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Microwave filter is a two port network
providing signal transmission at passband and signal reflection or
signal absorption at stopband. Microwave filters are widely used in
various microwave systems (radar, radio astronomy systems, base
stations…).
The topic of this paper is to present various possible approaches to
filter design provided by WIPL-D software on a simple example.
Filter Designer
Filter Designer (Fig. 1) is a user-friendly, wizard-style application
with straightforward two-stage approach to filter design. It is intended
for automated design of lowpass, highpass, bandpass and bandstop filters
of Chebyshev and Butterworth type. Currently supported filter
implementations are microstrip (stepped impedance and stub resonators),
ideal transmission line and lumped LC ladder networks. A
simulation-ready circuit model in WIPL-D Microwave is automatically
generated based on Filter Designer output.
Filter specs and design parameters are given in Tab. 1. Filter type is
lowpass and approximation is Chebyshev.
Three models of a low-pass filter, derived from the same specs, are
created using Filter Designer. The first model is implemented as a LC
ladder (Fig. 2), the second as transmission line (Tx) stubs (Fig. 3).
The third model is implemented as a microstrip stub filter (Fig. 4).
These three models are simulated in WIPL-D Microwave circuit simulator
using closed-form equation components.
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Figure 1. WIPL-D Filter Designer
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Table 1. Filter specification (spec) and design parameters
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Figure 2. LC ladder filter in WIPL-D
Microwave circuit simulator
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Figure 3. Transmission line ideal filter
(stub type) in
WIPL-D Microwave circuit simulator
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For purposes of verification, full EM simulation in WIPL-D Pro will be
performed. The EM model of the complete filter is shown in Fig. 5.
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Figure 4. Microstrip filter in WIPL-D
Microwave circuit simulator
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Figure 5. Microstrip filter in WIPL-D Pro
3D EM solver
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WIPL-D Simulation
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Dielectric characteristics for the microstrip model are given in Tab. 2.
H is substrate height, TgD is loss tangent, while T is metallization
thickness.
Frequency range of interest is from 0.014 GHz up to 2.8 GHz, and it is
scanned in 40 points in our simulations except for 3D EM simulation
where a more narrow band was 0.014-2.2 GHz was scanned in 20 points. The
band was narrowed for EM simulation because the closed-form models
showed this is enough to capture the dynamics of the filter.
Parameters S11 and S21 of the LC, microstrip and transmission line
circuit models are compared in Figs 6-7. Microstrip closed-form
component models were used in the corresponding circuit model (Fig. 4).
Parameters S11 and S21 of the microstrip circuit model and the full EM
model are compared in Figs 8-9.
The 3D EM filter model has the same dimensions as the model in circuit
simulator. Differences in the results exist, which is expected because
closed-form equations do not take into account all the physical effects
that EM simulation does. Some of these effects are mutual EM coupling
between discontinuities and radiation of some circuit elements.
Simulation times are given in Tab. 3. Computer used for these
calculations is Pentium® Dual-Core CPU E5200 @ 2.50 GHz 2 GB RAM, except
for 3D EM simulation where Intel® Xeon® CPU E5345 @2.33 GHz with 23.9 GB
RAM is used. |
Table 2. Dielectric characteristics
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Figure 6. S11 parameter of LC, Tx and
microstrip models in WIPL-D Microwave circuit simulator
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Figure 7. s21 parameter of LC, Tx and
microstrip models in WIPL-D Microwave circuit simulator
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Figure 8. S11 parameter of microstrip
model simulated in WIPL-D Microwave circuit simulator
and WIPL-D Pro 3D
EM solver
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Figure 9. S21 parameter of microstrip
model simulated in WIPL-D Microwave
circuit simulator and WIPL-D Pro 3D
EM solver |
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Conclusion
Using WIPL-D Filter Designer we can easily create initial filter models
that approximately satisfy design specs. Circuit models automatically
generated in WIPL-D Microwave can be simulated using closed form
component models from library so initial S-parameter results can be
obtained in seconds. However, we know that closed-form models have
limited validity ranges and don’t take into account all the physical
effects. Hence, models using closed-form equations can not be considered
accurate enough. As a first step, accuracy can be increased if
closed-form models are replaced by library EM component models in the
circuit simulator. However, EM coupling between circuit components can
be taken into account only in the EM model of the entire circuit, so it
is important to verify the results in this way.
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Table 3. Analysis characteristics
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If you have a
specific problem in mind and you are not sure if WIPL-D software can
handle that problem,
contact us. We will analyze your needs
and try to help, and if our products satisfy your requirements, we will
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