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October 2016 • The PCB Design Magazine 51 ROCK STEADY DESIGN The stackup configuration in Figure 2 has been set up for multiple single-ended and dif- ferential impedances. The material selected in this case is Isola 370HR, which is a high-per- formance E-glass fabric for applications where maximum thermal performance and reliabil- ity are required. You should work closely with your preferred fab shop to ensure they stock the selected materials. As mentioned, the dielec- tric constant and dissipation factor vary with frequency so I have selected the data from the library at 1GHz to match the maximum band- width required. Blind and buried via spans have also been defined. Figure 3, is a target impedance chart that in- cludes characteristic, edge-coupled and broad- side-coupled impedance that is exported as part of a fabrication drawing. This drawing also includes dielectric material definitions, trace/ clearance properties and via spans. 2. Impedance Matching Now that the stackup is finalized, let's look at impedance matching the source to the trans- mission lines. In a previous column, I pointed out all of the possible termination strategies. The easiest way is to use a resistive element. Ter- mination can be made at the source or at the load. Series termination is excellent for point to point routes (such as data signals), one load per net. It works well for traces that are electrically short and is also used to fanout multiple loads radially from a common source. Whereas, par- allel termination is preferred for address buses. The impedance of the trace is extremely important, as any mismatch along the trans- mission path will result in a reduction in sig- nal quality and possibly the radiation of noise. Mismatched impedance causes signals to reflect back and forth along the lines, which causes ringing at the load. The ringing (as displayed in Figure 4) reduces the dynamic range of the re- ceiver, eats into the noise budget and can cause false triggering. As signal rise times increase, consideration should be given to the propagation time and reflections of a routed trace. If the propagation time and reflection, from source to load, are longer than the edge transition time, an elec- trically long trace will exist. If the transmission line is short, reflections still occur but will be overwhelmed by the rising or falling edge and may not pose a problem. But even if the trace is short, termination may still be required if the load is capacitive or highly inductive to prevent ringing. Generally, when the trace length ex- ceeds one sixth of the electrical length of the rising edge rate, then termination is required. Unfortunately, the driver impedance is al- ways lower than 50 ohms and as such, does not Figure 3: Target impedance chart for all required variations.