Mixed-mode S-parameters and Conversion Techniques

Differential signaling in analog circuits is an old technique that has been utilized for more than 50 years. During the last decades, it has also been becoming popular in digital circuit design, when low voltage differential signaling (LVDS) became common in high-speed digital systems. Today LVDS is widely used in advanced electronics such as laptop computers, test and measurement instrument, medical equipment and automotive. The reason is that with increased clock frequencies and short edge rise/fall times, crosstalk and electromagnetic interferences (EMI) appear to be critical problems in high-speed digital systems. Differential signaling is aimed to reduce EMI and noise issues in order to improve the signal quality. However, in traditional microwave theory, electric current and voltage are treated as single-ended and the S-parameters are used to describe single-ended signaling. This makes advanced microwave and RF circuit design and analysis difficult, when differential signaling is utilized in modern communication circuits and systems. This chapter introduces the technique to deal with differential signaling in microwave and millimeter wave circuits.