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28 DESIGN007 MAGAZINE I MAY 2018 smaller package, e.g., 0201 (i.e., 10 mil pad width) to minimize the discontinuity. (Equation 6) Where: L o = intrinsic loop inductance per unit length of the transmission line (in nH/cm) C o = intrinsic capacitance per unit length of the transmission line (in pF/cm) Z o = characteristic impedance (in ohm) (Equation 7) Where: C = capacitance (in pF) e r = relative permittivity of substrate e 0 = permittivity of vacuum w = width of SMT pad (in cm) l = length of SMT pad (in cm) d = distance between mounting pad and reference plane underneath (in cm) F. Providing continuous reference plane When the PCB trace crosses over a gap between 2 split planes, an inductive imped- ance discontinuity or non-continuous refer- ence is encountered. This phenomenon is governed by Equations 6 and 8 [15] respec- tively. When the gap between split planes is crossed over, the distance between the trace and reference underneath (i.e., gap depth) is increased, this leads to the rise of inductance, which in turn causes the trace impedance to increase over the split. With reference to simu- lated plots of differential insertion loss in Fig- ure 8, where crossing over split planes results in higher insertion loss due to larger induc- tive impedance discontinuity, with minimum 0.1dB attenuation at 20GHz and beyond. (Equation 8) Where: L = parasitic inductance of copper trace (in nH) d = distance between copper trace and reference underneath (cm) w = width of copper trace (cm) t = thickness of copper trace (cm) x = length of copper trace (cm) In addition to that, non-continuous reference plane or return path also creates larger loop areas that increase mutual inductance and sig- nal crosstalk [16] . Hence, it is essential to ensure a solid reference plane along the entire trace length path. Figure 7: Differential insertion loss for varying surface mount pad width in series with 500 mil long PCB trace, simulated in Keysight EMPro. Figure 8: Differential insertion loss for 500 mil PCB trace with solid reference versus crossing over split planes simulated in Keysight EMPro.