IN CONTRAST to silicon-based passive components, package-based passives provide insufficient signal couplingespecially at higher frequencies. At IBM Microelectronics (Essex Junction, VT), however, a 77-GHz Marchand balun was designed and fabricated with the firm's five-metal-layer, BiCMOS technology by Guoan Wang, Amit Bavisi, Wayne Woods, Hanyi Ding, and Essam Mina.
To implement the balun's coupled sections, the team used broadside coupling. As a result, the shorted differential lines provided the ground reference for the top coupled lines. The balun was matched to 50-Ohm single-ended and 100-Ohm differential ends using multiple metal layers in silicon technology. At 77 GHz, the balun exhibited insertion loss of 1.8 dB with amplitude and phase imbalance of 1.8 dB and 1.5 deg., respectively.
The designers also investigated the effects of parasitic reactance in the load terminations and dummy metal fills on the balun's performance by means of electromagnetic (EM) simulations.
Due to the non-idealities in the custom termination, the balun's bandwidth was reduced by 8 GHz. Beyond 50 GHz, additional loss of 0.4 dB at both ports was attributed to the metal fill. The loss was found to be a combination of impedance mismatches caused by the fill structures and resistive losses in those structures. For the balun's performance to closely resemble the expected simulation results, the metal fill must be avoided directly below the signal lines and spaced away from the weakly radiative open end of the unbalanced line. See "A 77-GHz Marchand Balun for Antenna Applications in BiCMOS Technology," Microwave And Optical Technology Letters, March 2011, p. 664.