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May 2015 • The PCB Design Magazine 15 CONTROLLED IMPEDANCE DESIGN continues that, the "effective" dielectric constant and loss can be extrapolated. Unfortunately, most ma- terial datasheets specify the dielectric constant (Dk) and dielectric loss (Df) at 100MHz. This is the traditional test parameter, however, that is now changing with the next generation of high- speed, low-loss laminates that are specified up to 10GHz or more. Some low loss microwave materials are measured at 100GHz. Typically for a digital design, a characteristic impedance of 50–60 ohms is used. But, this be- comes more important as the edge rates become faster and different technologies have their spe- cific requirements. For example: Ethernet is 100 ohm and USB 90 ohms differential, DDR2 is 50/100 and DDR3/4 is 40/80 single-ended/dif- ferential impedance. So controlling impedance with a number of different technologies can become a challenge. Also, as operating voltages are reduced, the associated noise margins are also reduced, making it even more important to match the impedance. Figure 4 illustrates the ICD Stackup Plan- ner's unique differential pair calculation. In this case, digital, DDR3 and USB technologies are all accommodated on Nelco N4000-13, 2.5GHz material. With differential impedance, there comes a (coupling) point whereby increasing the trace separation or the dielectric thickness has lit- tle or no further effect on impedance. At this point, the impedance rolls off and the traces be- come uncoupled. This is also the point where crosstalk of unrelated signals begins to occur. For the microstrip stackup of Figure 4, Figure 5 shows this differential coupling point at 8 mils. So, where I have a 10 mil trace clearance for the 79.92 ohms differential impedance, I should have backed this off to just 8 mil trace clearance in order to maintain sufficient coupling other- wise the two traces begin to act as individual single ended signals of 41.75 ohms. For crosstalk, 8 mils (in this case) is also the minimum separation before coupling occurs. This gives you a defined clearance rule to con- strain routing, in order to avoid edge coupled crosstalk of long parallel trace segments. In conclusion, controlled impedance design is not just a matter of pushing a button to get the right trace width for the desired impedance. It is an interactive process of manipulating five variables in combination with the material your preferred fab shop stocks to achieve an educated result. Your product will not only be manufac- turable, but also exhibit improved signal quali- ty, reduced crosstalk and electromagnetic radia- tion and also perform reliably over many years. Points to Remember • A good transmission line is one that has constant impedance along the entire length of the line. • The impedance of the driver must match the transmission line to avoid reflections. • Drivers do not have the exact impedance to match the line (typically 10–35 ohms). • Impedance matching slows down the rise and fall times, reduces the ringing (over/under- Figure 4: Multiple differential pair technologies per substrate. beyond design