1. Chung-Hwa Wu, Chi-Hsueh Wang, and Chun Hsiung Chen,
Stopband-Extended Balanced Bandpass Filter Using Coupled Stepped-Impedance Resonators
IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 17, NO. 7, JULY 2007
Chung-Hwa Wu, Chi-Hsueh Wang, and Chun Hsiung Chen,
Adviser : Hon Kuan
Min-Hang Weng
Reporter : Yi-Hsin Su
Date : 2010/4/28

2. Outline Introduction Balanced Filter Structure
Design Of The Stopband-Extended Balanced Filter
Filter Responses
Differential-Mode Response
Common-Mode Response
Conclusion

3. Balanced Circuits
Balanced circuits play an important role in building a modern communication system.
Balanced circuits with differential operation show higher immunity to environmental noise
A well-designed balanced bandpass filter (BPF) should exhibit the desired differential-mode frequency response and should also be capable of reducing the common-mode signal at the same time.

4. Introduction
Recently, a fourth-order balanced coupled-line BPF using parallel-coupled-line structures was proposed, which presents both high selectivity and good common-mode rejection within the differential-mode passband.
However, This filter still has several drawbacks, such as requiring via-hole process.
A novel fourth-order balanced BPF based on λ/2 stepped-impedance resonators is proposed to realize the desired differential-mode characteristics in addition to overcoming the shortcomings
C.-H.Wu, C.-H.Wang, and C. H. Chen, “Novel balanced coupled-line bandpass filters with common-mode noise suppression,” IEEE Trans. Microw. Theory Tech., vol. 55, no. 2, pp. 287–295, Feb

5. Balanced Filter Structure
The proposed fourth-order balanced filter is composed of four symmetric λ/2 SIRs, The proposed balanced filter has used the balanced feed structure
This balance BPF is designed and fabricated on FR4 substrate having a thickness of 1mm, a dielectric constant of 4.4, and a loss tangent of 0.02.
Physical layout of the proposed fourth-order balanced filter using symmetric=λ/2 SIRs.

6. Balanced Filter Structure
Under differential-mode operation, a virtual-short (PEC) would appear along the symmetric-line, therefore each resonator, resonating at , may be treated as a shorted λ/4 SIR.
Under common-mode operation, a virtual-open (PMC) would present along the symmetric-line, thus each resonator, resonating at ( ), may be treated as a λ/2 resonator. ＞
differential-mode
common-mode

7. Design Of The Stopband-Extended Balanced Filter
differential-mode and common-mode external quality factors against the tap position of I/O resonators.
mixed coupling
( =1GHz and 3-db FBW=10%)

8. Design Of The Stopband-Extended Balanced Filter
( =1GHz and 3-db FBW=10%)

9. Filter Responses
The differential-mode response, the center frequency is at 1.02 GHz, with a minimum insertion-loss of 3.51 dB and a bandwidth of 12%.
The differential-mode spurious passband has been pushed up to

10. Filter Responses
The common-mode signal is suppressed with a level of dB around ,and almost below 30 dB from 0.5 to 6 GHz.
This spurious response has been suppressed due to the relatively weak signal along the main path and the poor common-mode external quality factor value associated with I/O resonators.

11. Conclusion
In this letter, a novel fourth-order balanced BPF using symmetric λ/2 SIRs is proposed.
To obtain the desired differential-mode frequency response and also to avoid the undesired via-hole process.
The open stub associated with tap position Lt has created an additional. transmission zero