[SI-LIST] Re: si-list: clifford@blackhole.net post needs approval

>
>
>Clifford wrote:
>
> At higher frequencies, when 
>the signal rise time becomes of the same order as the propagation delay 
>of the trace (the trace then looks like a classical transmission line), 
>the trace impedence appears to be 50 ohms (this impedence could be 
>resistive, reactive etc, depending on the frequency and trace geometry). 
>It is the edges (that contain high frequency components if one uses 
>Fourier decomposition) of the signal that see a 50 ohm impedence. 
>
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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