[SI-LIST] Re: SI Simulation of GHz signals
- From: Ray Anderson <reanderson@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 28 Jul 2004 19:40:09 -0700
May be bad netiquette replying to my own post, but in the earlier post
forgot to mention that the basic relationship for Zo of a transmission
line is Zo = sqrt(L/C) . With no frequency dependent terms (except
perhaps the frequency dependence of L and C wrt frequency which is a
second order effect) the Zo of a cable does not depend on the frequency
of the signal traveling along it.
-Ray
Ray Anderson wrote:
>Not totally correct. It is true that the line looks like infinite (or
>nearly so, actually 1/G) impedance at DC, but for AC signals a 50 ohm
>line presents 50 ohms impedance from very low frequencies to very high
>(ignoring for the moment frequency dependent effects that may cause the
>impedance to deviate a bit from 50 ohms). Imagine you have a 1 MHz
>signal with 100psec rise time. Both the 1 MHz component and the higher
>frequency components (~.35/100psec = 3.5 GHz) will see the line as a 50
>ohm impedance, not just the higher frequency components contributed by
>the Fourier components of the risetime.
>
>-Ray Anderson
>
>
>
>
>
.
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