Design and analysis of a tunable broadband 180-degree active coupler with low phase-error and high-directivity using staggering technique

This study presents the design and analysis of a 180 degrees tunable non-reciprocal active broadband coupler. To increase the bandwidth, the multi-section impedance transformation technique is utilised. The coupler includes two amplifiers, and three filters (phase-shifters) on the gate (drain) line, referred to as through-path (coupled-path). To achieve an accurate 180 degrees broadband coupler, the staggering technique is utilised for designing the filters. Lumped-element analysis, adopted here for the first time to analyse the active coupler, reveals the impacts of each element on directivity, output phase-shift, and phase-error. The design and post-layout simulation of the coupler are performed in 0.18 mu m CMOS technology over the frequency range of 10-20 GHz. An output phase of 180 degrees +/- 1.7 degrees, a directivity more than 27 dB, and a return loss better than 10 dB are achieved. The coupling gain is 7.7 dB at the centre frequency, the noise figure is 4.8 dB, and the power consumption is 22 mW. By tuning the bias voltage, the phase imbalance caused by process variations can be compensated. Also, a prototype of the coupler was fabricated and tested on a Rogers substrate for 8-10 GHz band, giving an output phase of 180 degrees +/- 2 degrees and a directivity >15 dB.