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

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: GrahamDavies@xxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Mon, 02 May 2005 15:37:13 -0700

Graham,   If you want to get to a reliable design, 1999 technology 
irrespective, someone with some at least minimal competency should review 
the design front to back.  If it is a simple, non-demanding design, it 
should not take long.  I see four distinct choices:

1. Educate yourself to be that person
2. Find a colleague or friend willing to be that person
3. Hire a consultant whose competency goes beyond nifty via art to be that 
person
4. Do nothing and probably fail.

50 ohm transmission lines work all day long with lots of logic.  The signal 
quality problems that you see are almost certainly a byproduct of lack of 
design process, and not the specific impedance of any trace segment.    I 
also suspect that the rise/fall times of your signals at the drivers are a 
lot faster than you think.  Unless you have a pretty fast scope and 
appropriate probe capability you can easily end-up seeing the low pass 
response of the combined probe and scope vertical amplifier and not your 
actual signals..

Steve
At 10:00 PM 5/2/2005 +0000, 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.
>
>
>
>
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