| Summary: | In this paper, we report a high-performance balun with characteristics suitable for future broadband sub-THz differential circuits. The idea of the balun is based on three asymmetric coupled lines, which enhance the odd mode capacitances to equalize the even/odd mode phase velocities. The inner line of the three asymmetric coupled lines is configured to form the open stub (<inline-formula> <math display="inline"> <semantics> <mi>λ</mi> </semantics> </math> </inline-formula>/2), while the outer lines form short stubs (<inline-formula> <math display="inline"> <semantics> <mi>λ</mi> </semantics> </math> </inline-formula>/4). To further reduce the phase imbalance, the short stubs in one of the arms of the balun are connected with vias and a lower metal layer. The balun is developed using the standard 130-nm SiGe BiCMOSback-end process and EM simulated with ADS momentum and Sonnet. The −10-dB reflection coefficient (S<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>11</mn> </msub> </semantics> </math> </inline-formula>) bandwidth of the balun is 136 GHz (88−224 GHz). It shows insertion loss (including RF pads) <1.5 dB, phase imbalance <7 degrees, and amplitude imbalance <1 dB at 94−177 GHz. Furthermore, a scaled-down version of the balun operates on the WR-6, WR-5, and WR-4 frequency bands without significant degradation in its performance. Such characteristics of the balun make it an ideal candidate for various broadband differential circuits.
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