Proceedings of 2010 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE 2010)

Xplore Compliant ©2010 IEEE

MULTIBAND MATCHED BANDSTOP FILTER

Siti Yuszaini Muhamad Hamzah, Badrul Hisham Ahmad, MIEEE

Fakulti Kejuruteraan Elektronik dan Kejuruteraan Komputer,Universiti Teknikal Malaysia Melaka (UTeM)

Hang Tuah Jaya, 76100 Durian Tunggal, Melaka

Peng Wen Wong, MIEEE Electrical & Electronic Engineering Department,

Universiti Teknologi Petronas (UTP) Bandar Seri Iskandar 31750 Tronoh, Perak

Abstract— This paper is to propose a multiband matched bandstop filter. A lossy resonator is proposed and it demonstrates the concept and design of a perfectly matched bandstop filter with a matched response. . The design is based on a half wavelength (λ/2) and quarter wavelength (λ/4) resonator with gap coupling, parallel with an all-pass nominally-90º-phase-shift element, which can be optimized to achieve high Q-factor. The filter design is cascaded to achieve multiband isolation. Theoretical analysis together with experimental results are presented in this paper.

Keywords-component; multiband bandstop filter, matched bandstop filter, lossy resonator

I. INTRODUCTION Band-stop filters [1] are key components in a microwave

communication front-end to isolate frequency band located within a wide pass-band [2]. In active circuit design such as oscillators and mixers, bandstop filters were applied to remove higher order harmonics and other spurious signal [3]. When the stop-band is narrow, it is called a band notch filter. In the past, most conventional resistor/inductor/capacitor (RLC) bandstop filters have included several types of open-circuited stubs or shunt stubs of a quarter wavelengths that suffer from a number of technical limitations which are associated with the use of discrete inductors. In a microwave filter, the energy is differentially reflected in order to realize frequency selectivity which is limited by the lossless associated with the technology used. Passive method approached that implement a narrowband bandstop filter with theoretically infinite stopband attenuation while being perfectly matched in both the passband and stopband. This design is based on K-inverter topology for lossy resonator which consist of a parallel coupled λ/2 short circuit transmission line that produced a nominally-90º-phase-shift element between the resonator couplings in one structured. Thus, this design approaches a perfectly match at all frequencies. In this design, multiband matched bandstop filter is designed to meet the requirement and to show that the perfectly matched bandstop filter topology allows construction of multiband filtering by simply cascading the two single-band matched bandstop filter. In this paper, the theory of lossy allpass network is presented which shows the realization of perfect notch for filter design.

II. THEORY

A. Bandstop Limiter Implementation (LOSS ALLPASS NETWORK) Bandstop limiter is realized at high frequencies. The lossy

nature of microstrip makes it difficult to achieve a high Q factor. The perfect-notched concept [4,5] is applied to improved the Q factor of bandstop limiter in this design. Based on a reflection mode filter, this concept makes use of two identical lossy resonators coupled to a 3-dB 90° hybrid coupler with correct coupling factors. At the centre frequency, the incident signals are critically coupled to the resonators and absorbed in the resistive part of the resonators leaving no reflected signals at the output. This theoretically gives infinite attenuation [6].

Figure 1 Hybrid circuit implementation of a perfectly-matched

notch filter. [6]

Figure 2 Even-mode admittance of a lossy resonant circuit. [6]

By considering a symmetrical two-port network defined by even and odd mode admittances Ye and Yo, The S- parameter is

(2) )1)(1(

S

(1) )1)(1(

1

12

11

eo

eo

eo

YYYeYo

YYYYS

++−=

++−=

Proceedings of 2010 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE 2010)

Xplore Compliant ©2010 IEEE

If YO = 1/Ye , hence S11 = 0 for all ω. The network is then perfectly matched,

(3) )1()1(

)1)(1()1)(1(

)1)(1(

S12e

e

e

e

eo YY

jYjY

YYYeYo

+−

=−++−

=++

−=

If the network is lossless, then Ye is a reactance function,

(5) jNDjN-D)(jSgiven , (4)

)()()( 12 +

== ωωωω

DNjjYe

And |S11|2=1 for all ω( an allpass work) By considering Ye is a lossy resonant circuit,

Let, RCp

LpYe ++= 1

Assuming Yo = 1/Ye , Then,

( )

7 41

1|12|

1R

(6) )/1()1()/1()1(|)(12|

2

0

0

2

2

22

222

⎥⎦

⎤⎢⎣

⎡−

+=

=−++−+−=

ωω

ωω

ωωωωω

Qu

S

CLRCLRjS

which is the transfer function of ideal lossless bandstop resonator, Qu is the unloaded Q,

(8) 0

RL

Qu

ω=

The loaded Q of the resulting band reject resonator is therefore half of the unloaded Q of the resonator.

B. Perfect Notch Concept The filter design implements the concept of a couple-

resonator model which represents a perfectly matched bandstop filter [4]. Figure 3 shows a generalized coupled-resonator model of a perfectly matched notch filter obtained by scaling nodes of the admittance matrix of 90º hybrid circuits. This design of matched bandstop filter consists the two parallel-coupled λ/2 short circuit transmission line that produced a nominally-90º-phase-shift element between the resonator couplings in one structured.

