[SI-LIST] Re: Middle (Star) Termination
- From: "Newton, Scott" <Scott.Newton@xxxxxxxxxxxxxxxx>
- To: "steve weir" <weirsp@xxxxxxxxxx>, <dbostan@xxxxxxxxx>,"Robert Haller" <rhaller@xxxxxxxxxx>
- Date: Thu, 26 Feb 2004 08:41:10 -0500
Thank you to everyone for the excellent support!
I wasn't aware of the Motorola document, thanks for the lead and the =
follow-up analysis.
Regards,
Scott
-----Original Message-----
From: steve weir [mailto:weirsp@xxxxxxxxxx]
Sent: Wednesday, February 25, 2004 6:33 PM
To: dbostan@xxxxxxxxx; Newton, Scott; Robert Haller
Cc: si list
Subject: Re: [SI-LIST] Re: Middle (Star) Termination
Dan, I agree. So far what Scott describes is just a series termination
with multiple legs, well documented by MOT. The impedance of the driver
plus any external resistance equals the 1/sum(Y1...Yn) of the driven =
legs
=3D Zline/N. Ie, it is the lumped equivalent of N series terminated =
drivers
of impedance Z in parallel.
If the driven legs are all equal electrical length then the reflections =
all
arrive back at the junction node at the same time, and all of the driven
transmission lines behave as ordinary series terminations. Mismatched
lengths lead to oscillations superimposed on the step.
Regards,
Steve.
At 01:50 PM 2/25/2004 -0800, Dan Bostan wrote:
>I think this termination technique is well explained
>in the Motorola data sheet for clock drivers driving
>multiple loads.
>/dan
>
>--- "Newton, Scott" <Scott.Newton@xxxxxxxxxxxxxxxx>
>wrote:
> > All,
> >
> > I haven't found the paper yet, but I've realized how
> > the star =3D
> > termination works, at least at a high level.=3D20
> > If anyone is interested, here is a summary:=3D20
> > Resistors with identical values are connected at the
> > junction of the =3D
> > star. The value is calculated such that the resistor
> > of the driven leg =3D
> > plus the parallel combination of the resistors in
> > the undriven legs plus =3D
> > the parallel combination of the impedances of the
> > undriven leg traces =3D
> > EQUAL the impedance of the driven leg trace. The
> > resistor value is =3D
> > calculated as (N-2/N)Zo where N is the number of
> > legs in the star and Zo =3D
> > is the characteristic trace impedance. The
> > equivalent circuit yields a =3D
> > matched impedance (almost) on the driven leg to the
> > parallel =3D
> > combinations of all of the undriven legs/resistors.
> > A half wave is =3D
> > propagated at the driven resistor (side closest to
> > the driver). When the =3D
> > wave hits the receivers, it doubles as it reflects
> > (off of infinite =3D
> > impedance juncture). The reflected wave propagates
> > back to the driver =3D
> > leg which looks almost like an infinite length
> > matched impedance, so =3D
> > very little reflection occurs. There are of course
> > details such as the =3D
> > output impedance of the driver, etc that are not
> > covered in this =3D
> > summary.
> >
> > Scott
> >
> >
> > -----Original Message-----
> > From: Robert Haller [mailto:rhaller@xxxxxxxxxx]
> > Sent: Wednesday, February 25, 2004 12:54 PM
> > To: Newton, Scott
> > Cc: si list
> > Subject: Re: [SI-LIST] Middle (Star) Termination
> >
> > Scott,
> > John Grebenkemper wrote an excellent paper
> > that is available in =3D
> > the
> > Designcon98 precedings "Network Topology Analysis
> > Using The Reflection
> > Coefficient" that does and excellent job of not only
> > explaining the
> > theory of middle termination but also presents the
> > math.
> >
> > Regards
> > Bob
> >
> > Newton, Scott wrote:
> > > Can anyone please provide a brief explanation
> > regarding the theory of =3D
> > a middle termination scheme?
> > > In the book" High Speed Digital Design" by
> > Johnson&Graham the topic =3D
> > seems to be glossed over. It essentially says that
> > providing resistors =3D
> > connected in the middle of a star network that have
> > impedance of 1/3 Zo =3D
> > work by halving the voltage at the node. I've used
> > Hyperlynx to simulate =3D
> > this arrangement and it works well. I've searched
> > the web and haven't =3D
> > found a satisfactory explanation.
> > >=3D20
> > > Thanks in advance.
> > >
> >
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> > --
> > Robert J. Haller (rhaller@xxxxxxxxxx)
> > Principal Consultant
> > Signal Integrity Software Inc.
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