# [SI-LIST] Re: Need advice on basic 6-layer stackup

• From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
• To: "Mike Brown" <bmgman@xxxxxxxxxx>, GrahamDavies@xxxxxxxx
• Date: Tue, 3 May 2005 06:56:26 -0700

```50 ohms comes from the RF world.  When manufacturing coaxial cable, this
impedance resulted in the optimum dimensions for both cable manufacture and
connector manufacture.  Since instrumentation was developed around this
impedance, the rest of us have adopted it.  It turns out that 50 ohms is
about the optimum impedance for manufacturing high layer count PCBs, as
well.  Just a coincidence.

The drivers didn't determine the impedance of transmission lines, it is the
other way around.

Lee W. Ritchey
Speeding Edge
P. O. Box 2194
Glen Ellen, CA 95442
Phone- 707-568-3983
FAX-    707-568-3504

I just used the energy it took to be angry to write some blues.
Count Basie

> [Original Message]
> From: Mike Brown <bmgman@xxxxxxxxxx>
> To: <GrahamDavies@xxxxxxxx>
> Cc: <si-list@xxxxxxxxxxxxx>
> Date: 5/2/2005 6:47:01 PM
> Subject: [SI-LIST] Re: Need advice on basic 6-layer stackup
>
> Graham,
>
> To understand where 50 ohms comes from, I suggest that you look at the
> IV curves of the troublesome drivers, with a load line of 40-50 ohms to
> the opposite voltage level.  In other words, plot the high state IV
> curve with a load line of 40-50 ohms to Vol, and vice versa.  The
> intersection of the IV curve and load line represents the launch voltage
> into the net of that impedance.  It's not surprising that the launch
> voltages are really close to the input threshold with low-Z loads.  A
> few ohms difference in load Z shifts the launch voltage considerably.
> Heebie-jeebies is an understandable response.
>
> Basically, 50 ohms is driveable with a large class of circuits. The
> output impedance of  many circuits can approach 50 ohms (depending on
> drive strength selection), giving a built-in series termination effect.
> Higher impedance traces are more easily driven but are more prone to
> crosstalk and are harder to fabricate (narrower traces, thicker
> dielectrics).
>
> The absense of  termination (or a design strategy that comprehends the
> consequences of not terminating)  really leaves you hanging out.  That
> long net of yours could ring 3 or 4 times, at ~1.5 ns per transit of the
> trace.  All depending on where the driver is on the vendor's process
> spread.   Meanwhile, some inputs are probably being overdriven with
> respect to Vin max or overshoot duration. The series terminated line is
> settled in 2 transitions, with minimum or no stress on the receiver.  If
> the ringing net is a clock or reset, you have lost control of the timing
> completely.  An extra threshold crossing on that class of net is fatal.
> If it is a clocked signal, you will be functionally OK if the ringing
> settles out before the next clock (minus setup time).
>
> One thing I can tell you,  if the board was designed some time back, the
> risetime of the parts on the board is probably faster than when it was
> initially designed and tested.  Nets that used to work OK can become
> unreliable when the risetime decreases.  Lots of boards "work" in spite
> of ignoring SI considerations.  For a while.
>
> Regards
>
> Mike
>
> - - - - -
>
> Graham Davies wrote:
>
> >--- In si-list@xxxxxxxxxxxxxxx, steve weir <weirsi@xxxx> wrote:
> >
> >
> >
> >>... the fact that your company's
> >>"slow"  "1999 era" product doesn't
> >>work, is testament that something
> >>in your shop is sorely lacking.
> >>
> >>
> >
> >This is accepted fact. Specifically what is lacking is a digital
> >circuit designer. More generally, what was lacking is any kind of
> >management of the engineering process. I am trying to rectify the
> >latter but the former will take more time.
> >
> >
> >
> >>Your board house has handed you a
> >>massively improved ... 50 ohm
> >>stack-up.
> >>
> >>
> >
> >Agreed. It almost works and can be made to work with a minor tweak.
> >The only thing that's troubling me, that I can't seem to get across,
> >is that I don't understand where this 50 ohms came from and why it is
> >the right choice. A number of the chips on the good-stackup board
> >seem to be having trouble driving their nets, leaving signal levels
> >dangerously close to the threshold while reflections rattle around.
> >This just gives me the heebie-jeebies.
> >
> >
> >
> >>That you still have questions ...
> >>should tell you to seek education ...
> >>
> >>
> >
> >That's exactly what I'm doing here.
> >
> >
> >
> >>Lee Ritchey's "Right the First Time".
> >>Dr. Bogatin's "Signal Integrity Simplified"
> >>Dr. Johnson's  "High Speed Digital Design".
> >>Tom Granberg's book "Handbook of Digital Techniques
> >>for High-Speed Digital Design..."
> >>
> >>
> >
> >Thank you for these recommendations.
> >
> >Graham.
> >
> >
> >
> >
> >------------------------------------------------------------------
> >To unsubscribe from si-list:
> >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >
> >or to administer your membership from a web page, go to:
> >http://www.freelists.org/webpage/si-list
> >
> >For help:
> >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> >List FAQ wiki page is located at:
> >                http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> >
> >List technical documents are available at:
> >                http://www.si-list.org
> >
> >List archives are viewable at:
> >             http://www.freelists.org/archives/si-list
> >or at our remote archives:
> >             http://groups.yahoo.com/group/si-list/messages
> >Old (prior to June 6, 2001) list archives are viewable at:
> >             http://www.qsl.net/wb6tpu
> >
> >
> >
> >
> >
> >
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List FAQ wiki page is located at:
>                 http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
> List technical documents are available at:
>                 http://www.si-list.org
>
> List archives are viewable at:
>               http://www.freelists.org/archives/si-list
> or at our remote archives:
>               http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
http://www.si-list.org

List archives are viewable at:
http://www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu

```