LF Analysis (lf)
Description
The LF analysis begins by linearizing the circuit about an operating point. This analysis detects poles that may potentially cause stability problems, and identifies loops that associate to potentially problematic poles.
Syntax
Name lf parameter=value ...
Parameters
Sweep interval parameters
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start
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0
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Start sweep limit.
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stop
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Stop sweep limit.
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center
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Center of sweep.
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span
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0
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Sweep limit span.
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step
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Step size, linear sweep.
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lin
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50
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Number of steps, linear sweep.
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dec
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Points per decade.
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log
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50
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Number of steps, log sweep.
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values
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[...]
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Array of sweep values.
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valuesfile
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Name of the file containing the sweep values.
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Sweep variable parameters
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dev
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Device instance whose parameter value is to be swept.
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mod
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Model whose parameter value is to be swept.
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param
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Name of parameter to sweep.
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State-file parameters
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readns
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File that contains estimate of DC solution (nodeset).
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write
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DC operating point output file at the first step of the sweep.
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writefinal
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DC operating point output file at the last step of the sweep.
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Output parameters
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oppoint
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no
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Determines whether operating point information should be computed:if yes, where should it be printed (screen or file). Operating point information is not printed if the operating point computed in the previous analysis remains unchanged. Possible values are no, screen, logfile and rawfile.
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Convergence parameters
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restart
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yes
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Restart the DC solution from scratch if any condition has changed. If not, use the previous solution as initial guess. Possible values are no and yes.
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Annotation parameters
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annotate
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sweep
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Degree of annotation. Possible values are no, title, sweep, status and steps.
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Miscellaneous parameters
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order
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Order of expansion around each pole to generate residue information. This option is used for model order reduction in node impedance computation.
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sensitivity
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3
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Sensitivity of the krylov method of pole detection and the accuracy of impedance computation. Higher values increase runtime while reduce the chance of missing a pole and improve the accuracy of impedance values. Possible values are 1, 2, 3, 4, and 5.
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solver_method
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auto
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If set to krylov, the modified rational krylov method is used for pole detection; if set to direct, QZ method is used; if set to auto, loopfinder chooses the method for pole detection automatically. Possible values are auto, direct and krylov.
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romsize
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6000
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Minimum number of nodes for applying the Krylov method. This option is used if solver_method=auto.
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zmin
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0.1 Ω
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Minimum DC impedance of interest in loop identification.
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dampmax
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0.7
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Maximum damping ratio, i.e. negative value of cos(phi) where phi is the argument of the corresponding pole, for targeting loops. This option is used in pole detection.
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freqtol
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(Hz)
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Relative error tolerance for natural frequencies of loops if modified rational krylov method is used for pole detection.
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freqmax
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1e10 Hz
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Maximum natural frequency for targeting loops.
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freqmin
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1e-2 Hz
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Minimum natural frequency for targeting loops.
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emptyloop
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yes
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If set to yes, empty loops, if any, are printed in the output; if set to no, only non-empty loops are printed. Possible values are no and yes.
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doublekrylov
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If set to no, krylov flow searches the region of interest once; if set to yes, krylov flow searches the region of interest twice to minimize the change of missing poles. Possible values are no and yes.
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print_all_poles
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no
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If set to yes, print full list of poles to the log file; if set to no, do not print full list of poles. This option is used in direct solver method. Possible values are no and yes.
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prevoppoint
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no
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Use the operating point computed on the previous analysis. Possible values are no and yes.
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force
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none
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The set of initial conditions to use. Possible values are none, node, dev and all.
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readforce
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File that contains initial conditions.
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skipdc
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no
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Skip DC analysis. Possible values are no and yes.
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useprevic
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no
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If set to yes or ns, use the converged initial condition from previous analysis as ic or ns. Possible values are no, yes and ns.
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By default, this analysis computes the operating point, if it is not known, or recomputes it if any significant component or circuit parameter has changed. However, if an operating point was computed during a previous analysis, you can set prevoppoint=yes to avoid recomputing it. For example, if prevoppoint=yes and the previous analysis was a transient analysis, the operating point is the state of the circuit at the final time point.
Nodesets help find the DC or the initial transient solution. You can specify nodesets in the circuit description file with nodeset statements or in a separate file by using the readns parameter. When nodesets are specified, Spectre computes an initial guess of the solution by performing DC analysis, while forcing the specified values on to nodes by using a voltage source in series with a resistor whose resistance is rforce. Spectre then removes these voltage sources and resistors and computes the required solution from this initial guess.
Nodesets have two important uses. First, if a circuit has two or more solutions, nodesets can bias the simulator towards computing the required solution. Second, this is a convergence aid. By estimating the solution of the largest possible number of nodes, you might be able to eliminate a convergence problem or significantly speed up convergence.
When you simulate the same circuit multiple times, it is recommended that you use both write and readns parameters and assign the same file name to both parameters. DC analysis then converges quickly even if the circuit has changed since the last simulation, and the nodeset file is automatically updated.
The loopfinder analysis does not work with frequency-defined or distributed devices properly. If there are frequency-defined or distributed devices, poles and node impedance are computed by approximating those devices as equivalent conductances and capacitances evaluated at 1Hz.
The default value of options freqtol, order and doublekrylov are controlled by the setting of the option sensitivity. The effect of sensitivity on other parameters is shown in the following table.
sensitivity freqtol order doublekrylov
-------------------------------------------------
5 1.0e-6 7 yes
4 1.0e-4 5 no
3 1.0e-3 3 no
2 3.0e-3 2 no
1 3.0e-3 2 no
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