Frequency Domain Solver Problem Handling
This page contains a list of the most important warning messages together
with a detailed explanation of the meaning and proposal for handling and
resolution.
The message is displayed by the Frequency Domain
Solver with tetrahedral mesh if farfield monitors and "open"
or "open (add space)" boundary conditions are defined. Different
kinds of open boundaries are available for the solver. Each realization
of the open boundary condition has its particular advantages and disadvantages.
The default "standard impedance boundary condition" provides
results at low computational costs. In order to increase the accuracy,
other choices are available in the solver
specials. Please consider enabling the "add space before mesh
generation" option, which adds more surrounding space to increase
the distance from the structure to the open boundary even beyond the visible
bounding box with "open (add space)" boundary conditions. Even
higher accuracy (lower artificial reflection at the open boundary) can
be obtained at higher computational cost by using the PML. See the section
Open boundary frame in the solver specials
for further details.
One of the three recommendations is displayed (the
first condition is considered as an error in the setup) if the relation
between two accuracy settings of the solver needs to be improved: first
of all, the linear equation system solver's relative residual threshold
"yyy" (referred to as "Accuracy"
in the Solver settings frame of
the Frequency Domain
Solver Parameters, and second, the broadband sweep's convergence threshold
"xxx", see the S-parameter
error threshold value in the Frequency
Domain Solver Sampling dialog.
Please follow the proposal displayed in the text
or choose an even lower threshold for the linear equation system solver's
accuracy. If you change the settings accordingly, the broadband frequency
sweep possibly requires fewer frequency samples. Usually this results
in an increased overall solver performance.
Similar statements hold, as an example, for the
S-parameter threshold of the adaptive mesh refinement. It should be chosen
good enough for the broadband frequency sweep, but no better than the
linear equation system solver's accuracy setting allows.
This warning is displayed if frequency sampling
intervals have been defined with some "gaps" where the solver
is not allowed to place new automatically chosen frequency samples. Please
check your settings in the Frequency
samples frame of the Frequency
Domain Solver Parameters dialog.
For instance, in a global frequency range from 5
to 10 GHz, imagine that two sampling intervals have been defined: the
first one from 5 to 7 GHz and the second from 9 to 10 GHz. The solver
can place new frequency samples in those intervals only. The S-parameter
values in the "gap" from 7 to 9 GHz are then influenced by the
frequency samples in the two intervals only. Hence, the S-parameter sweep
may fail to converge, or: the S-parameter's accuracy might be low in these
"gaps" of the global frequency range. It is recommended to run
two simulations in that case, one for each interval individually.
A second example is if there is only one automatic
frequency sampling interval and one automatic adaptive mesh refinement
sample that is not contained by the automatic frequency sampling interval.
With the global frequency range of 5 to 10 GHz as an example, the default
mesh adaptation frequency will be 10 GHz. Now if the automatic frequency
sampling interval is limited to the range 5 to 7 GHz, there is a "gap"
from 7 GHz to the adaptation frequency at 10 GHz. In that case, please
move the mesh adapatation frequency into the interval.
Those messages or warnings are displayed in the
context of the adaptive mesh refinement. For the sake of accuracy, it
is important to have a mesh adaptation sample at some frequency where
power is delivered into the structure, for instance in the passband of
a filter. If the mesh adaptation frequency is defined at a frequency where
most of the input power is reflected, the error indicator will not "see"
the possibly more important interior parts of the structure, and the mesh
refinement will focus on the terminals of the structure rather than refining
in the inner regions.
The solver may therefore stop the adaptive mesh
refinement if the minimum input reflection of all S-parameters at the
present adaptation frequency seems to be too high. It attempts to insert
new adaptation frequencies with a trial-and-error approach that covers
the whole frequency range, starting with monitor frequency samples, if
any. The number of attempts to "move" the automatic adaptation
frequency samples is limited. If no suitable frequency is found, the adaptive
mesh refinement will continue at the first adaptation frequency again.
In this case, please choose and define a suitable constant adaptation
frequency in the Frequency samples frame
of the Frequency
Domain Solver Parameters dialog.
Mesh adaptation samples which are not defined as
"Automatic" and hence are constant will not be moved, for instance
single frequencies or equidistantly spaced samples. If an "Automatic"
sampling interval definition has more than two samples, its lowest and
its highest frequency will be calculated in any case because the S-parameters
at those frequencies are required for the broadband frequency sweep.
