Re: [SI-LIST] : Some Semiconductors are Unnecessarily Fast

Mike Degerstrom (degerstrom.michael@mayo.edu)
Wed, 14 Jul 1999 17:11:40 -0500

Hans,

On Jul 14, 1:22pm, Hans Mellberg wrote:
> Subject: Re: [SI-LIST] : Some Semiconductors are Unnecessarily Fast
> Using a more resistive metal will attenuate the signal rather than slow
> down the edge rate. In the transmission line, the resistance will only
> serv to attenuate, whereas the L/l and C/l determine the transmission
> line characteristics and behavior.

I would have to disagree. The edge rate will be slowed down
more and more as it travels down the transmission line due
to skin effect. A very simple approximation to the distributed
resistance along a transmission line is R=Rdc+Rac*sqrt(freq).
So it stands to reason that the higher frequency components
will be filtered more than the lower frequency components. Thus
the edge rate will be slowed. One exception is where extremely
thin metals are used and the frequency dependant resistance
begins to dominate the DC resistance above the highest frequencies
of your digital pulse. In that case, your above statement is correct.

Mike

>
> Besides, not to many other metals have affinity for solder. You would
> be restircted to metals such as tin or zinc which have lower
> conductivities than copper.
>
> --- Philip Gantt <pgantt@cisco.com> wrote:
> > Has anyone considered that if a more resistive
> > material (other than copper) were used for PCB
> > laminates, the inherent resistance of the conductors
> > would slow the edge rates and also reduce EMI? This
> > might eliminate the need for
> > external resistors as well. Just a thought...
> >
>
> ===
> Best Regards
> Hans Mellberg
> EMC Consultant
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>
>-- End of excerpt from Hans Mellberg

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