Spice simulation

[ingrast]

Originally posted by Bob Cordell

.......
Have you ever compared speed of Micro Cap and LTspice? I've heard that for most simulations LTspice is significantly faster than PSPICE or HSPICE. Supposedly, this was in part due to the difficulty of simulating switching power supplies over a large number of cycles, prompting a re-write of some critical routines by the LTspice team to speed it up.

Thanks,
Bob

I used Micro Cap for several years before switching to LTSpice. It is nice to use once the usual idiosicracies are sorted out. Magnetics are a pain in terms of speed, and FFT is not as easily worked with - tough this may be lack of expertise in my part, which also applies to LTSpice.

Models inserted in the spice text are surprisingly strightforward to work with unless you want to mess around with libraries.

I guess Bob is right about LTSpice, in fact I recall something about critical code rewriting for switching environments in the help section.

Rodolfo

PD. I know this is abusive, but Andy, could you refresh us about tricks regarding clean FFT's ....i.e. proper timestep selection, hacks etc....

[andy_c]

Originally posted by ingrast
I know this is abusive, but Andy, could you refresh us about tricks regarding clean FFT's ....i.e. proper timestep selection, hacks etc....

Hi Rodolfo,

Sorry I missed your post earlier. The only thing I do is a trick I learned in the LTSpice users' group. That is to first figure out the integer number of cycles over which the FFT will be taken. Then take the time span for this and divide it by (number of points in the FFT - 1) and choose that as the maximum time step for the transient sim. Then, when doing the FFT, choose an FFT stop time equal to the stop time of the transient sim, and the FFT start time as the stop time minus the time span for the integer number of cycles chosen. I usually simulate about twenty cycles total and take the last four or so. But this is only for viewing the spectrum plot.

If it's just harmonic distortion that you're concerned with, just use a "dot FOUR" directive in the schematic. This makes LTSpice automatically choose the last cycle of the transient sim for the FFT - freeing you from having to specify it. So in that case, I just take the time of one period and divide it by (number of points in FFT - 1) to get the maximum time step for the transient sim. The results from "dot FOUR" can be seen as text using "View, SPICE error log" after the transient sim completes. The text can be copied and pasted into Excel, which will parse them into columns nicely.

The idea of the time step choice is to make the FFT points coincident with the simulation points so LTSpice doesn't have to interpolate to get the FFT points. Of course, one must use ".options plotwinsize = 0" to disable waveform compression.

That's as far as I go with it. I've seen plots from JCX that have a residual way lower than mine, but I don't know what technique he's using. Maybe he could chime in. I'm not a DSP person, so I"m not familiar with all the windowing options and their advantages and disadvantages.

That's all I know

[GK]

Wouldn't it be worth creating a permanent SPICE thread for all of this stuff?

Cheers,
Glen

[Nelson Pass]

Originally posted by G.Kleinschmidt
Wouldn't it be worth creating a permanent SPICE thread for all of this stuff?

I vote for that. It would be great to develop a nice library of
models for parts.

[andy_c]

Originally posted by Nelson Pass
I vote for that. It would be great to develop a nice library of
models for parts.

Maybe a "SPICE Tips, Tricks and Models" sticky? I've noticed some very interesting SPICE posts in the Tubes forum and the Power Supplies forum too. So maybe Solid State would not be the best place for it, if such a sticky were created.

[andy_c]

Since there hasn't been much response beyond the initial discussion about a SPICE sticky thread, let me propose something.

"Electronics and Parts" has traditionally been the focal point of general CAD discussion, mainly about PCB design software. Also, there have been a lot of good posts about SPICE techniques that cover all device types - tubes and solid state, magnetics and other passives, scattered among the tubes forum and the solid state forum, and the power supplies forum as well. I'm proposing a post in "Electronics and Parts" that would introduce the idea of a focal point for SPICE-related tips, tricks and models - including model extraction techniques. It could start with a post containing links to other threads with useful information in this category, as well as links to external web sites with useful information.

If there is interest in such a thing, I'd be willing to serve as an email focal point for the initial post containing the links. What I'd like to see would be a link to the discussion or information in question, a short description of what the discussion is about, and a short description of why you think the discussion is important. Then I could condense this into a post that would kick the thread off. Maybe some of the previous posts in this thread, with tips about FFT techniques for distortion analysis, could be moved there, since they're off-topic for "BJT vs. MOSFET". I would ask, in return for this effort, that the moderators make this post a sticky.

Regarding the Wiki, I got the impression that the whole Wiki concept was born of the not-too-swift computer science concept of "learn a new computer language for every task to be performed". That's fine if, in learning the language, one can re-use the acquired skills for other useful things. Learning HTML is a good example of this. As I get older though, I become much less tolerant of abuses of my time, and the Wiki appears to be to be a classic example of that. Of course, I could spend the time to learn the Wiki techniques, but it appears their only application is yet further time abuse. So I am less than enthusiastic about the Wiki, unless there were some breakthrough that would greatly simplify the process.

[ingrast]

Originally posted by andy_c
Since there hasn't been much response beyond the initial discussion about a SPICE sticky thread, let me propose something.
......

A Spice and related issues clearinghouse should be a valuable addition to this forum, whatever the form it takes as long as it is friendly to use.

Rodolfo

[andy_c]

Just to kick things off, I wanted to add a link to a post from the Tubes forum about a program called CurveCaptor. Here is the link:

Vacuum tube modeling software - beta testers wanted

I'd love to have something like this for creating models of solid state devices. It would be cool to be able to specify the graph area, data limits on x and y axes, and whether each axis is linear or log. Then, to be able to get just the V-I values from the graph would be great. When I do my own models, I have to painstakingly read the values from the graph manually. Ugh!

As you can see, the tube guys are doing some fantastic work in the area of SPICE model development.

Edit: Looks like the Engauge Digitizer might be just the ticket for this application.

[ingrast]

Andy,

What about SpiceMod?

The free version may be useful as long as the models generated are accurate. Do you have an evaluation?

Rodolfo

[bogdan_borko]

Enyone here using Circuit Maker? I`d love to pass to LT spice but i never maneged to add mosfet models to library... Could someone help me?

Could I copy someone`s folder with models from LT spice program files? Could you send your folder here Andy_C?