 The solver has detected that regions exist
which are not electrically connected to any other part of the model. This
often is an unwanted side effect of the modelling. If the regions are
separated and solved one at a time, or if a small cavity can be "filled",
the structure is simplified and thus easier to solve. An isolated region
which does not contain any source is free of fields and can be filled
by perfect electric conducting (PEC) material without changing the solution.
As an example, a coaxial line (gray) is sketched, the inner conductor of
which is plugged into some socket (brown). This leaves a vacuum cavity
between the inner conductor and the socket, which is highlighted in light
blue in this illustration.
Please consider removing this cavity for instance by adding a PEC inset.
This hint is displayed for the fast resonant Frequency
Domain solver with tetrahedral mesh to refer to a way to combine the general
purpose solver's adaptive tetrahedral mesh refinement with the fast resonant
solver. Instead of running the fast resonant solver directly with its
own mesh refinement, there is an option to run the fast resonant solver
after the general purpose solver's mesh refinement:
Switch to the "General Purpose" Method in the corresponding
frame of the Frequency
Domain Solver Parameters. Click on Properties...
next to Use broadband frequency
sweep (enable if necessary) to bring up the Frequency
Domain Solver Sampling dialog.
Enable Run
after adaptive mesh refinement if applicable in this dialog to
let the general purpose solver stop after the adaptive mesh refinement
without performing the interpolative sweep. Afterwards, the fast resonant
solver method will be executed to calculate the broadband results. This
is an alternative and efficient way of using the fast resonant solver
method, especially if the faster single point adaptive mesh refinement
of the general purpose method is preferred.
If a required feature prevents the fast resonant
solver from being applied, the default interpolative sweep is used. The
solver log file provides a hint about the missing features, which possibly
can then be removed (for instance, some field monitors which are unsupported
by the fast resonant solver.)
Most of the prerequisites have been checked if this
message is displayed. There is one possible exception for those cases where the
port mode calculation detects modes other than TEM, TE, or TM.
The fast resonant solver will still be used for the sweep in that case (in order
to avoid those modes being approximated, please turn off
Run after adaptive mesh refinement
if applicable to force the interpolative sweep.)
This information is displayed when the iterative
solver is used with up to "xxx" excitations calculated in parallel.
While the direct solver usually benefits from all the cores available
on the CPUs in the computer and thus uses as many threads as cores are
available, the iterative solver may choose to use less threads. This is
because of mainly two reasons: the memory requirements for the parallel
computation of excitations increase with the number of excitations, and
the memory bandwidth of the computer may limit the number of parallel
excitations for which an increase of performance is seen.
The default number for "xxx" is twice
the number of sockets in the computer, and was found to be a good choice
for many hardware architectures.
However, advanced users can overwrite the behavior
of the parallel iterative calculation of excitations by calling the "SetCalculateExcitationsInParallel"
method of the FDSolver object. See the Visual Basic (VBA) Language online
help for details.
As the iterative solver is running in parallel for
up to "xxx" excitations at the same time, there is no text output
of the relative residual norm per excitation into the message window.
But the progress of the iterative solver can be accessed in the Navigation
Tree, see "1D Results/Convergence/Solver/Residuals".
These messages are for instance shown with plane wave excitation only.
S-parameter results are used as a stopping criterion for the adaptive tetrahedral mesh refinement
in the general purpose frequency domain solver. In the course of the interpolative broadband frequency sweep, the choice
of "automatic" frequency samples is based on S-parameters, too.
Thus, if no S-parameters are defined, the solver will by default stop if the
maximum number of adaptive mesh refinement passes is reached.
However, any kind of 0D result template can be used as a stopping criterion with a few
modifications of the solver's mesh adaptation properties:
First define a result template which extracts a 0D result, for instance the magnitude of the electric field at some location
(see the Template Based Post Processing Overview for details.)
Then open the Adaptive Tetrahedral
Mesh Refinement properties. Deactivate the criterion "S-parameter" below "Check at discrete adaptation samples."
Activate the "0D Result Template..." below "Check after broadband calculation" instead and choose the
result template from the drop down list. If required, change the relative threshold and the number of checks. The solver will then
use the 0D result template as a stopping criterion for the adaptive mesh refinement.
"Automatic" frequency sample definitions are ignored if no S-parameter results are generated
during the simulation. For plane wave excitation this usually means that all frequencies of the field monitors will be calculated,
and no additional frequency samples are placed elsewhere unless the sampling definition is changed to "single",
"equidistant", or "logarithmic", as described in the
Frequency Domain Solver Overview.
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