A 180° Phase Shifter With Small Phase Error for Broadband Applications

In commonly used multi-bit high-/low-pass phase shifters, the phase error is mostly due to the first bit, which provides the 180deg phase shift. This paper explores the design of a 180deg phase shifter that combines a high-pass filter with a transmission line to reduce the phase error over a large bandwidth. Compared to the conventional high-/low-pass phase shifter, the proposed phase shifter shows better performance in both phase error and amplitude balance over a broad bandwidth. To illustrate the principle, a 180degphase shifter using this topology is designed, fabricated and measured. The phase error is measured to be plusmn2deg, plusmn3.5deg and plusmn4.5deg over a bandwidth of 31.8%, 43.4% and 49.3% respectively. The measured amplitude imbalance of the two branches is within 0.4 dB from 840 to 1310 MHz and the return loss is found to be better than 17 dB inclusive of the effect of the switches and the discrete components.