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

  • From: "Ray Anderson" <ray.anderson@xxxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Tue, 3 May 2005 07:56:33 -0700

Slightly tangential to the topic, but look at the following web page on
Howard Johnson's web site for a number of anecdotal  stories on why 50
ohms has been chosen to be a standard for many transmission lines and
some of Howard's comments on the topic:



Raymond Anderson
Senior Signal Integrity Staff Engineer
Product Technology Department
Advanced Package R&D
Xilinx Inc.

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Lee Ritchey
Sent: Tuesday, May 03, 2005 6:56 AM
To: Mike Brown; GrahamDavies@xxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Need advice on basic 6-layer stackup

50 ohms comes from the RF world.  When manufacturing coaxial cable, this
impedance resulted in the optimum dimensions for both cable manufacture
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
other way around.=20

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
> the opposite voltage level.  In other words, plot the high state IV=20
> curve with a load line of 40-50 ohms to Vol, and vice versa.  The=20
> intersection of the IV curve and load line represents the launch
> into the net of that impedance.  It's not surprising that the launch=20
> voltages are really close to the input threshold with low-Z loads.  A=20
> few ohms difference in load Z shifts the launch voltage considerably.


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