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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 can be lowpass,
bandpass, bandstop and highpass. They can also be passive or active.
Here, we will focus on passive filters implemented in microstrip
technology. Microwave filters are used in every microwave system (radar,
radio astronomy systems, base stations…). Main characteristics of
microwave filters are
• Non-independent magnitude–phase response,
• Signal attenuation because of dielectric presence,
• Relatively difficult full 3D numerical analysis.
WIPL-D Simulation
Two models of band pass filters, simulated using WIPL D Pro 3D EM
solver, are presented here. Interdigital filter, which is an array of
resonators with electrical length of 90° at midband frequency, short
circuited at one end and open-circuited at other end alternatively, is
shown in Fig. 1. The edge-coupled half-wavelength (at the center of
range) resonator filter is shown in Fig. 2. Both models are implemented
in microstrip technology. Finite size substrates have been used in both
models. Ground plane as well as top metallization is considered to be
perfectly conducting.
Our aim is to inspect simulation times,
memory requirements, and S-parameters of analyzed models. We will
observe output results for interdigital filter starting from 1.9 GHz up
to 2.6 GHz, while for edge coupled resonators filter, output results
will be inspected from 2 GHz up to 2.7 GHz (D and E bands–NATO band
classification). Dielectric characteristics for simulated models are
given in Tab. 1.
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Figure 1. Interdigital band pass
filter |
Figure 2. Edge-coupled resonators
band pass filter |
Table 1. Dielectric
characteristics
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Parameters S11 and S21 for simulated
models, calculated in 15 points within the frequency ranges of interest,
are shown in Figs 3-4. We can see the expected behavior of these
filters, with a fractional bandwidth of about 10 %. Number of unknowns,
memory requirements, and simulation times for the whole frequency range
of interest are given in Tab. 2. Computer used for these calculations is Intel® Core2 Quad CPU @ 2.83 GHz.
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Figure 3. S-parameters for
interdigital filter |
Figure 4. S-parameters for
edge-coupled resonator filter |
Table 2. Analysis characteristics
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Model |
Number of unknowns |
Time (whole range) [sec] |
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Interdigital |
904 |
237 |
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Edge-coupled resonators |
452 |
27 |
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Conclusion
Microwave filters are very widely used in various application fields.
Using full EM simulation, we are able to get accurate results and thus
verify the filter design. Full 3D EM model takes into account EM
coupling between discontinuities and possible radiation of all circuit
elements. Results given here by WIPL-D Pro coincide with theoretical
expectations.
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