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Designers Notebook: Layer Stackup Planning for RF Circuit Boards
Impact of Serpentine Routing on Multi-gigabit Signal Transmission
November 29, 2018 | Chang Fei Yee, Keysight TechnologiesEstimated reading time: Less than a minute
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 1Gbps (i.e., lower-speed grade) and 10Gbps (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 (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 characteristic impedance of the PCB trace in differential mode will rise, as governed by Equations 1 and 2. This leads to impedance discontinuity, signal reflection, and ultimately, attenuation. The signal attenuation is heavily dependent on the number of segments and intra-pair spacing of the serpentine. Besides impedance discontinuity, increasing the intra-pair spacing at serpentine segments also loosens the electromagnetic coupling within the differential signal pair, and eventually worsens the differential to common-mode conversion that weakens the immunity of the channel against common-mode noise or crosstalk.
To read this entire article, which appeared in the October 2018 issue of Design007 Magazine, click here.
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