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

[Ivor Bowden <ivor@xxxxxxxxxxx>]

Fred, I am using the SIMetrix Intro v5.0f program ( 
http://www.catena.uk.com/index.html ). I did try changing the step size 
and it seems to make no difference for the lossless line.
The netlist I have (generated by the schematic GUI) is
*#SIMETRIX
T2 Probe3_NODE 0 Probe4_NODE 0 Z0=50 TD=50n
V1 V1_P 0 PWL 0 0 500n 0 500n 1 1.5u 1 1.5u 0
.GRAPH Probe3_NODE curveLabel="Probe3-NODE" nowarn=true ylog=auto xlog=auto
.GRAPH Probe4_NODE curveLabel="Probe4-NODE" nowarn=true ylog=auto xlog=auto
R4 0 Probe4_NODE 50
R3 Probe3_NODE V1_P 50
.TRAN 100p 2u

This results in a perfect square wave 500mV amplitude at probe3 
(transmission line input), and at probe4 (transmission line output) 
there is rise / fall time of around 20nS with over 100mV overshoot.

So is it that the transmission line model is the appropriate way to 
implement a PCB trace or cable, but the Spice model or program I am 
using isn't doing it well? Or have I made some horrible error?

Thanks,

Ivor

on 2/3/2005 4:03 PM Fred Balistreri wrote:

>Ivor, you did not say which SPICE program you are using. And presumably
>you are using the T element and not cascaded LC networks. The original
>T element in SPICE is quite poor. Most modern day SPICE programs
>will show the correct result for lossless lines. That is a delay without any
>signal degradation only if the line is perfectly matched (which you claim).
>I have to conclude that the SPICE program you have is very poor for
>transmission line analysis. Using the GEAR method may clean up some of
>the overshoot you see but it is not really the solution. Since your line is
>50ns long controling the time step may help this situation. This will slow
>down the simulation but may help. Something like
>this is required:
>
>.tran .1ns 200n 0 100p
>
>Where .1ns is the initial time step, 200n is the stop time, 0 is the time
>to start recording the data and 100p is the maximium time step allowed.
>
>Best Regards,
>Fred
>
>----- Original Message ----- 
>From: "Ivor Bowden" <ivor@xxxxxxxxxxx>
>To: <alan@xxxxxxxxxxxxxxxxx>
>Cc: <si-list@xxxxxxxxxxxxx>
>Sent: Thursday, February 03, 2005 12:20 PM
>Subject: [SI-LIST] Re: Spice
>
>
>  
>
>>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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>
>
>
>
>  
>


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