A design of a ring bandpass filters with wide rejection band using DGS and spur-line coupling structures

A novel inductive coupling (L-coupling) structure is presented to easily control the coupling value and to suppress the harmonics in ring bandpass filters using defected ground structure (DGS). DGSs are placed below the 50Ω microstrip feed lines at the input and output. This DGS is operated as inductor for L-coupling and stop resonator to suppress the harmonics. We also embedded spur line into the coupling section to suppress more harmonics at the input/output port. Therefore, coupling section consists of DGS and spur line. This coupling section inhibits signal propagation in the stopband frequency, allowing the desired passband. To demonstrate these novel techniques, we have designed and fabricated two ring bandpass filters with 150MHz bandwidth at center frequency, 2.3GHz. Experimental results show good harmonic suppression, up to the fourth harmonic with rejection levels near -23dB. Since this L-coupling does not need very narrow coupling gap, this approach can easily control the coupling level, be fabricated in the coupling section, and suppress the harmonics.