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30 The PCB Design Magazine • March 2014 Martyn Gaudion, Ceo of Polar instruments ltd., began his career at Tektronix in the early 1980s. he joined Polar instru- ments ltd. in 1990 and was ap- pointed Ceo in 2010. Gaudion also writes occasional articles for a number of PCB industry publications and regularly con- tributes to iPC high-speed, high-frequency standards development activities. To contact Gaudion click here. TOLERANT OF TOLERANCE? continues feature It is interesting that spread glass is one of those rare win-win coincidences as it was origi- nally developed to ease controlled-depth laser drilling, the resin being far easier to ablate than the glass. A side effect of the improved cloth is reduced regional variation in E r . There is a third win here which no one saw coming: At very high (GHz) frequencies, the periodic variation in E r caused by weave mitigation can cause high-frequency variation in the insertion loss characteristics, but that's for another article. Summary So, when specifying any measurement, you should ensure that the design authority (or sup- ply chain procurement management) specifies realistically: that means both measuring within the limits of the capability of your measure- ment systems and recognising that in some cases an appropriate sample measurement test vehicle (coupon or sample material) is an im- portant part of the measurement system. Finally, the key to keeping tolerance as tight as economically possible is an understanding of the materials and processes that are the key drivers of the property you are measuring. And if you are still wondering if the AVO survived its traumatic measurement experience, well, yes, it did. PCBDESIGN References 1. A Designer's Guide to PCB Tolerancing, Polar Instruments Application note AP521. CliCk To View Video Interview Guest editors Kelly Dack and Mark Thompson have a lively discussion about the often underestimated value of keeping in touch with your suppliers through on-site visits. Mark and Kelly on Value-Added Vendor Visits by Real Time with... PCBDesign007

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