A HDMI design guide for successful high-speed PCB design
Controlled Impedance Transmission Lines
Controlled impedance traces are used to match the differential impedance of the transmission medium, e.g., cables, and the termination resistors. Differential impedance is determined by the physical geometries of the signal pair traces, their relation to the adjacent ground plane and the PCB dielectric. These geometries must be maintained across the
entire trace length.
Figure 5 depicts the parameters relevant for impedance calculation for both, Microstrip traces (outer layer traces), and stripline traces (traces within the layer stack, typically sandwiched by two ground planes).
To calculate the trace geometries in Figure 5 for a 100 differential impedance TMDS signal pair, the closed-form equations 1 ” 6 can be applied.
- For loosely coupled striplines, s > 12 mils, the number 0.748 might be replaced with 0.374
- For W < 2h the maximum error is 3%
- For best accuracy keep b ” t > 2W and b > 4t, where b is the dielectric thickness between ground planes
With regards to the distance between a different signal pair and its environment, Figure 5 shows a trace X that is not associated with the current flow in the adjacent “+” and “”" conductors. X can be a trace of another signal pair, a ground shield trace or a TTL/CMOS trace.
For adjacent signal pairs and shield traces, make distance d 3 s. Running the shield trace (preferably ground) on one side potentially creates an imbalance that can increase EMI. Ground trace shields should have a scattering of vias to the underlying ground plane.
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