From: Chuck Hill (email@example.com)
Date: Wed Jan 24 2001 - 12:35:17 PST
The context here is power transferred or dissipated in the transmission
line. At gigahertz speeds, the driver is impedance matched to the
line. Nevertheless, if one had a current source driver the power
transferred into the would be V * I = (I * Z0) * I = I^2 Z0. So for a
constant current drive, the power transferred increases linearly with
Z0. And as you noted, loss is not a linear function of Z0. Thus, higher
Z0 reduces the skin effect losses. The issue of the power transferred at
the driver into the transmission line is really a source impedance mismatch
issue, separate from the transmission line loss.
At 11:44 AM 1/24/01 -0800, Farrokh Mottahedin wrote:
>There seems to be a phenomenon that on a differential transmission line,
>an increase in the characteristic impedance (Zo) will help to reduce IR
>losses due to skin effect.
>Now, we know that Zo = sqrt(L/C). Likewise the IR losses due to skin
>effect can be summarized generally as 4.34 (R/Zo+GZo) in dB/meter. R and
>G are the load resistance and admittance.
>Conceptually, it also makes sense that if a transmitter sees a larger Zo
>(the transmitter does not see the load directly, but only sees the line
>ahead), less current will flow, and since the load doesn't change, there
>will be less power loss. But if the drivers are current sources, then the
>current should be constant, and a larger Zo serves only to cause more IR
>loss. Here I am looking for some math to clear all this up rather than to
>rely on intuition.
>Cisco Systems, Inc.
>Gigabit Systems Business Unit
>170 W Tasman Drive
>San Jose, Ca 95134-1706
>Phone: (408) 525-9185
>Fax: (408) 527-8254
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