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July 2017 • The PCB Design Magazine 77 the trace and the proportion will depend on the proximity, of the trace, to the upper and lower reference planes that will share the return cur- rent. Essentially, routing in symmetric stripline structure doubles the current carrying capacity, of conductors, at high-frequencies. There are two frequency-dependent process- es that attenuate signals in a transmission line: the series resistance through the signal and re- turn path conductors, and the shunt resistance through the lossy dielectric material. Dielectric losses can also be broken down into two components: DC and AC losses. Since dielectric materials, used to fabricate PCBs, are not perfect insulators, there is a small DC loss associated with the DC drop across the mate - rial, between a signal conductor and reference plane(s). The direct current that flows in the ca- pacitor, formed by the structure, is referred to as leakage current. However, the conductor losses are dominant and the DC loss is usually negli- gible. But as frequency increases, the AC loss also increases. Subsequently, it is important to under- stand the fundamental mechanisms that cause the dielectric losses to vary with frequency. AC dielectric loss is the dissipation of en- ergy, through the movement of charges, in an alternating electromagnetic field as polarization switches direction. When a voltage is applied across a capacitor, an electric field is generated. This field will cause the dipoles, in the dielec- tric, to align with the field. The motion of these dipoles, alternating from one electrode to the other, appears as a transient current through the material. At high frequency, the conductiv- ity increases due to the increased motion of the dipoles. The measure of the number of dipoles, in a material, and how far each can rotate, in the applied field, is called the dissipation factor (Df) or loss tangent (d) of the material. Another important property of dielectric materials is the dielectric constant (Dk) or rela- tive permittivity (Er). Dk is the ratio of the amount of energy stored in a material by an ap- plied voltage, relative to that stored in a vacu- um. If a material with a high dielectric constant is placed in an electric field, the magnitude of that field will be measurably reduced within the volume of the dielectric. Therefore, a lower Er is desirable for high-frequency design. Also, dielectric loss tends to be lower in materials with lower dielectric constants–which is also beneficial. Wit h so many materials to choose from, which are the best for your specific application? There are a number of factors to be considered: • Low cost generally means low quality. • The price of poor yield drives up the final material cost. TRANSMISSION LINE LOSSES Figure 4: Taiwan Leader, FR4135 vs. Isola, I-Speed material (Source: iCD Design Integrity).