Monte Carlo analysis is a statistical analysis of a system to check the possible variations in its characteristics due to the variations of its components. The MicroCap program can perform such an analysis of the circuits that are entered into it. If you plan to use Monte Carlo analysis on a circuit you must first specify a tolerance model for that component. The model statement must be typed on the text page. The first line on the text page shown below is a model for ±1% tolerance resistors.
This model must then be selected for each component in the circuit that is to have that characteristic. If other resistors are to have different tolerances then a Model statement must be included for each different tolerance to be used. If other components are to have tolerances then they must have Model statements. For each component that is to have a tolerance the appropriate model statement must be selected in the component specification window. Any resistor model statements that you have included inthe text page will appear by name in the lower right windor of the resistor window.
After you have completed the circuit and are ready to analyze it, go to the appropriate analysis window and select the analysis limits you want to use. Then run the analysis. This will give you a plot of the operation with the nominal values of the components. Each of the analysis output windows has Monte Carlo listed in its menu at the top of the screen. Select Monte Carlo and then select Options.
From the Monte Carlo Options box you can turn the Monte Carlo analysis on by clicking the on status button. You can then select Uniform, Gauss, or Worst Case as the type of statistical distribution to use for the variations of the components. Then you can select the number of cases to be run.
Uniform will randomly select values that are uniformly distributed between the limits specified for each component. Gauss causes a Gaussian or normal (bell curve) distribution to be used. Worst case causes each component to randomly receive either the maximum or the minimum value in the tolerance range. In all cases the first plot case will be made using the nominal values of the components than all the following cases will receive different random selections for each component that has a tolerance specified. This simulates the random insertion of parts from a bin of parts along the production line in a manufacturing plant. The resulting analysis gives statistical information on the expected variations you will see between systems selected at random as they come off the production line. The full version of Micro Cap allows you to plot histograms of the variations of max and min values for most of the variables, but the Demo version limits this to rise times only.