Has anybody considered that the discrepancy between the calculation and
measurement can actuall be due to poor measurements? I am not necessarily
talk about impedance measurements (although, that too), but measurements
and variations of board thickness and trace width, thickness etc.?
Thanks,
-Dima Smolyansky
----------
> From: Jim Lyke <[email protected]>
> To: fabrizio=zanella%eng%[email protected]
> Cc: casperdd <[email protected]>;
[email protected]; [email protected]
<[email protected]>
> Subject: Re: [SI-LIST] : Trace impedance
> Date: Thursday, August 20, 1998 12:18 PM
>
> Most of these formulae were based on conformal transforms, often based on
> zero thickness conductors. In a lot of modern work, those assumptions
are
> not very good, particularly when integrated circuits or multichip modules
> are involved. I compared a lot of these expressions (Wheeler,
Hammerstad,
> Jensen, Chang, etc) against real-life measurements on 10u and 25u t-line
> structures of different types in my Master thesis work and got results
all
> over the map.
>
> jim
>
> At 12:25 PM 8/20/98 -0400, fabrizio zanella wrote:
> >Gentlemen, formulae for calculating impedances of stripline/microstrip
> >traces are only somewhat accurate for certain geometries. The formulae
> >were developed many years ago, when line widths were .010"+, dielectric
> >thicknesses differed from today's designs, and most designs used 50 ohm
> >transmission lines. For high impedance tlines, (80+ ohms), the accuracy
of
> >the formulae gets terrible.
> >If you want to match your TDR measured impedances to calculations, you
need
> >to use a field solver tool. There are many good tools available that
use
> >boundary element or finite element analysis. Try Apsim's 'RLGC' tool,
or
> >Ansoft's Maxwell EZ2D.
> >
> >Regards,
> >Fabrizio Zanella
> >EMC Corporation
> >[email protected]
>
> Jim Lyke
> Air Force Research Lab
> AFRL/VSSE
> 3550 Aberdeen Ave SE
> Bldg 887, Room 3
> Kirtland AFB NM 87117-5776
> 505-846-5812/5815
>
>
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