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This letter proposes a design of single-layer microstrip ultrawideband (UWB) 90° differential phase shifter. The operating bandwidth covers the UWB frequency range from 3.1-10.6 GHz. The design is optimized using a transmission line model and an electromagnetic model, and have been fabricated using a plotter dealing with generation of printed circuits. The measured S11 is better than -10 dB and S21 is better than -0.96 dB within the UWB frequency range. The simulated and measured phase deviation of the differential phase shift are 90°±5.23° and 90°±9.02°, respectively. The advantages of the proposed design are the simple fabrication, low insertion loss, and broad bandwidth.