PIERS Proceedings, Moscow, Russia, August 18–21, 2009
1214
A Novel Bandpass Defected Microstrip Structure (DMS) Filter for
Planar Circuits
M. Kazerooni1 , A. Cheldavi1 , and M. Kamarei2
1
College of Electrical Engineering, Iran University of Science and Technology, Tehran, Iran
2
Faculty of Electrical and Computer, University of Tehran, Tehran, Iran
Abstract— A new defected microstrip structure (DMS) unit lattice is proposed in order to
perform a bandpass filter (BPF). The proposed DMS provides the good cutoff and passband
characteristics. Also in this paper we extract the model of BPF. In order to show the improved
the parameters, several circuits were designed and simulated. The BPF has a bandwidth more
than 39%.
1. INTRODUCTION
Recently, the defected ground plane structures (DGS) and defected microstrip structures (DMS)
have been proposed for suppression of spurious response in the microstrip filters [1–4]. However,
the existing DGS and DMS configurations provide only the bandstop characteristic. In this paper,
we report a new DMS bandpass filter in a defected microstrip line. We also report its circuit model.
The desired and tuned frequency resonance can be obtained by changing the T-cell dimensions and
by the gap space at the beginning and end of structure.
2. BANDPASS DMS CIRCUIT AND EQUAIVALENT CIRCUIT EXTRACTION
The bandstop DMS with its parallel resonance equivalent circuit is reported in [1]. It behaves as
a lumped inductor at frequency below the parallel resonance frequency. The proposed bandpass
DMS configuration, shown in Fig. 1 is created by a coupling gap (g1 ) on a microstrip line. The
proposed structures are designed on a substrate with relative permittivity εr = 2.33 and thickness
h = 0.787 mm.
The parallel and series-parallel combined circuit models for bandstop and bandpass DMS configuration are shown in Figs. 2–3.
At frequency below the series resonance, the bandpass DMS configuration behaves as a capacitor.
The bandpass configuration shows both the series and parallel resonance. According to [4] we can
obtain the capacitance Cp in pF, the inductance Lp in nH and the resistance RP in ohm of the
equivalent circuit of a bandstop DMS shown in Fig. 2:
C=
fc
200π(f02 − fc2 )
(1)
Once the capacitance value of the equivalent circuit is extracted, the series equivalent inductance
50 ohm microstrip line
T-cell section
g1
g1
h
g
Substrate
Figure 1: Bandpass DMS configuration.
Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 18–21, 2009 1215
Figure 2: Bandstop DMS configuration and related equivalent circuit.
Figure 3: Bandpass DMS configuration and related Equivalent circuit.
and resistance for the given DMS bandstop unit section can be calculated as:
L =
1
(2)
4π 2 f02 C
µ
¶
1
1
1
Y =
=
+ j ωC −
Z
R(ω)
ωL
zin = z + z0
zin − z0
1
S11 =
=
zin + z0
1 + 2z0 y
1
q
R(ω) =
1
1
1
2
2z0
(s11 (ω))2 + (2z0 ( ωL − ωC)) − 1
(3)
(4)
(5)
(6)
where f0 and fC are the resonant frequency and 3-dB cutoff frequency, respectively. However in
resonance phenomena role of RP is important but in the lower and upper of this phenomena roles
of inductor and capacitor are important. For simplicity we ignore the frequency dependence of RP
and use a constant value for RP obtained for ω = ω0 . Using
S21 =
2z0
,
2z0 + z
(7)
1 − S21 (ω0 )
S21 (ω0 )
(8)
for ω = ω0 , z = RP and then RP is given as
RP = 2z0
PIERS Proceedings, Moscow, Russia, August 18–21, 2009
1216
To validate the circuit model, this model has been simulated using Ansoft Designer v3 (a circuit
simulator). The extracted RP , LP , and CP values from aforementioned equations are 3.0949 kΩ,
0.81297 nH, and 0.10471 pF, where fC and f0 from Fig. 4 are 11.25 and 17.25 GHz, respectively,
and S21 (ω0 ) = 0.0313. However these models are very simple but the error is acceptable. Fig. 4
shows good agreement of results of the circuit model with results of the Ansoft HFSS simulator.
In case of a bandpass DMS configuration, the equivalent inductance and coupling capacitance
form a series resonance circuit. However, when the gap spaces are used in the bandpass structure,
its pole fP frequency is changed by a small amount. We assume that its 3-dB cutoff frequency, fC
for the bandstopt DMS is also 11.25 GHz.
