these=3D20=20
>>results =3D3D but=3D3D20 I thought it worth while to at least show the =
=3D
>> =20
>>
>basis=3D20
> =20
>
>>for my statements.=3D3D20
>>
>>I would be happy to evolve the simulation environment if someone=20
>>has=3D20 a=3D3D20 suggestion...=3D3D20
>>
>>I have put together a document that I can post to an ftp site or email
>> =20
>>
>
> =20
>
>>=3D3D if=3D3D20 anyone would like a copy...=3D3D20
>>
>>Kind regards,=3D3D20
>>Steve=3D3D20
>>
>>Stephen P. Zinck=3D3D20
>>Interconnect Engineering Inc.=3D3D20
>>P.O. Box 577=3D3D20
>>South Berwick, ME 03908=3D3D20
>>Phone - (207) 384-8280=3D3D20
>>Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx=3D3D20
>>Web - www.interconnectengineering.com=3D3D20
>>
>>
>>
>>
>> =20
>>
>
> =20
>
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