[SI-LIST] How to use S-parameter accurately

Dear all
          I have some s-parameter simulation problem. I have s-parameter
file extracting from board file and 
simulate it by Hspice. According to the input signal, I extracted the
s-parameter with sufficient 
bandwidth and points (input signal: 125MHz, S-parameter bandwidth : 5GHz
1000pts/linear). 
When simulating, the output waveform will vary with input rise/fall time
enormously and also 
simulation time. The most accurate waveform will occur when the input
rise/fall time set to 0.8ns
and simulation time set to some specific value. Even though the
rise/fall time set to larger or smaller 
than 0.8ns the output waveform will get inaccuracy. 
          The s-parameter bandwidth is larger than knee frequency of
input signal and I also filtered out high harmonic 
component of input signal (input signal is an idea 50/50 duty pulse) by
5 order low-pass filter ( cut-off 
frequency set to 10 times the input knee frequency). Why the s-parameter
can just simulate at specific input 
condition accurately? How could I use s-parameter more accurately?
Thanks. 
 
Terence
 
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