A 60 GHz 3-dB tandem coupler using offset broadside-coupled lines on a silicon substrate

In this letter, a 60 GHz tandem coupler using offset broadside-coupled lines is proposed on a silicon-based integrated passive device technology. Over the frequency band of 57-66 GHz, the measured insertion loss, amplitude imbalance, input return loss, isolation and phase error of the designed tandem coupler are better than 0.67 dB, 0.31 dB, 27.9 dB, 29.7 dB and 3.7 degrees respectively. To the best of our knowledge the proposed coupler achieves the lowest reported insertion loss and amplitude imbalance for a 3-dB coupler on a silicon substrate at 60 GHz band. Furthermore using the even-and odd-mode analysis, the reason for a high directivity of edge-coupled lines and a low coupling level of broadside-coupled lines on the adopted technology is investigated and a solution for increasing the coupling level of broadside-coupled lines is proposed.