Micro-Cap v7.1.6 / RM

Stepping summary

The most important things to remember when using stepping include:

1.The parameters of these components may not be stepped:

•Transformer

•User sources

•Laplace sources

•Function sources

•SPICE dependent sources (E,F,G,H sources)

•Old switches (S and W switch parameters can be stepped)

The behavior of User, Laplace, Function, and SPICE sources are embodied in algebraic formulas and numeric tables, and thus have no parameters to be stepped. You can step symbolic parameters which are used in Laplace, Function, and SPICE source functions. For example, you could use

.DEFINE TAU 5

A Laplace function source whose LAPLACE attribute is:

1/(1+TAU*S)

could then be changed by stepping TAU.

Similarly in a Laplace table source like

.DEFINE RVAL 2.0

.DEFINE TAB (1k,0,RVAL)

you could change the behavior of the source by stepping RVAL.

The User source gets its data from an external data file and has no parameter that can be stepped.

2. In Component mode, stepping affects one parameter of one device only if the PRIVATEANALOG and PRIVATEDIGITAL options (Global Settings) are enabled. In Model mode, stepping affects one parameter of all devices that use that model name. Thus you are potentially affecting many devices.

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This is true regardless of the state of the PRIVATEANALOG or PRIVATEDIGITAL flags. In model stepping, AKO models track stepped parameters in the parent model and all of the temperature model parameters are available for stepping.

3.The MOSFET Level model parameter may be stepped, but an error will occur if the model statement has been created with parameters for level 1, 2, or 3 and the level changes to a BSIM model (Level 4, 5, or 8). Each BSIM model uses model parameter names that are different from the other BSIM models as well as the level 1, 2, and 3 models, so the parameter names would be meaningless.

4.Linear stepping starts with the From value and adds the Step Value until it reaches the To value. Log stepping starts with the From value and then multiplies by the Step Value until it reaches the To value. A Step Value of 2 is often convenient and is called octave stepping. A Step Value of 10 is sometimes referred to as decade stepping. List stepping simply uses the values specified in the From field.

5.If multiple parameters are to be simultaneously stepped, they must each specify the same number of steps. If there is a mismatch, an error message is generated.

6.At least two parameters must be varied to create 3D plots where a performance function is chosen for the Y axis.

7.Stepping a resistor, capacitor, or inductor that uses an expression for its value, replaces the value calculated from the expression with the step value. In other words, the step value takes precedence over the calculated value.

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Chapter 16 Monte Carlo Analysis

What's in this chapter

After successfully simulating a circuit, a user may want to know how the circuit performance is affected by parameter variation. Monte Carlo analysis provides a means of answering that question.

During Monte Carlo analysis, multiple runs are performed. For each run, a new circuit is generated from components whose numerical parameter values are randomly selected. The selection process is based upon user-specified parameter tolerances and distribution types. MC7 extracts performance characteristics from each run and displays the information graphically in the form of histograms and numerically in the form of statistical parameters.

Monte Carlo-related features new in Micro-Cap 7

•Ability to identify curve branches by labeling them and by selecting them with the ALT key and a mouse click or the Go To Branch feature.

•Ability to use performance function expressions in histograms instead of single performance functions.

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distribution. DEV tolerances require the use of private libraries, regardless of the state of the PRIVATEANALOG or PRIVATEDIGITAL flags. These flags are set in Options / Global Settings.

This sample specifies a 10% absolute and 1% relative BF tolerance:

.MODEL N1 NPN (BF=300 LOT=10% DEV=1%)

In this example, assuming a worst case distribution, the first N1 model is randomly assigned one of the two values 270 or 330. These two values are calculated from the mean value of 300 and 10% LOT tolerance as follows:

BF = 270 = 300 - .1•(300)

BF = 330 = 300 +.1•(300)

Suppose that the LOT toleranced BF value was randomly chosen to be 330. Then all N1 transistors, including the first, are randomly given one of these values, based upon the 1% DEV tolerance:

327 = 330 - .01•300

333 = 330 + .01•300

If the LOT toleranced BF value had been randomly chosen to be 270, then all N1 transistors, including the first, would be randomly given one of these values, based upon the 1% DEV tolerance:

267 = 270 - .01•300

273 = 270 + .01•300

Assuming a worst case distribution, all BF values in any particular run would be chosen from the set {267, 273, 327, 333}.

