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46 The PCB Design Magazine • August 2017 WHEN LEGACY PRODUCTS NO LONGER PERFORM To improve signal integrity, hence EMI, one needs to slow down the rise time of the signal to reduce the high-frequency components. This is easily achieved by placing a termination re- sistor in series with the transmission line at the source. The value of this terminator is deter- mined by extracting the IV curves from the IC IBIS model, and then the required series termi- nation resistance is calculated, based on a dis- tributed system, to match the transmission line for the selected layer as shown in Figure 4. So, to fix that legacy product that now exhibits in- termittent operation, simply add a series termi- nator. In fact, it is always good design practice to allow space for a series terminator by adding a zero-ohm resistor to the critical interconnects to future-proof the design. Points to Remember: • Faster switching intensifies signal integrity issues by producing spurious signals exhibiting excessive overshoot and ringing. This also has a direct impact on radiated emissions. • Established products can suddenly stop performing reliably due to a new batch of ICs. The cause of this problem is rise time shrinkage. • Even when the package hasn't changed and the clock speed hasn't changed, a problem may exist for legacy designs. • The emissions from sub-nanosecond rise times, can easily exceed the FCC/CISPR Class B limits for an unterminated transmission line. • The ratio of signal rise time to physical delay, of an interconnect, determines how the circuit behaves. • An ideal square wave clock signal has fre- quency components only at odd multiples of the clock frequency. • In practice, the signal rise time has an im- pact on the maximum signal bandwidth. • The upper knee frequency of 0.5/Tr, forms a translation between time and frequency domains. • When selecting the most appropriate di- electric materials for a design, one should con- sider the bandwidth up to the 5 th harmonic. • For high-frequency digital, RF and micro- wave design alternative materials that exhibit lower losses need to be considered. • Although the amplitude of each harmon- ic drops off with the inverse of frequency, the ability to radiate increases linearly–so all har- monics contribute equally to EMI. Figure 4: Series terminator value determined for an FPGA clock (source: iCD Design Integrity).