In this work, on-chip single-to-balanced bandpass filters with good selectivity are proposed. The functions of bandpass filter and balun are integrated using a simple multicoupled line structure along with capacitive loadings. The use of inductor is avoided such that the proposed single-to-balanced bandpass filters can achieve very compact size when implemented on chip. They can be designed with the desired bandpass response and reference impedances through the proposed equivalent-circuit models and design equations. In addition, by introducing two cross-coupled capacitors, one tunable transmission zero can be achieved in either the lower or upper stopband to improve the selectivity. Specifically, two third-order single-to-balanced bandpass filter designs in commercial pHEMT process on 4-mil GaAs substrate with different transmission zero locations are presented. The circuit size is less than 1.02 mm x 0.86 mm for a center frequency of 5.5 GHz and a bandwidth of about 18.18%. The corresponding electrical size is only 0.054 lambda(g) x 0.045 lambda(g) at the center frequency. Good selectivity and balanced performance are demonstrated.
IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES