PCB007 Magazine

PCB-Jan2015

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42 The PCB Magazine • January 2015 by Karl Dietz Karl DieTz ConsulTing llC Signal Loss Karl's tech talK Signal loss (attenuation) in transmission lines can have a variety of causes [1] . • Radiative loss • Coupling to adjacent traces • Impedance mismatches • Conductor loss • Dielectric loss According to Reference 1, radiative loss is only a small component of loss contributors. Coupling to adjacent lines, on the other hand, can cause signal rise time degradation. It can be modeled quite accurately and proper circuit lay- out can minimize it. Impedance Discontinuities Impedance discontinuities (i.e., variations or changes) along the transmission line cause signal degradation. The cross section of the con- ductor should not change along the transmis- sion line. Such changes affect the impedance. They can be caused by plating height variation, or etch non-uniformity, or localized imperfec- tions such as conductor "dish down" or "mouse bites." Less understood is the change of the proper- ties of the dielectric that contributes to imped- ance mismatch, namely the dielectric constant. Typical dielectric structures are glass weave, re- inforced epoxy resins. Both the dielectric con- stant of the resin as well as that of the glass are bulk properties of the materials and are quite constant throughout the material. The dielec- tric constant of FR-4 resin is about 3.2 and the dielectric constant of glass is about 5.6. Since the glass weave reinforcement has a pattern, there are glass-rich and glass-lean domains in the dielectric. Therefore there are domains of higher and lower dielectric constant surround- ing the signal path, which means the imped- ance changes along the signal path (i.e., there Column

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