The bandpass structure has series resonance fS = 7.4 GHz and parallel resonance fP = 18.1 GHz.
They provided CP = 0.089 pF, LP = 0.868 nH and Cg1 = 0.532 pF for the circuit model. We have
assumed that the 3-dB cutoff frequency, for the bandpass case is also 6.12 GHz. Fig. 4 shows
Comparison of circuit model and full wave analysis
Comparison of circuit model and full wave analysis
0
0
fC
Magnitude(dB)-S11,S21
Magnitude(dB)-S11,S21
-10
S11
-10
Circuit model
-15
-20
S21
-25
S21
-15
S11
fP
0
5
10
Circuit model
fp
-20
-25
fs
-30
-30
-35
Full wave analysis
-5
Full wave analysis
-5
-35
15
20
freq(GHz)
25
30
35
-40
0
2
(a)
4
6
8
10
12
freq(GHz)
14
16
18
20
(b)
Figure 4: Magnitude frequency response due to circuit modeling and full wave analysis with a = 2.2756 mm,
b = 0.5728 mm, c = 1.3572 mm, g = 0.2 mm, g1 = 0.01 mm and W = 2.33 mm. (a) Bandstop DMS frequency
response. (b) Bandpass DMS frequency response.
0.00
Name
m1
m2
m3
m4
m5
-10. 00
S 11
X
Y
9.8000 -10.3659
9.3000 -14.4179
7.4000 - 20.5878
18.1000 -32.9536
18.5000 -61.1246
m1
m2
m3
Frequency Response(dB)
-20. 00
-30. 00
S 21
m4
g1=0.2mm
-40. 00
g1=0.01mm g1=0.1mm
-50. 00
-60. 00
m5
-70. 00
0.00
5.00
10. 00
Freq [GHz]
15. 00
Figure 5: S11 and S12 of bandpass DMS element response with a = 2.2756 mm, b = 0.5728 mm, c =
1.3572 mm, g = 0.2 mm and W = 2.33 mm.
Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 18–21, 2009 1217
p
0.00
Name
X
S11
Y
m1
6.7000
-20.1865
m2
6.0000
-16.8455
-10.00
Frequency Response(dB)
m2
m1
-20.00
S 21
-30.00
-40.00
g1=0.01mm
g1=0.01mm
g=0.01mm
g=0.2mm
a=4.675mm
a=4.675mm
-50.00
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
Freq [GHz]
Figure 6: More tuning of resonance frequency using change dimensions of T-cell.
satisfactory agreement of results. Also this figure shows that the bandwidth of BPF is more than
39%.
3. TUNING OF BANDPASS DMS FILTER
The responses for several values of g1 are shown in Fig. 5. As can be seen in this figure, by
decreasing the gap spaces, the resonance response goes to the lower frequencies, insertion loss is
improved and pass band become wider.
Addition to change of gap spaces for more tuning of resonance frequency of bandpass DMS filter,
we can displacement this frequency by changing the T-cell dimensions. As can be seen in Fig. 6,
by increasing a and decreasing g the resonance frequency tend to be lower frequency and then this
frequency can be tuned.
4. CONCLUSION
We have introduced a new DMS bandpass component. By using the bandpass DMS components,
we have developed a compact BPF of superior performance.
REFERENCES
1. Ahn, D., J. S. Park, C. S. Kim, J. Kim, Y. Qian, and T. Itoh, “A design of the low-pass filter
using the novel microstrip defected ground structure,” IEEE Trans. Microwave Theory and
Tech., Vol. 49, 86–93, Jan. 2001.
2. Yang, F. R., Y. Qian, and T. Itoh, “A novel uniplanar compact PBG structures for filter and
mixer applications,” IEEE Microwave Theory and Tech. Symp., 919–922, 1999.
3. Park, J. S., J. Kim, J. Lee, and S. Myung, “A novel equivalent circuit and modeling method
for defected ground structure and its application to optimization of a DGS lowpass filter,”
IEEE Microwave Theory and Tech. Symp. Dig., 417–420, 2002.
4. Kazerooni, M., G. Rezai Rad, and A. Cheldavi, “Behavior study of simultaneously defected
microstrip and ground structure (DMGS) in planar circuits,” PIERS Proceedings, 895–900,
Beijing, China, March 2009.