# [SI-LIST] Re: Transmission Line question

• From: steve weir <weirsi@xxxxxxxxxx>
• To: darshanmehta2k@xxxxxxxxx, si-list@xxxxxxxxxxxxx
• Date: Wed, 09 Feb 2005 03:19:22 -0800
```Darshan ask yourself what happens when an unterminated line is lambda /
100?  What about lambda / 10?  Lambda / 6?  Lambda / 5 ?  Lambda /
4.00000001?  Lambda / 3.99999999?   Is there a critical um less than which
the line acts like a simple delay, and beyond which all that stored energy
in the line becomes problematic?

Steve
At 01:45 AM 2/9/2005 -0800, Darshan Mehta wrote:
>Hello Guys,
>
>I have small doubt about Transmission line.
>
>As we know if the rise time of the Driver is greater than the Round trip
>time, the signal will see the interconnect as Open and there will not be
>any need for termination Resistor. I was trying to calculate the critical
>length for particular driver. Here are the input data I have;
>
>Er = 4.5
>Magnetic Permeability = 1
>Driver Rise time = 1.9 ns
>
>So now velocity of signal = v = 12/sqrt(Er) = 12/sqrt(4.5) = 5.78 inches/ns
>
>Now if I want to calculate the critical length, L = 5.78 * (to and fro
>time for signal) = 5.78 * (rise time/2) = 5.78 * (1.8/2) = 5.2 inches
>
>But in simulation tool, it says that Driver to receiver length should be <
>1.762 inches. That means if we have a trace with length of 1.762 inches or
>less, there will not be any need for Termination resistor as the trace
>will not behave like Open.
>
>So my question is, why my calculation differs from Simulator result. Is I
>am doing something wrong?
>
>Thanks,
>Darshan Mehta
>
>
>---------------------------------
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