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PCBD-Oct2017

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60 The PCB Design Magazine • October 2017 parameters such as loss tangent which has a directly proportional effect on the insertion loss. A rounding to 2 decimal places of a TanD say, from 0.015 to 0.02 (quite legitimate, you may think) would actually lead to the model- ing of insertion loss being overpredicted by a massive 33%. You can see the effect when using a PCB field solver's frequency-dependent calculation feature to model loss in an offset stripline 1B1A controlled impedance structure (Figures 1-3). Rounding the Value of Loss Tangent How will the dielectric loss value change if the value of the loss tangent is rounded from 0.015 to 0.02? First, change the value of the loss tangent to 0.02 (Figure 4). Note that rounding from 0.015 to 0.02 actu- ally represents an increase in loss tangent value of 33%. This increase in turn results in a correspond- ing increase (Figure 6) in dielectric loss of 33%. It might have been easier to visualize the real difference in loss tangent (and avoid unex- TANGENTIAL THOUGHTS: LOSS TANGENT VALUES Figure 1: A field solver modeling loss in an offset stripline 1B1A controlled impedance structure. Figure 3: Finally, calculate and display the losses against frequency, in this case, a loss value of -14 dB. Figure 2: Next, specify a frequency range of 500MHz – 20GHz and a value of 0.015 for loss tangent (TanD).

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