Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 9, No. 1, June 2010
50
Notch Implementation in Planar Band Stop
Filter using Step Impedance Resonator
Approach
Kamaljeet Singh and Surendra Pal
ISRO Satellite Center, Bangalore, India
Deepak Bhatnagar
Department of Physics, University of Rajasthan, Jaipur -302004
Email: dbhatnagar_2000@rediffmail.com
Abstract— This paper demonstrates the implementation of pass band
notch in a band stop filter. The structure is constructed by
incorporating stepped impedance resonator topology in dual behavior
resonator. Dual behavior resonator brings two attenuation zeros in
erstwhile band-stop characteristics. An addition of coupled quarterwavelength line at one end of the port brings notch characteristics.
Highly selective pass band notch has been demonstrated and measured
results show band pass notch characteristics having return loss better
than 10 dB in 5% bandwidth.
Index Terms— Dual behavior resonator, band stop filter, attenuation zero, stepped
impedance resonator (SIR), open stubs
I. INTRODUCTION
Band stop filters are extensively employed in a wide range of instruments including ground based
satellite communication systems. In instrumentation applications, existence of unwanted or spurious
signals from the active circuitry necessitates blocking of the signals for increasing the dynamic range of
the system. This also holds true for retaining the purity of the signal by blocking harmonics. In satellite
communication, ground based reception chain of the earth terminal also uses band stop filter to attenuate
the uplink frequency band required to avoid cross-talk from nearby terminals. Reported conventional
topologies like L shaped resonator [1], quarter-wavelength resonator [2] and asymmetric spur-line
topology [3] yield narrower bandwidth whereas present day requirements are for wider bandwidth to
block harmonics. Designs using classical theory are band limited therefore to have higher bandwidth in
band stop filter, new topologies have to be employed [4]. This article deals in two aspects; having wider
bandwidth and implementing notch in the mid-band. Notch filter is used to remove / pass narrow band of
Brazilian Microwave and Optoelectronics Society-SBMO received 21 Jan., 2010; revised 24 Feb., 2010; accepted 23 June, 2010
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Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 9, No. 1, June 2010
51
frequencies from the signal path of a receiver or transmitter. Demand for these type of system increases
after the introduction of UWB systems in which interference from wireless frequencies are prominent.
For a conventional notch filter, maximum attenuation or notch depth occurs at a single frequency midway
between the specified edges of the lower and upper pass bands. Unloaded quality factor (Qu) of the filter
resonator limits both notch depth and selectivity. As quality factor is generally proportional to resonator
volume, the desire for a deeper, more selective notch is at odds with the size miniaturization.
Work reported so far deals with the notches in the band pass filter [5]. Present work reports for
the first time a notch in the ultra wide planar band stop filter topology. This adopted approach yields size
miniaturization by incorporating dual band resonator with asymmetrical stubs. These stubs having
different widths can be assumed as step impedance resonator providing higher bandwidth with extra
degree of freedom for controlling zeros. Presented filter with band pass notch has been implemented
using microstrip topology. The concept of the notch along with its implementation has been verified by
comparing simulated and measured results.
II. METHODOLOGY
Dual band resonator uses two open ended stubs (acting as resonator), working at upper and lower
end of frequencies. Main feature of the proposed topology is to have impedance variation (stepped
impedance) within dual band resonator topology to have control of the attenuation zero’s at the either side
of the stop band frequencies [6]. This approach applied at one end of the tightly coupled line yields notch
performance in the traditional band stop filter. The use of stepped impedance resonator in place of
uniform impedance resonator yields better control of filter characteristics. Step-impedance resonator is
formed by open stub (characteristics impedance Z1, electric length θ1) in series with a section of high
impedance section (characteristics impedance Z2, electric length θ2) [7]. As shown in figure 1, the simple
transmission line has been modified as the coupled line, giving better matching and transmission
characteristics (shown as T). Length of the open stubs plays an important role in deciding the
performance of filter at the desired frequency and indicates whether it is working as band stop or bandpass. Conventionally, by taking quarter-wavelength stub, the response becomes band-stop whereas by
taking half-wavelength stub, it becomes band-pass for same impedance values (50 Ω in our case). Instead
of that, if dual band resonator approach with stepped impedance is employed, than notch behavior i.e.
