On-chip single-to-balanced bandpass filters with wide upper stopband

In this work, on-chip single-to-balanced bandpass filters with wide upper stopband are proposed. The proposed filter integrate the functions of bandpass filter and balun using a simple multicoupled line structure along with loaded capacitors. The use of inductor is avoided such that they can achieve very compact size when implemented on chip. They can be designed according to the desired bandpass response and reference impedance. Specifically, two single-to-balanced filters of 2nd-and 3rd-order bandpass response are demonstrated using the GaAs pHEMT process. Their chip sizes are 1×1 mm² and 1.5×1 mm² for a center frequency of 5.5 GHz and a bandwidth of about 18%. The corresponding electrical sizes are only about 0.036λ_g × 0.045λ_g and 0.062λ_g × 0.043λ_g, respectively. Good balanced performances are observed with common-mode rejection ratio better than 29 dB. Notably, very wide upper stopbands up to the highest measurement frequency of 67 GHz are also achieved.