Fig. 3 Generalized coupled-resonator model of a matched notch

filter

Fig. 4 Shape of Single Band Matched Bandstop Filter Design

A multiband matched bandstop filter is designed by simply cascading the two single matched bandstop filter at center frequency of 1GHz and 2GHz. This design is to show that the perfectly matched bandstop filter allows the construction of a multiband matched bandstop filter. The design implements a narrow band of single frequency response and matched at all frequency.

Fig. 5 Shape of Multi-Band Matched Bandstop Filter Design

III. EXPERIMENTS Simulation is done using ADS where FR4 board is chosen

as substrate with εr of 4.7, substrate thickness of 1.6 mm and metal thickness of 17.5 μm.

A. Singleband Match Bandstop Filter Fig. 6 shows the layout of Single-band Matched Bandstop filter and Fig. 7 shows the result obtained.

Proceedings of 2010 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE 2010)

Xplore Compliant ©2010 IEEE

Fig. 6 Photograph of the single band matched bandstop filter.

0.95 1.00 1.05 1.10 1.150.90 1.20

-40

-30

-20

-10

-50

0

freq, GHz

dB(S

(1,1

))dB

(S(1

,2))

dB(S

(3,3

))dB

(S(3

,4))

Fig. 7 Single band matched bandstop filter. Simulation Result

Table 1; Simulation and measured S parameter performance

The fabrication process is referred on ADS momentum frequency response that gives 90 % accurate result with measurement. From table above, it summarized the analysis of frequency response performance. The insertion loss S12 and return loss of simulation give about 75 % and 31.65% better tuned response than the measurement. The selectivity of quality factor of simulation gives high quality factor. The frequency response is tuned by a coupled line of resonator to get the notch single frequency response. However, from the result, this design shows that the network is matched at all frequency which means that target of return loss is lower than -10 dB. It will minimize the signal loss of network which maximizes the signal to pass through the network.

B. Multiband Match Bandstop Filter Fig. 8 is the layout of Multiband Matched bandstop filter and Fig. 9 is the response.

Fig.8 Photograph of multiband matched bandstop filter design

Fig.9 Multiband matched Bandstop Filter Simulation Result

Matched

Bandstop Filter simulation measurement

Insertion Loss

(S12) dB -49.002 -12.235

Return Loss

(S11) dB -42.740 -29.212

Unloaded Q

factor 50 24.6

S21 measured s11 simulation

S11 measured s21 simulation

Proceedings of 2010 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE 2010)

Xplore Compliant ©2010 IEEE

Table 2; Simulated and measured response for multiband filter.

Table above summarized the performance analysis of simulation and measurement result. This design consists of two single band matched bandstop filter at center frequency of 1GHz and 2GHz. Figure 4.1, shows the comparisons of S12 and S11 measured and simulation result. Both of the result is nearly same and accurate. From table 4.3, the simulation results give 20% better performance of unloaded quality factor. This quality factor is determined by 3dB selectivity.

IV. DISCUSSION This design prove that, perfectly matched bandstop concept allows the construction of multiband matched bandstop filter design by simply cascading two single band matched bandstop filter. The center frequency of simulation is 1.004 GHz and 1.99GHz. The achievement of this design proves that the network is perfectly matched at all frequency. It gives a single notch frequency response which implements a narrowband bandstop filter with infinite stopband attenuation. The injected signal is fully pass through the network and minimized reflecting power in the network. This multiband design will increase in size when the number of center frequency is increase. The higher center frequency will give a poor of frequency response performance however it perfectly matched at all frequency.

V. CONCLUSION AND RECOMMENDATION The theoretical and measurement analysis is proven which show that the perfectly matched topology allows the construction of multiband matched bandstop filter by simply cascading the two single band matched bandstop filter at center frequency of 1GHz and 2GHz. This design proves by using the topology, the perfectly matched at all frequency is achieved interm of insertion loss and return loss frequency response. This design project shows that a narrowband bandstop filter with theoretically infinite stopband attenuation can be implemented. For future work, the filter can be designed by using other methods or adding another topology for manufacturing process design by considering the size and higher selectivity besides perfectly matched at all frequency. For simulation, other software can be used for example microwave office software or HSFF software.

VI. REFERENCE

[1] D. M. Pozar, Microwave Engineering. Hoboken, NJ: John

Wiley & Sons, 2005. [2] R. Levy, R. V. Snyder, and S. Shin, “Bandstop Filters

With Extended Upper Passbands,” IEEE Transactions on Microwave Theory and Techniques, vol. 54, no. 6, June 2006.

[3] M. –Y. Hsieh and S. –M. Wang, “Compact and Wideband Microstrip Bandstop Filter,” IEEE Microwave and Wireless Components Letters, vol. 15, no. 7, July 2005.

[4] D.R. Jachowski, "Passive Enhancement of Resonator Q inMicrowave Notch Filters," IEEE MTT-S Int. Microwave Symp. Dig., vol. 3, pp. 1315-1318, 2004.

[5] A.C. Guyette, I.C. Hunter, R.D. Pollard, D.R. Jachowski, "Perfectly-Matched Bandstop Filter using Lossy Resonators," IEEE MTT-S Int. Microwave Symp. Dig., pp. 517-520, 2005.

[6] Padisan Phudpong and Ian C.Hunter, “Nonlinear Matched Reflection Mode Bandstop Filters for Frequency Selective Limiting Applications”, IEEE MTT-S Int. Microwave Symp. Dig., vol 57, 09 January 2009