highly selective band pass topology comes out. Step impedance resonator approach gives extra degree of
freedom in choosing the line lengths and the widths. CAD oriented simulation has to be carried out to
choose the optimum line lengths / widths of the step impedance resonator. To make symmetrical band
stop characteristics and for generating pass band notch at either side of stop band, a coupled quarterwavelength line is inserted at one end of the port as discussed above. Further, length of this coupled line
can change the location of the notch frequency, without affecting the notch characteristics.
Brazilian Microwave and Optoelectronics Society-SBMO received 21 Jan., 2010; revised 24 Feb., 2010; accepted 23 June, 2010
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Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 9, No. 1, June 2010
Fig 1: Proposed filter with stepped impedance resonator implementation
Fig 2: Dual band resonator with the stepped impedance resonator approach and its equivalence
The step impedance resonator can be analyzed with the equivalent circuit shown in figure-2. The
single stub is modeled as L and C as shown in the above equivalent figure. Size of step impedance
resonator can be decreased further by changing aspect ratio of Z2 and Z1 whereas θ2 and θ1 changes the
location of the zeros, thus affecting the selectivity of the filter. Optimum ratio and length are chosen
keeping fabrication tolerances and desired characteristics into consideration. Impedance of one section of
step impedance resonator can be written as
Z SIR =
1 + s 2 Lr C r
s(C r + CG + s 2 Lr C r )
(1)
where
Cr = C0 + Ch;
C0= jZ1. tan θ1/s;
Cg = Cs1 + Cs2 + Ch;
Ch = j (1-cosθ2) / (sZ2sin θ2);
L1= 2Z0/πω0, C1=1 /
(ω02L1)
= π / (2Z0 ω0);
Lr = jZ2. sin θ2 /s;
ω0 = 1/√LC; s= jω
Brazilian Microwave and Optoelectronics Society-SBMO received 21 Jan., 2010; revised 24 Feb., 2010; accepted 23 June, 2010
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Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 9, No. 1, June 2010
Frequency tuning can be carried out by varying impedance ratio. Present topology is constructed
by considering the ratio Z2/Z1 >1 (Z2= 71Ω, Z1= 39 Ω), tuning at the desired specified center frequency. If
the ratio is Z2/Z1< 1, then considerable frequency shift towards the lower end takes place.
III. CHARACTERIZATION
Figure-3 shows layout of the proposed topology implementing the notch. The structure is loaded with
the asymmetrical stubs construed as step impedance resonator. Length (L) of the coupled line is chosen
equal to λ / 4 at the desired frequency. Gap between the coupled lines (g) is optimized and finally chosen
to be 0.67 mm which provided best compromise between fabrication limitation and performance. Low
and high frequency stubs are having the lengths of 6.13 mm and 4.82 mm with non-uniform symmetrical
widths of 0.39 mm and 0.1 mm respectively.
Fig 3: Layout of the proposed topology with dimensions
In order to allow the structure to generate a narrow pass band (notch), inside wide stop band, the
loading stub should have unequal lengths i.e.
(θ1+θ2) ≠ (θ3+θ4)
(2)
The phase response of the simulated filter is shown in figure 4. The phase transit at the specified
frequency shows the resonance at the desired frequency in C-band, resulting in the notch formation. The
bandwidth of the notch can be easily controlled by optimizing the length difference of dual band resonator
sections. On increasing the effective difference, the notch bandwidth increases dramatically.
