[SI-LIST] Re: Middle (Star) Termination

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 =
> termination works, at least at a high level.=20
> If anyone is interested, here is a summary:=20
> Resistors with identical values are connected at the
> junction of the =
> star. The value is calculated such that the resistor
> of the driven leg =
> plus the parallel combination of the resistors in
> the undriven legs plus =
> the parallel combination of the impedances of the
> undriven leg traces =
> EQUAL the impedance of the driven leg trace. The
> resistor value is =
> calculated as (N-2/N)Zo where N is the number of
> legs in the star and Zo =
> is the characteristic trace impedance. The
> equivalent circuit yields a =
> matched impedance (almost) on the driven leg to the
> parallel =
> combinations of all of the undriven legs/resistors.
> A half wave is =
> propagated at the driven resistor (side closest to
> the driver). When the =
> wave hits the receivers, it doubles as it reflects
> (off of infinite =
> impedance juncture). The reflected wave propagates
> back to the driver =
> leg which looks almost like an infinite length
> matched impedance, so =
> very little reflection occurs. There are of course
> details such as the =
> output impedance of the driver, etc that are not
> covered in this =
> 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 =
> 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 =
> a middle termination scheme?
> > In the book" High Speed Digital Design" by
> Johnson&Graham the topic =
> seems to be glossed over. It essentially says that
> providing resistors =
> connected in the middle of a star network that have
> impedance of 1/3 Zo =
> work by halving the voltage at the node. I've used
> Hyperlynx to simulate =
> this arrangement and it works well. I've searched
> the web and haven't =
> found a satisfactory explanation.
> >=20
> > Thanks in advance.
> >
>
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> 
> --
> Robert J. Haller (rhaller@xxxxxxxxxx)
> Principal Consultant
> Signal Integrity Software Inc.
> 6 Clock Tower Place, Suite 250
> Maynard, MA 01754
> Phone: (978) 461-0449, ext 15
> 
>
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