Cross-Coupled YBCO Filters With Spurious Suppression Using Tap-Connection Technique and Skew-Symmetric Feeds

Narrow-band microstrip cross-coupled band-pass filters based on a quadruplet geometry are designed for wireless-communication applications. We have fabricated the high-Tc (HTS) superconducting filters by patterning doubie-sided deposited YBa
₂
Cu
₃
Oy (YBCO) films. The 4-pole 15-mm-square and 20-mm-square filters with skew-symmetric feeds are designed with center frequencies of 2.45 and 1.90 GHz, respectively. The inductive non-adjacent coupling between the resonators produces a pair of transmission zeros near the pass-band edge. Also observed is an additional pair of transmission zeros that enhance the out-of-band rejection characteristics, which is originated from the skew-symmetric-feed structure. Furthermore, a tap-connection technique by tapping the input/output microstrip lines at certain locations is applied to the band-pass filters so as to suppress the second and/or the third passbands, which are spurious and undesirable. In this paper, it is confirmed that tapping somewhere along the lambdainput/output microstrip lines with lambda /4 open-circuited stub or a lambda/2 short-circuited stub, where lambda refers to the guided wavelength at 2f
_o
or 3f
_o
, may effectively suppress harmonics at 2f
_o
and/or 3f
_o
by more than 20 dB. The realized YBCO filters show a low insertion loss with high spurious suppressions. The temperature-dependent microwave properties are discussed.