Brazilian Microwave and Optoelectronics Society-SBMO received 21 Jan., 2010; revised 24 Feb., 2010; accepted 23 June, 2010
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Fig 4: Phase response of the band stop filter
IV. IMPLEMENTATION
The proposed filter is implemented on the 10 mil alumina substrate (εr = 9.9). The lengths ℓ0 and ℓ1 as
shown in figure 1 are kept as 1.0 mm whereas lengths ℓ2, ℓ3, ℓ4 and ℓ5 are taken as 1.65mm, 1.85 mm,
3 mm and 1.85 mm respectively. The width w1 is kept equal to 0.39 mm giving impedance close to 39 Ω.
The impedance ratio (Z2/ Z1) is kept around 1.8. The proposed filter has compact size of 2.4 mm ×
5.1 mm. The overall size is less than λg/3, where λg is the guided wavelength of 50 Ω lines at the center
frequency of the notched pass band of the circuit. To facilitate the measurement, a 10 mm line at the input
and the output port is added. The electromagnetic simulation of this circuit is carried out by using MoM
technique [8].
Fig 5: Comparison of the measured and simulated results for S11 & S12
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(a)
(b)
Fig 6 (a): Reflection & Transmission response curves of the filter (b) Group delay characteristics
The measured results show stop band loss of less than 1 dB and attenuation of more than 15 dB. The
notch pass band shows nearly 10 dB return loss with around 1.5 dB insertion loss. The comparison of the
simulated and measured results is shown in figure-5. Results show good analogy with the measured data.
Conductor losses can be further minimized by decreasing the line lengths at the input/output port.
Reflection and transmission response of the filter at center frequency is shown in figure 6(a) while group
delay characteristics are shown in figure 6(b). Response at center frequency shows a very flat group delay
with measured variation of less than 1 ns in the desired band.
V. CONCLUSIONS
A simple concept of pass band notch in the wide band planar band stop filter topology has been
developed and presented in this paper. This approach of clubbing dual band resonator approach with the
Brazilian Microwave and Optoelectronics Society-SBMO received 21 Jan., 2010; revised 24 Feb., 2010; accepted 23 June, 2010
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asymmetric step impedance resonator stubs provides high level of selectivity. The proposed structure is
simple, reliable and can be implemented in the variety of the substrates. This circuit concept will find
applications for wide range of frequency blocking as well as selective passage of the RF signals. Similar
approach can be easily implemented in MIC / MMIC technology and can be employed for very
narrowband band pass filter requirement.
REFERENCES
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[2] E. H. Fooks and R.A. Zakarevicius, Microwave Engineering Using Microstrip Circuits, Prentice Hall,
1990
[3] G. Matthaei, L. Young & E. M. J. Jones , Microwave Filters, Impedance-matching networks &
Coupling Structures, Artech House,1980
[4] M. Hsieh and S. Wang, “Compact and Wideband Microstrip Band stop Filter”, IEEE Microwave and
Wireless Letters, Vol. 15, pp. 472-474, 2005.
[5] H. Shaman, J Hong, “Asymmetric Parallel-Coupled Lines for Notch Implementation in UWB Filters”,
IEEE Microwave and wireless Components Letters, Vol. 17, pp. 516-518, 2007.
[6] E. Rius, C. Quendo, C. Person, A. Carlier, J. Cayrou, J L Cazaux, “High Rejection C-Band Planar
Band-Pass Filter For a Spatial Application”, 33rd European Microwave Conference, Paris, pp. 10551058, 2005.
[7] Y. Z. Wang and M. L. Her, “Compact microstrip bandstop filters using stepped-impedance resonator
(SIR) and spur-line sections”, IEE Proc.-Microw. Antennas Propag., vol. 153, pp. 435-440, 2006.
[8] Advanced Design System, ADS 2002C, Agilent Technologies
Brazilian Microwave and Optoelectronics Society-SBMO received 21 Jan., 2010; revised 24 Feb., 2010; accepted 23 June, 2010
Brazilian Society of Electromagnetism-SBMag
© 2010 SBMO/SBMag
ISSN 1516-7399