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October 2017 • The PCB Design Magazine 61 pected errors) if the values had been expressed in engineering notation. The actual value of loss tangent, 0.015, is expressed in engineer- ing notation as 15 x 10 –3 , but the rounded val- ue, 0.02, is expressed as 20 x 10 –3 , an obvious increase of 33%. So perhaps a better way of displaying loss tangent would be in scientific notation instead of 0.015. You could say TanD = 15x10 -3 or 15 milli TanD. But for some reason that I am un- aware of, it seems that only tangible units (i.e., weight, mass, wavelength and frequency) are referred to in this way. Quantities that are ra- tios tend to be shown in fractional or decimal format. Loss tangent is one of those "mystery" char- acteristics that isn't easy for fabricators to visual- ise or measure. The simplest way of thinking of loss tangent is to look at it as the ability (albeit undesired) to turn precious RF energy into heat. This is excellent if you are designing microwave ovens, but not so helpful if you are attempting to transmit small-amplitude high-speed signals from point A to point B along a PCB transmis- sion line. Because it is a tricky animal to measure, it comes as no surprise that there are a variety of measurement methods, and some more appro- priate to some applications than others. Split post resonator methods, for example, are ideal for bulk measurement of loss tangent when manufacturing base materials. When choosing a value of loss tangent for use in signal integ- rity applications you will get best results if you use a value derived by using transmission line techniques. The loss tangent in a data sheet may have been measured in a variety of ways depending on application and frequency of measurement—most data sheets note this—but if in doubt, ask. Figure 5: Recalculate and note the new loss value of -19 dB at the 20GHz data point on the dielectric loss series. Figure 4: Change the loss tangent value to 0.02. TANGENTIAL THOUGHTS: LOSS TANGENT VALUES

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