[SI-LIST] Re: 90 degree bend
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- Date: Mon, 30 Aug 2004 10:08:56 -0700
Well put!
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: Scott McMorrow <scott@xxxxxxxxxxxxx>
> Cc: <si-list@xxxxxxxxxxxxx>
> Date: 8/30/2004 9:47:52 AM
> Subject: [SI-LIST] Re: 90 degree bend
>
> Alright newbies, the electron-as-marbles theory of 90-degree bends
> really does not fly. (Pun intended.) What is propagating down a trace
> is an electromagnetic wavefront, not physical electrons. Please go back
> to your Electromagnetic and Physics textbooks.
>
> As for 90 degree bends, the impact that these have on signals is limited
> by the width of the trace and the Er of the material used.. Microwave
> boards often have very wide traces on very thick substrates. Excess
> capacitance (or excess inductance, depending on the frequency being
> analyzed) is proportional to the length of the corner discontinuity. A
> 100 mil trace, as might be used in a microwave design, will have a
> discontinuity length of 1.414 X 100 mil or 141 mils. On a ceramic
> substrate with an Er of 10, this leads to an electrical length of about
> 33 ps, which is a big delay error and a big impedance bump which causes
> high insertion loss at some high frequencies. However, for digital
> boards where traces tend to be relatively narrow, the length of these
> corners is small, say 14 mils, for a 10 mil trace width, limiting the
> discontinuity and delay error to 2 ps or less. A 5 mil trace would have
> a potential delay error of about 1 ps/corner.
>
> Now a 1 or 2 ps delay error might be considered a problem by some, but
> there are some mitigating factors:
>
> 1) All modern-day PCB CAD tools mitre corners, eliminating 90 degree
> bends, and reducing the overall corner error by a significant amount.
> Let's just say there is about a 4:1 reduction in the delay introduced
> for starters. Thus, for a 5 mil trace this would limit the delay error
> to 250 fs, which I defy most of you to measure accurately. (Please do
> not confuse this with any additional skew introduced by serpentined
> trace coupling, which can also introduce additional delta skew.)
>
> 2) Most digital boards are fabricated with FR-4 or other
> fiberglass-epoxy laminate materials. These materials have significant
> global and local Er variations, due to trace orientation over the
> underlying weave, which can account for a 2 to 4 ps/in delay variaiton
> between any two traces on an FR-4 PCB. As a result, any delay due to
> corner bends tends to be insignificant when compared to other sources of
> trace delay error.
>
> As a result, corners are just not that interesting any more.
>
>
> best regards,
>
> scott
>
>
> >
> >
> >
>
> --
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> (401) 284-1827 Business
> (401) 284-1840 Fax
> (503) 750-6481 Cellular
> http://www.teraspeed.com
>
> Teraspeed is the registered service mark of
> Teraspeed Consulting Group LLC
>
>
>
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