188.8.131.52.4. Right-Angle Bends Related to a Transmission Path
To minimize impedance discontinuities on the transmission line, avoid using right-angle bends. At the bend, the effective transmission line width increases, this result in an impedance discontinuity, increasing the capacitance.
Instead of 90° bends, use mitered 45° bends. Mitered 45° bends reduce reflection on the signal by minimizing impedance discontinuities. Right-angled bends also look like antennas. Figure 64 shows a 60-mils transmission line immersed in FR-4 dielectric (εr = 4.1, loss tangent = 0.022) with dimensions for 50-Ω impedance. The 90° and 45° bend (refer to Figure 65) traces are simulated using SPICE models. The parasitics are extracted with a 3D field solver.
Figure 66 shows the board’s cross section.
A 1-ns (rise time) signal is fed to one side of the trace and the output is observed at the other end. Because of the extra capacitive loading, the output of the 90° bend has a slight delay and more ringing on it. When driving through long traces or other stressful conditions, even a little bit of ringing is destructive. For instance, adding more closure to an almost closed eye can result in the receiver failing to recognize some bits of data. The 90° bend affects signals running at 3.125 Gbps even more severely. Figure 67 shows the effects of bends on signals.
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