[SI-LIST] Re: SI Simulation of GHz signals

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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