[SI-LIST] Re: Length matching of source synchronous busses.

  • From: steve weir <weirsp@xxxxxxxxxx>
  • To: jonpowell@xxxxxxxxxxxx, <rhaller@xxxxxxxxxx>,<jmartinson@xxxxxxxxxxxxxxxxx>
  • Date: Wed, 12 Mar 2003 08:10:12 -0800

Jon,

I am pretty sure that I saw a paper on this a few years back on the same 
point.  There is also some coverage of it in Dr. Johnson's book showing how 
tight meanders cause dispersion through adjacent coupling.  A group of 
tight meanders can also make a pretty effective patch antenna.  You can 
avoid this by using wide meanders.

Regards,


Steve.
At 01:01 AM 3/12/2003 -0800, Jon Powell wrote:

>My problem with 10 mil matching and serpentines nets is I think it
>is all a self deception. The many curves of the serpentines and the self
>coupling
>envolved (tell me you serpentine with ground trace guards) adds change of
>delay to the
>nets that far out weigh any 10 mil length matching. I think the only way the
>length matching is
>going to be accurate to anything like 10 mils is if you do near perfect
>mirroring of the the matching trace routes.
>
>Not saying you shouldn't do the length matching. Just don't believe you are
>really matching to 10 mils.
>
>so shoot me.
>
>jon
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Robert Haller
>Sent: Tuesday, March 11, 2003 5:40 AM
>To: jmartinson@xxxxxxxxxxxxxxxxx
>Cc: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Length matching of source synchronous busses.
>
>
>
>I too find myself compelled to add to this interesting thread. I see a
>paralel
>to the controlled impedance coupon thread.
>
>First off if timing  specifications are critical , design and build a
>simple test board
>representative of your actual board and characterize the technology.
>
>Having spent a few summers working in a Board shop (prior to becoming an
>engineer)
>I got a good appreciation for boards manufacturing processes and
>variables first hand.
>There is no replacement for a good symmetrical stackup design, with
>input from your
>manufacturing engineer.
>
>I have measured good designs that yielded variations of less then 1
>ps/inch across multiple
>layers, lots and boards at a particular edge rate.  When working at very
>high speeds or with
>source synchronous  interfaces,  you will find that (propagation delay)
>Tpd is one of the
>most important electrical parameters.
>
>The best way to Measure Tpd is to send an edge down a line of some know
>edge rate.
>The edge rate defines the frequency content and the Tpd is a function of
>frequency.
>The challenge is zeroing out the contribution of fixturing.
>
>I designed controlled impedance boards with ECL back in the early 80's .
>Initially we
>found many board manufacturers resistant to impedance and propagation
>specifications.
>Subsequently we specified mechanical dimensions and listed impedance and
>prop delay
>targets and ranges as are reference. Eventually with the onset of source
>synchronous
>(i.e. Clock forwarding as the term was originally call  back at DEC, )
>critical timing
>specifications (i.e. Z0 and Tpd) became paramount and requirements.
>
>There is a tendency in the industry for people to over design for
>characteristic impedance
>when propagation delay is often a more critical parameter.
>
>So get into the Lab early and Often, and you will avoid  sitting in the
>lab debugging problems
>with a manager sitting over your shoulder asking for daily status
>reports :-)
>
>my $.02
>
>bob
>
>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
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>Jerry Martinson wrote:
>
> >While reading this thread, I was reminded of an issue I had seen about
> >two years ago that goes along the same line of needing to take
> >everything into account when trying to length or delay match nets.  When
> >matching rules are as close as 0.010", many non-obvious effects are
> >often more significant than the residual length difference.
> >
> >
>...
>
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