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Design of Compact and Sharp-Rejection Ultra Wideband Bandpass Filters Using Interdigital

Stepped-Impedance Resonators

IEICE TRANS. ELECTRON., VOL.E90–C, NO.8 AUGUST 2007Cheng-Yuan HUNG,, Min-Hang WENG,, Yan-Kuin SU,

Ru-Yuan YANG, and Hung-Wei WU.

Adviser : Hon Kuan Min-Hang Weng

Reporter : Yi-Hsin SuDate : 2010/3/24

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Outline

Introduction UWB-BPF: Schematic and Principle Experimental Results and Discussion Conclusion

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Introduction A small size and high selectivity microwave bandpass filter (BPF)

is widely used to enhance the performance of radio frequency (RF) front end.

In the past, conventional microstrip interdigital bandpass filters (IDT-BPFs) using quarter-wavelength resonators were compact while they require short-circuit connections with via holes.

J. S. Hong and M. J. Lancaster, Microstrip Filters for RF/MicrowaveApplications. New York: Wiley, 2001, ch. 3.

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Introduction New type of miniaturized IDT-BPFs by using pseudo-IDT

structure (PIDT) without via holes ground These microstrip PIDT-BPFs fabricated on PCB have a relatively

small bandwidth, since the coupling level of interdigital electrode using conventional design methods were limited.

This paper develop a very compact UWB-BPF with FBW larger than 95% based on PIDT-SIRs. It is able to place two transmission zeros near the passband edge so that higher selectivity with fewer resonators could be obtained.

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UWB-BPF: Schematic and Principle

The proposed UWB-BPF using PIDT-SIR on commercial PCB RT/Duroid 5880. This filter basically consists of two identical SIR and two tapped I/O at the two sides.

The SIR is symmetrical and has two different characteristic impedance lines, low-impedance (Z1) line in center and two identical high-impedance (Z2) lines in two sides.

Practical layout of the designed UWB-BPF designed on a 0.787 mm-thick substrate with a dielectric constant of 2.2.

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UWB-BPF: Schematic and Principle

The typical SIR with R = Z2/Z1 < 1, has shorter electrical length and shifts the first spurious response mode fs1 to higher frequency.

The proposed UWB-BPF, the first two resonant modes ( f0 and fs1) are used and taken into account together and they are applied to create a wide passband. Therefore, the SIR with R = Z2/Z1 > 1.

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UWB-BPF: Schematic and Principle

1 2tan tanR

1 2cot tanR

1

1

2tx

or

2 2tx

The low-impedance wavelength θ1 is not equal to high impedance wavelength θ2.

The stepped percentage x is defined as the portion of θ1 and θ2 to the total wavelength of the SIR (θt = 2(θ1 + θ2)).

(1)

(3)

(2)

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UWB-BPF: Schematic and Principle

It is found that the first spurious response mode fs1 goes toward the fundamental response mode f0 as stepped percentage x is around 0.76.

The first spurious response mode fs1 in SIR with more lager R value is closer to the fundamental response mode f0.

The first two resonant modes can be combined together to create a wide passband.Resonant electric length of first spurious

response mode versus stepped percentage xwith impedance ratio R = 1, 1.2, 2, 3 as a parameter.

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UWB-BPF: Schematic and Principle

Three cases are designed and simulated, including case 1, case 2 and case 3 of SIR with the parameters.

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UWB-BPF: Schematic and Principle

In case 2 with R = 2 and x = 0.76, a satisfied band at center frequency f0 = 7.1 GHz and bandwidth of 3.74–10.43 GHz (3-dB FBW= 94%) is obtained.

The frequency skirts of the passband edge are very sharp, since two transmission zeros are produced from the multi-path effect of the PIDT-SIRs

The simulated frequency responses of case 1, case 2 and case 3.

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Experimental Results and Discussion

The fabricated filter consists of two SIRs having the low-impedance (Z1=69Ω) line section with a strip width of 1.44mm and the high-impedance (Z2=138Ω) line sections with a strip width of 0.3 mm.

At the stepped percentage x = 0.76, the length L is 11.2 mm, t is 7mm

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Experimental Results and Discussion The predicted and measured results o

f designed UWB-BPF at center frequency f0 = 7.1 GHz.

The measured results of the fabricated BPF have low insertion loss of −0.5±0.4 dB and bandwidth of 3.68–10.46GHz (3-dB FBW= 95%).

The finite transmission zeros occur in the lower side of passband edge at 3.26 GHz with −35 dB attenuation, and in the higher side of passband edge at 11.3 GHz with −38 dB attenuation.

Predicted and measured frequency response of the fabricated UWB-BPF. The designed parameters are those in case 2.

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Conclusion New type of miniaturized IDT-BPFs by using pseudo-IDT

structure (PIDT) without via holes ground. UWB-BPF using pseudo-interdigital stepped impedance resonators

(PIDT-SIRs), with improved skirt characteristics and very compact size.

The first two resonant modes of the SIR with stepped impedance ratio larger than 1 are taken into account together and they are applied to make up a wide dominant passband.

The multi-path effect of the PIDT-SIRs is a very useful feature for enhancing the attenuation characteristics in the stopband of the filter.