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article influence of via stub length and antipad size on the insertion loss profile continues Figure 11: Raw data for S21 magnitude. Figure 12: Average data for S21 magnitude. Figure 13: Additional loss caused by the via stubs. Figure 14: Maximum acceptable stub length. this data with a de-trend operation and plotted in Figure 13. As an example, a maximum additional insertion loss of 5 dB might be acceptable at frequencies up to 20 GHz. Using the chart in Figure 13 and adding a forbidden zone (red hatched box), it can be found that the stub lengths SE08, SE09 and SE10 are too long and therefore add too much insertion loss. The stub length SE07 is barely acceptable in this example, whereas all shorter stub lengths pass the requirement (Figure 14). Beside evaluating the magnitude of insertion loss, the return loss was also plotted (Figure 15). Obviously, the effect of stub length and antipad size is much less pronounced than in the insertion loss charts. Because of the wide maxima at the resonance frequency in the return loss chart, no numerical evaluation was performed here. However, plotting the insertion loss and the return loss in one chart confirmed the expected alignment of the dips in insertion loss (IL) and maxima in return loss (RL) regarding frequency, which is shown in Figure 16 for the longer stubs. The charts provided a good overview about the influence of the stub length and the antipad sizes, but to get quantitative data on the level of influence, ANOVA evaluations were performed. The first ANOVA shows the influence of the pa- 52 The PCB Design Magazine • January 2014