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发表于 2021-7-8 07:47:58
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如果电路中有较为理想或简单的模型, 可能导致波形不连续, 从而引起很小的time step. 如果有这个可能, 可以试试设置spectre tran的cmin为一个恰当的值, 比如1fF.
4.3.7.1 Discontinuities
There are two situations for which the simulator’s natural tendency
to reduce the time step does not improve convergence. First, if the
models for the nonlinear capacitors are not continuous, Newton’s
method can get hung up on the discontinuities and never converge.
Shrinking the time step actually makes things worse because it results
in the discontinuous capacitors dominating over the presumably
continuous linear and nonlinear resistors. Unfortunately, discontinuous
models are a fault found in circuit simulators too often. The
only real solution to this problem is for the simulator to be fixed so
that it supplies continuous models.
The second situation for which reducing the time step does not improve
convergence is if the waveforms exhibit discontinuous jumps.
This most often occurs in circuits with overly simplified models that
exhibit positive feedback, such as relaxation oscillators or Schmitt
triggers. Jumps can occur when the circuit contains nodes that do
not have a capacitive path to ground. In this case, shrinking the time
step does not bring the previous time point into the region of convergence
for the current time point because the transition is infinitely
fast. In practice, waveforms generated by real circuits cannot jump
discontinuously, and so the circuit must be incompletely modeled.
Generally, a small capacitor from the troublesome node to ground
solves the problem. Spectre provides the transient parameter cmin,
which when nonzero connects a small capacitor from every node to
ground. This slows down the transitions enough so that Spectre can
follow them without convergence problems.
K. S. Kundert, The designer’s guide to SPICE and Spectre. Boston: Kluwer Academic Publishers, 1995.
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