Lowpass Filter Design by Circuit-EM Co-simulation
Microwave filter is a two port network providing signaltransmission at passband and signal reflection or signal
absorption at stopband. Microwave filters are widelyused in various microwave systems (radar, radio
astronomy systems, base stations…).

Filter specs and design parameters are given in Tab. 1.
Filter type is lowpass and approximation is Chebyshev.
Table 1. Filter specification (spec) and design parameters

SpecParameter

Value[Unit]

DesignParameter

Value[Unit]

F pass

1 [GHz]Filter order5Filter Designer

F stop1.4 [GHz]R source

50 [Ohms]

Filter Designer (Fig. 1) is a user-friendly, wizard-style
application with straightforward two-stage approach tofilter 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 (steppedimpedance and stub resonators), ideal transmission line
and lumped LC ladder networks. A simulation-readycircuit model in WIPL-D Microwave is automaticallygenerated based on Filter Designer output.

Pass-band
attenuation

1 [dB]

R load50 [Ohms]

Stop-bandattenuation

20 [dB]

Ripple1 [dB]The topic of this paper is to present various possible
approaches to filter design provided by WIPL-Dsoftware on a simple example.

Three models of a low-pass filter, derived from thesame specs, are created using Filter Designer. The firstmodel is implemented as a LC ladder (Fig. 2), thesecond 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.

Figure 2. LC ladder filter in WIPL-D Microwave circuit
simulator

Figure 3. Transmission line ideal filter (stub type) in
WIPL-D Microwave circuit simulator

Figure 1. WIPL-D Filter Designer
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because the closed-form models showed this is enoughto capture the dynamics of the filter.
Parameters S11 and S21 of the LC, microstrip andtransmission 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 modeland the full EM model are compared in Figs 8-9.
Figure 4. Microstrip filter in WIPL-D Microwave circuitsimulator

For purposes of verification, full EM simulation in
WIPL-D Pro will be performed. The EM model of thecomplete filter is shown in Fig. 5.

The 3D EM filter model has the same dimensions as themodel 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 somecircuit 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.

Figure 5. Microstrip filter in WIPL-D Pro 3D EM solver

WIPL-D Simulation
Dielectric characteristics for the microstrip model aregiven in Tab. 2. H is substrate height, TgD is loss
tangent, while T is metallization thickness.

Figure 6. S11 parameter of LC, Tx and microstrip models
in WIPL-D Microwave circuit simulator

Table 2. Dielectric characteristics

Parameter

Value [Unit]

Er

3.38

H

0.508 [mm]Sigma

58.8 [MS/m]

TgD

0.0027

T0.017 [mm]Frequency range of interest is from 0.014 GHz up to
2.8 GHz, and it is scanned in 40 points in oursimulations except for 3D EM simulation where a morenarrow band was 0.014-2.2 GHz was scanned in 20
points. The band was narrowed for EM simulation

Figure 7. s21 parameter of LC, Tx and microstrip models
in WIPL-D Microwave circuit simulator

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Table 3. Analysis characteristics

Figure 8. S11 parameter of microstrip model simulated in
WIPL-D Microwave circuit simulator and WIPL-D Pro
3D EM solver

ModelTime @ 0.0142.8 GHz [sec]

LC

<1

Tx

<1

Microstrip<1

Microstrip/3D
EM

5227

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 afirst 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 accountonly in the EM model of the entire circuit, so it isimportant to verify the results in this way.

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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