Hints on using individual LTspice commands and things the Help Manual sometimes may not help you with

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Revision as of 00:10, 10 August 2009 by Analogspiceman (talk | contribs) (.Param and Curly Brackets)

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.Param and Curly Brackets


One of the more confusing aspects of using .param (at least when I began using LTspice) was where and when do you use the curly barckets { } ?

Beginning with the simplest definition, LTspice doesn't require curly brackets:

.param res1 = 1000

This could be used to define a resistor of 1000 Ohms.

However, in the real world, a parameter sometimes needs to be defined from other parameters.

To get results and not pull errors within LTspice, it becomes useful to use curly brackets. Without them - you can only nest (calculated) .param values, one parameter calculation deep.

Going any deeper (where one calculation depends on another etc.) will invite LTspice to complain with an error.

Correct Method (doesn't pull errors)

* Comment line : Calculate capacitance

* -------------------------------------

.param length = {3e-2}
.param width  = {4e-3}

.param area = {length * width}

.param gap = {40e-6}

.param Eo = {8.85e-12}

.param Capa = { (Eo * gap) / area}

.MEAS Capa_  PARAM Capa

The answer (from the error log ) is:

capa_: capa=2.95e-012

To see the results in LTspice use the Error Log (a better name would have perhaps been Results Log or Output Log)

Incorrect method (pulls errors)

.param length = 3e-2 

.param width  = 4e-3

.param area = length * width

.param gap = 40e-6

.param Eo = 8.85e-12

.param Cap =  (Eo * gap) / area

.MEAS Capa_  PARAM Capa


Measurement "capa_" FAIL'ed

The quickest way to check a calculation before you commit it to a library is to create a section of text within the LT schematic. Set the text as LTspice directive (command), then run the schematic

Finding the measure results of a simulation Check your results in the error log Found from the toolbar : View/Spice error Log

Suggestion: Set the Tools/Control panel/ Operation / Generate Expanded Listing and tick (yes) to see a more comprehensive output listing - helps with debugging