Simple dual-mode balanced bandpass filter with high selectivity and extended common-mode noise suppression
A new miniaturised dual-mode balanced bandpass filter (BPF) is presented in this letter. To achieve the required differential-mode (DM) operation with common-mode (CM) suppression, an octagonal microstrip dual-mode loop resonator is employed. By symmetrically locating four high-low impedance sections inside the free area of the loop resonator, the proposed balanced BPF can realise compact size, wide stopband under DM operation, and extended CM suppression, simultaneously. Furthermore, a pair of coupled lines is introduced to readily achieve two transmission zeroes for realising high selectivity of DM response. For demonstration, a dual-mode balanced BPF with the passband centred at 2.39 GHz is designed and measured. Both the simulated and measured results are provided with good agreement.
