HDMI Design Guidelines -- > Control Impedance, Discontinuities

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.

A more accurate, and in the long term cheaper approach, is to use a 2D or better field solver. This is a software tool that solves Maxwell’s Equations and calculates the electric and magnetic fields for an arbitrary cross-section transmission line.

From these, it also calculates the electrical performance terms, such as characteristic impedance, signal speed, crosstalk and differential impedance. Some field solvers can also calculate the current distributions inside conductors. The advantage a 2D field solver wields over an approximation is the flexibility to consider almost any arbitrary cross-section geometry. In addition to the first-order terms such as line width, dielectric thickness and dielectric constant, second- order terms such as trace thickness, solder mask and trace etch back can be considered.

Discontinuities

Discontinuities are locations in the signal path where the differential trace impedance deviates
from its specified value (of 100 15 per cent for HDMI), and assumes either higher or lower impedance values.

Discontinuities cause signal reflections due to impedance mismatch compromising signal integrity. These are primarily the result of changes in the effective trace width or in the line-to-line spacing caused either by unavoidable transitions in the trace geometries along the signal path, or by poor routing of the signal traces.

Potential locations for discontinuities are:

• where the solder pads of the HDMI connector meet the signal traces
• where signal traces meet vias, component pads of resistors,or IC-pins
• 90 degrees bends in signal traces
• where a signal pair is split to route around an object

Discontinuities are detected during differential impedance, TDR, tests. A TDR, (time-domain reflectometer), is an electronic instrument used to characterise and locate faults in metallic conductors. A TDR transmits a fast rise time pulse along the conductor. If the conductor is of uniform impedance nd properly terminated, the entire transmitted pulse will be absorbed in the far-end termination and no signal will be reflected back to the TDR. But where impedance discontinuities exist, each discontinuity will create an echo that is reflected back to the reflectometer (hence the name). Increases in the impedance create an echo that reinforces the original pulse while decreases in the impedance create an echo that opposes the original pulse.

The resulting reflected pulse that is measured at the output/ input to the TDR is displayed or plotted as a function of time and, because the speed of signal propagation is relatively constant for a given transmission medium, can be read as a function of trace length.

The goal in PCB design must be to minimise discontinuities wherever possible, thus eliminating reflections and maintaining signal integrity. Following a minimum set of routing guidelines helps avoiding unnecessary discontinuities. The remaining, unavoidable discontinuities should be lumped, that is their areas should be kept small and placed together as close as possible.

The idea is to concentrate the points of reflection to a certain area rather than having them distributed across the entire signal path.

FIG 5. TDR display revealing the locations of discontinuities

The magnitude of the discontinuities seen using a TDR are directly effected by the edge rate of the pulse used by the TDR. The faster the TDR edge the more discontinuities will show up, and the larger the impedance spike will appear. With the HDMI specification they have defined the edge rate that is to be used to be 200ps. Figure 5 illustrates this point. The lower line on the graph was taken using a 30ps edge rate and the upper line was taken with a 200pf filter. The discontinuities created by the SMA launch onto the TPA board that show up on the low line are completely invisible when the 200ps edge rate filter is applied.