Resistors, capacitors, and inductors must be toleranced through their multiplier model parameter. This example provides a 10% LOT tolerance and a 1% DEV tolerance for a resistor.

.MODEL RMOD RES (R=1 LOT=10% DEV=1%)

Any resistor that uses the RMOD model will be toleranced, since the toleranced R value will multiply its resistor value.

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[t&d] specifies the tracking and distribution, using the following format:

[/<lot#>][/<distribution name>]

These specifications must follow the keywords DEV and LOT without spaces and must be separated by "/".

<lot#> specifies which of ten random number generators, numbered 0 through 9, are used to calculate parameter values. This lets you correlate parameters of an individual model statement (e.g. RE and RC of a particular NPN transistor model) as well as parameters between models (e.g. BF of NPNA and BF of NPNB). The DEV random number generators are distinct from the LOT random number generators. Tolerances without <lot#> get unique random numbers.

<distribution name> specifies the distribution. It can be any of the following:

If a distribution is not specified in [t&d], the distribution specified in the Monte Carlo dialog box is used.

To illustrate the use of <lot#>, suppose we have the following circuit:

Figure 16-1 Uncorrelated RE values

In this example, Q1's RE value will be uncorrelated with Q2's RE value. During the Monte Carlo runs, each will receive random uncorrelated tolerances.

232 Chapter 16: Monte Carlo

Now consider this circuit:

Figure 16-2 Using <lot#> to correlate RE values

Here, the presence of LOT/1 in both RE tolerance specs forces the LOT tolerance of the RE values to be the same. The values themselves won't be the same since their nominal values (1.0 and 2.0) are different.

DEV can also use [t&d] specifications. Consider this circuit.

Figure 16-3 Using <lot#> in DEV and LOT

Here the LOT toleranced RE values will track perfectly, but when the DEV tolerance is added, the values will be different due to the use of different generators for DEV.

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Tolerancing symbolic parameters

Symbolic parameters, those created with a .DEFINE statement, may also be toleranced. The format is as follows:

.DEFINE [{lotspec devspec}] <varname> <expr>

where the format of lotspec is the same as for other parameters:

[LOT[t&d]=<value>[%]]

and the format ofdevspec is similar:

[DEV[t&d]=<value>[%]]

[t&d] specifies the tracking and distribution, using the usual format:

[/<lot#>][/<distribution name>]

For example,

.DEFINE {LOT/1/GAUSS=10%} RATE 100

This defines a variable called RATE that has a Gaussian distribution with a LOT tolerance of 10% and its tolerances are based on random number generator 1.

Here is another example:

.DEFINE {LOT/1/GAUSS=10% DEV/2/UNIFORM=2%} VOLTAIRE 100

This defines a variable called VOLTAIRE that has a Gaussian distribution with a LOT tolerance of 10% and a uniform distribution with a DEV tolerance of 2%. Its LOT tolerance is based on random number generator 1 and its DEV tolerance is based on random number generator 2.

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Tolerances and public vs. private libraries

In order to accomplish relative DEV tolerancing, it is necessary to have private libraries. Consider this case:

.MODEL N1 NPN (BF=300 LOT=10% DEV=1%)

Because of the DEV tolerance, each instance of a BJT using the N1 model will have a unique BF. This can only happen with private libraries. Thus, when a model uses a DEV tolerance, the PRIVATEANALOG or PRIVATEDIGITAL flags are enabled, forcing the use of a private library for all parts which use the model. The flags are not affected for other models.

To summarize, for all parts which use the model statement:

If DEV is used, then all devices have their own private model parameter set, regardless of the PRIVATEANALOG or PRIVATEDIGITAL flag settings, and may have different parameter values if the tolerances are not zero.

If DEV is not used, and PRIVATEANALOG or PRIVATEDIGITAL are disabled, then all devices that use the same model name will have the same parameter values, since public libraries are being used.

If DEV is not used, and PRIVATEANALOG or PRIVATEDIGITAL are enabled, then all devices that use the same model name may have different parameter values, if the tolerances are not zero.

This table summarizes how the parameters of two parts using the same model vary depending upon DEV use and the PRIVATE flags.

This table summarizes how the parameters of two parts using the same model vary depending upon LOT use and the PRIVATE flags.

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