Issue link: https://iconnect007.uberflip.com/i/886239
62 The PCB Design Magazine • October 2017 Taking this one step further, if your data sheet uses one TanD measurement method at 100MHz, and another at 1GHz and a third at 10GHz, which should you use when modeling high-speed designs? Should you: a) guess; b) se- lect the highest; c) select the one closest to your clock frequency; or d) use all three? Note that in the previous paragraph I ex- plained that some measurement methods are less appropriate than others for transmission line modelling, so if there is a measurement us- ing a transmission line, then that is the best ap- proach. I can hear you asking, "But what if it is not at my clock frequency?" Fortunately, there are some mathematical techniques that can help you out here. Svensson-Djordjevic modeling uses mathematical models of the substrate to predict the value of both TanD and Er over fre- quency when given a single known frequency input (see Figure 7). Using this type of modelling in your field solver to predict Er and TanD from a known spot frequency and measured with an appropri- ate model for use in a transmission line applica- tion is the optimum solution when faced with a variety of TanD values, methods and frequen- cies. Back to the original message: The point I make here is that, whether you are concerned about insertion loss or any other quantity that is presented as a small ratio, it is vital that you use the number expressed with enough signifi- cant digits to yield an appropriately accurate calculation and a value obtained from a method appropriate to your application. PCBDESIGN References 1. Dielectric constant: Think of this in layman's terms as simply a measure of how well a substrate can store electrical charge, often referred to with a variety of symbols as ε r, Er, Dk, K. 2. Loss tangent: Think of this in layman's terms as simply the undesirable ability of a sub- strate to turn useful signal energy into heat (un- Figure 6: An increase in loss tangent value of 33% in turn results in a corresponding increase in dielectric loss of 33%. TANGENTIAL THOUGHTS: LOSS TANGENT VALUES