Design of high-order wideband planar balun filter in S-plane bandpass prototype

Yun Wei Lin*, Jhe Ching Lu, Chi-Yang Chang

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

22 Scopus citations


A new high-order wideband planar balun filter is proposed in this paper. The balun filter is exact synthesized based on S-domain bandpass prototype using Richards' Theorem to fit the user-defined specifications. By analyzing the redundant element in the equivalent circuit, the output impedance of the balun filter can be arbitrarily assigned. To use an S-plane bandpass prototype other than a high-pass prototype can shrink the circuit area and improve the stopband performance. The lumped capacitor approximation suppresses the spurious response in the stopband and further reduces the circuit size. A high-order balun filter with a bandwidth of 100% and a reflection coefficient of -15 dB is implemented by combining of microstrip lines, slotlines, and coplanar striplines. The simulation and measurement results match well.

Original languageEnglish
Article number6193187
Pages (from-to)2124-2130
Number of pages7
JournalIEEE Transactions on Microwave Theory and Techniques
Issue number7
StatePublished - 8 May 2012


  • Balun filter
  • bandpass prototype
  • impedance transformation
  • planar structure
  • synthesis

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