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March 2014 • The PCB Magazine 83 can now calculate the velocity factor for a con- ductor placed directly over a glass fibre yarn as 0.39x the speed of light and that of a conduc- tor placed in area containing only resin as 0.53x the speed of light. These values represent the ex- tremes, however the difference is massive. The use of differential pairs in signalling is very com- mon in order to eliminate common electromag- netic noise, the transmitted information being the difference between the traces which contain equal amplitude but opposite polarity signals. It is easy to visualise the effect of the velocity factor table 3: traditional woven glass fabric styles. Figure 5: glass fabric images. DEVELOPMENTS IN GLASS YARNS AND FABRIC CONSTRUCTIONS continues