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46 The PCB Design Magazine • February 2015 InSERTIOn LOSS: A BIggER COnCERn In HIgH-SPEED DIgITAL? continues In reality, digital pulse genera- tion is more elaborate than shown here, but Figure 1 gives an example why high-speed digital applications have more concern with high-fre- quency RF components. One big concern with high-frequency RF components is insertion loss and if the RF wave has more loss for the 7 th harmonic as compared to the 3 rd harmonic, then digital pulse will not be well formed. The example given in Figure 1 assumes a 10 Gbps application and if a high-speed digital application is at 28 Gbps then the corresponding frequencies will be 14 GHz, 42 GHz, 70 GHz and 98 GHz. This example is using very high-frequency RF components and the losses are typi- cally much worse as the frequency increases. In this example the differ- ence for insertion loss between 14 GHz and 98 GHz can be extreme. The insertion loss difference can cause the amplitude of one RF signal to be different than another, which can cause poor digital pulse forma- tion. PCB fabricators typically use a time domain reflectometer (TDR) to test PCBs for impedance. However, very few fabricators have the equip- ment necessary to test PCBs for in- sertion loss. Although a network analyzer is typically used for inser- tion loss characterization, some test methods have defined the use of TDR to obtain insertion loss. The circuit materials needed to support these high-speed digital ap- plications, where insertion loss is critical, are typically defined by dis- sipation factor, although copper sur- face roughness is another key factor for insertion loss. For existing and previous digital applications, circuit materials that are considered mid- loss have been used. Mid-loss sub- strates have dissipation factors of lightning speed laminates Figure 1: Adding sine waves of different frequencies can form a digital pulse.