RE: [SI-LIST] : MECL System Design Handbook

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From: Issa, Elie (Elie.Issa@compaq.com)
Date: Tue May 15 2001 - 08:50:21 PDT


Mr. Ritchey:
I think this is good opportunity to take advantage of
your knowledge and expertise at no charge.
Regarding those bends, whether visible or not,
their parasitics are there which would contribute to the overall
parasitics of the line. So if a bend have an extra capacitance,
and the engineer happen to have in his routing few of them,
don't you think that high speed logic designer should worry
that it might eventually adds up, change the overall impedance
and contribute into slowing the signal?
It could be small, but sometime it is all what we need!

Also regarding the 18ps edge, is it really the edge injected from the
fixture
to the board, or only out of the pulse head of the TDR?
Usually TEK SD24 are capable of 17-18ps but degrade through the fixture!

What are your comments on the graphs provided by Mr.Swanson
which demonstrated a differnce in performance for
return loss between the 90degree bend & the optimized
design even at lower frequency?

Best Regards
Elie

-----Original Message-----
From: Ritchey Lee [mailto:leeritchey@earthlink.net]
Sent: Tuesday, May 15, 2001 7:14 AM
To: Chris Padilla
Cc: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : MECL System Design Handbook

Chris,

If there is "some kind of effect" the claimer should be able to show it in a
repeatable way. I have never see any such demonstation. When I ask for
one, I get
hand waving much like that in you response.

I used an 18 pSEC edge for my tests. The first harmonic of such an edge is
well
above 20 GHz and even then there is no visible effect of the bend.

Is in meaningful t speculate about there might be some effect at some very
high
frequency when the context of the hadbook and this discussion is high speed
logic
design?

Lee

Chris Padilla wrote:

> Lee,
>
> It is not a myth, there is "some kind of effect" from right angle bends.
>
> The real question is, I believe, "Do I care, given the speed and/or
> frequency content of the signals I am dealing with?"
>
> For most of us right now, on this list, we probably *should not* care as
> you point out.
>
> If one is designing a 50 GHz antenna, then it might make a world of
difference!
>
> Out of curiosity, what kind of rise time were you pushing through that
> right angle bend? 100 ps? I am farily certain that I've seen some small
> dips (aka capacitance) through right angle bends but I cannot honestly say
> that it was directly attributed to the bend.
>
> Thanks----->Chris
>
> >Any of you who want to know how the myth about right angle bends got
> >started, look
> >at figure 7.17 on page 155. This alleges that right angle bends can be
> >seen by a
> >TDR. I've done this measurement dozens of times and coiuld never see a
right
> >angle bend.
> >
> >A few years ago, I called Mr. Blood the author of the book and asked
> >about the
> >diagram. His reply was that he knew the diagram was flawed, but there
> >wasn't time
> >to fix it before the book went to press.
> >
> >As a result, thousands of engineers have spend countless time worrying
> >about right
> >angle bends.
> >
> >When we publish technical information such as this, it is important to
> >insure it
> >is accurate.
> >This applies especially to applications notes, whic often contain
entirely
> >false
> >data.
> >
> >Lee
>
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