FreeLists / si-list / [SI-LIST] Re: Spice

[Ivor Bowden <ivor@xxxxxxxxxxx>]

Thank you for the kind comments.
I probed the input (after the series resistor) and output (at the 
termination resistor) of the transmission line. I thought that since I 
used a lossless line, I thought the end point should be a perfect square 
wave, delayed by the line (input & output look the same). I used a 50 
ohm 50nS transmission line. What I see is the transmission line input is 
perfect square wave, at the output the rise/fall time is slower and 
there is overshoot on both edges. If I change the generator to have some 
risetime (like in the real world) the overshoot goes away, but still the 
rise/fall time is greater than the input. Varying the terminations 
produced steps and reflections as expected.

What really confused me is I set up a lossy line based on the L/C 
characteristics of a 50 ohm PCB trace. The output of that seemed clean, 
but the lossless node waveforms changed! Even when I disconnected and 
grounded the lossy line the output of the lossless line was dramatically 
changed. I think simply having a lossy line in the circuit affects the 
algorithm, but I didn't expect such a dramatic difference.

I would like to model a signal source driving a short PCB trace, then 
through a connector to several feet of cable into a load, so I can see 
the results of changing the signal source circuit. The frequency isn't 
too high, rise/fall times of a few nS. I'm willing to start with 
ignoring the trace and connector for a first attempt. Do you have any 
suggestions?

I have a lot to learn..

Thank you,

Ivor

on 2/3/2005 10:15 AM Alan Hilton-Nickel wrote:

>Ivor,
>
>I could be wrong, but I don't know of a better place to ask SPICE 
>questions. As for newbie questions, you've gone about it the right way 
>by indicating you've invested some effort before asking a pretty 
>specific question. It sounds like your simulation model is correct, at 
>least on paper, but I don't know what you mean when you say the waveform 
>isn't what you expected. You should be getting a "step" waveform, with 
>the voltage rising initially to the Vin/2 level and staying there (since 
>you're perfectly terminated). If you vary the termination at the far end 
>you should see some reflections. What waveform did you get? And where in 
>the circuit are you probing?
>
>Alan
>
>
>Ivor Bowden wrote:
>
>  
>
>>Hi SI Experts,
>>
>>I have finally decided to get to know Spice, after many years of circuit 
>>design work, mostly digital. To this end I downloaded the SIMetrix Intro 
>>v5.0f program, and also acquired a copy of PSPICE 9.1 Student Version. I 
>>have only worked with SIMetrix so far.
>>
>>I realize this is probably not the place to ask newbie questions about 
>>basic SPICE operations, so my first question is, can anyone suggest any 
>>good resources (url, list server, news group, etc) to find useful 
>>information and / or ask these types of questions? Web search finds 
>>overwhelming hits to pick through, specific recommendations are appreciated.
>>
>>That being asked, my next question is what is the appropriate way to 
>>model a coax cable or a PCB trace, driven by a pulse? I set up a pulse 
>>voltage source (which I assume to be "perfect", that is no source 
>>impedance), a 50 ohm series resistance to represent the source 
>>impedance, a 50 transmission line grounded on both sides of one 
>>conductor, and a 50 ohm resistor to ground at the end. But the resulting 
>>waveform was not exactly what I expected..
>>
>>Thanks for any comments,
>>
>>Ivor
>> 
>>
>>    
>>
>
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