Compact slot-line bandpass filter using backside microstrip open-stubs and air-bridge structure for spurious suppression

In this paper, a slot-line bandpass filter with spurious suppression using broadside coupled microstrip open-stub and air-bridge structures is proposed. Miniaturization is achieved by terminating the slot line with a double-spiral inductive termination at both ends. By properly adjusting the quarter-wavelength (¿/4) broadside coupled microstrip open-stubs with vias to the other side of the common substrate, one could suppress the spurious at 3f₀, where f₀ stands for the center frequency of the passband. Furthermore, air-bridge structures are applied to the slot-line bandpass filter in order to suppress the spurious at 5f₀. To verify the efficiency of these structures to suppress spurious, a slot-line bandpass filter is implemented using the broadside coupled microstrip open-stub and air-bridge structures. Measured and simulated results demonstrate the feasibility of the proposed bandpass filter. The measured results show that the implemented bandpass filter has a center frequency of 2.44 GHz, 3-dB passband bandwidth of 80 MHz, typical insertion loss of 1.89 dB, and minimum return loss of 24.3 dB. Meanwhile, good selectivity at center frequency ranges from less than 160 MHz at a 15 dB bandwidth to less than 630 MHz at a 30 dB bandwidth is achieved. Specifically, the fabricated bandpass filter possesses stopband up to 6.55f₀ with a satisfactory rejection greater than 18 dB.