Design of Compact 90 $^{\circ}$ and 180 $^{\circ}$ Couplers With Harmonic Suppression Using Lumped-

Compact 3-dB 90° and 180° couplers with harmonic suppression using lumped-element bandstop resonators are proposed. The bandstop resonators are not only used to provide the high-pass and low-pass characteristics of conventional couplers, but also have a significant harmonic band rejection at the same time. Therefore, the required lumped-element inductors with a low quality factor are reduced as much as possible, enabling significant circuit size reduction. Further, minimum bulky via holes are employed in these topologies, thus carrying them out on the monolithic microwave integrated circuit is beneficial. Detailed design and theoretical analysis are presented for the proposed couplers with harmonic rejection. To validate the design concept, two proposed couplers implemented on an FR-4 printed circuit board at 2.45 GHz are demonstrated. Promising agreements between the simulation and measurement can be observed. Experimental results reveal that the 90° and 180° couplers suppressed the second harmonic below 26 and 30 dB, respectively, within the 0.6-dB amplitude imbalance and 2° phase error at the center frequency.