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Design007-Aug2019

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50 DESIGN007 MAGAZINE I AUGUST 2019 rents flow in the same direction, then they are additive (think two parallel trace segments). When the currents flow in the opposite direc- tion (think a trace over a return plane), the currents cancel. This implies that two individ- ual traces should be kept well apart to reduce crosstalk whilst a signal trace should be tightly coupled to its return path (plane) to increase coupling and reduce inductance. Parasitic inductance is often an afterthought in high-speed design. A substrate consisting of conducting and dielectric materials will have some parasitic inductance, possibly leading to problems like crosstalk, induced currents, noise coupling, and other effects that degrade signal quality. Unfortunately, parasitic capacitance and in- ductance in a PCB are unavoidable. A PCB is composed of a number of parallel conducting elements that are separated by an insulator, ba- sically forming a capacitor. Likewise, conduc- tors on a PCB will inevitably form complete loops, creating an equivalent inductor. While making dielectric layers in the stackup thin- ner will decrease the loop area and the para- sitic inductance, it will also increase parasit- ic capacitance. Therefore, one needs to choose the sweet spot where inductance is minimized, and capacitance is maximized. In high-speed digital applications where mul- tiple data lines can run at tens of Gbps, parasit- ic capacitance and inductance can produce im- pedance mismatch along the signal path. Any mismatch caused by parasitics will produce re- flections along the transmission line, ultimate- ly increasing timing jitter and bit error rates. Figure 1 shows the near (NEXT) and far- end (FEXT) crosstalk for the victim traces ad- jacent to the aggressor trace (1.5V at 1 GHz). In this case, the traces are 4 mils wide with 4-mil spacing and have a 40-ohm impedance. As the victim trace gets farther away from the aggressor, the crosstalk decreases. The self-in- ductance line rings are those field line rings around a trace that arise from its own current only, whilst the mutual inductance line rings are the magnetic field line rings complete- ly surrounding a trace that arise from anoth- er trace's current; these cause the crosstalk. Crosstalk creates ringing, which creates elec- tromagnetic radiation. 2. Inductance of the Power Distribution Network Also, as the frequency and rise times in- crease, the AC impedance of the PDN increases due to the inductance of the bypass and decou- pling capacitors attached to the planes. Every capacitor has an equivalent series inductance (ESL), which causes its impedance to in- crease at high frequencies. Bulk bypass capaci- tors provide low impedance up to ~10 MHz. High-frequency decoupling is provided by ce- ramic capacitors up to several hundred MHz, but above that, only the planar capacitance can reduce the PDN impedance. The power- to-ground plane capacitance of the PCB pro- vides an ideal capacitor in that it has no series lead inductance and little equivalent series re- sistance (ESR), which helps reduce noise at ex- tremely high frequencies, providing tight cou- pling (<5 mils) between planes creates valu- able capacitance at high frequencies. Capacitors reach their minimum impedance at their resonant frequency (Figure 2), which is determined by the capacitance, ESR, and ESL. To meet the PDN target impedance at a par- ticular frequency, a capacitor is chosen so that when mounted on the PCB, it will resonate at the desired frequency and have an impedance Figure 1: Near and far-end crosstalk for microstrip (simulated in HyperLynx).

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