Bandpass Filter Using Three Pairs of Coupled Lines With Multiple Transmission Zeros

A novel but very simple bandpass filter using just three pairs of coupled lines with multiple transmission zeros (TZs) and transmission poles is proposed in this letter. The filter structure can be seen as the two one-wavelength ring resonators coupled each other with
$\lambda _{g}/4$
coupled length and sandwiched between the
$3\lambda _{g}/4$
input and output feedlines. Good out-of-band suppression and sharp roll-off skirts can be realized by multiple fixed and tunable TZs without complicated coupling scheme. For further demonstration, a filter example with center frequency at
$f_{0}= 2.1$
GHz is implemented with generation of eight TZs at the frequency range from 0 to
$2f_{0}$
for theoretically calculated certification. Good agreement between simulation and measured results validates the design method.