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70 DESIGN007 MAGAZINE I OCTOBER 2018 Article by Chang Fei Yee KEYSIGHT TECHNOLOGIES Abstract This article discusses the impact of PCB serpentine routing on high-speed signal integrity in terms of impedance discontinuity, insertion loss and eye diagram opening and differential to common mode conversion for signal transmission at one Gbps (i.e., lower- speed grade) and 10 Gbps (i.e., higher-speed grade). This investigation is performed using Keysight ADS. Introduction Serpentine is a technique to minimize skew or misalignment of differential pairs [1] (Figure 1). The number of segments and intra-pair spacing of serpentining impacts the high-speed signal transmission. As the intra-pair gap is enlarged for serpentining, the characteris- tic impedance of the PCB trace in differential mode will rise, as governed by Equations 1 and 2 [2] . This leads to impedance discontinuity, signal reflection, and ultimately, attenuation. The signal attenuation is heavily dependant on the number of segments and intra-pair spac- ing of the serpentine. Besides impedance dis- continuity, increasing the intra-pair spacing at serpentine segments also loosens the electro- magnetic coupling within the differential sig- nal pair, and eventually worsens the differen- tial to common-mode conversion that weakens the immunity of the channel against common- mode noise or crosstalk. (Equation 1) (Equation 2) s = intra-pair spacing h = dielectric thickness Z 0 = PCB trace characterisric impedance in single-ended mode Impact of Serpentine Routing on Multi-gigabit Signal Transmission

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