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78 The PCB Design Magazine • July 2017 TRANSMISSION LINE LOSSES • Different materials are available locally compared to offshore. • Materials are cheap in mass production compared to prototypes. As an example, a low-cost FR-4 material such as Taiwan Leader FR4135 core has a Dk of 4.8 and Df of 0.022 specified at 1 MHz. On the other hand, a high-speed, 20 GHz material, Isola I-Speed (spread glass weave) material has a Dk of 3.55 and a Df of only 0.0059. Spread weave glass also helps eliminate skew and has far less loss than the standard FR-4 material. Se- lection of the most appropriate dielectric mate- rial is in important consideration for product performance. Figure 4, shows the properties of both materials in the iCD Dielectric Materials Library of over 31,000 rigid and flexible materi- als to 100 GHz. Finally, while radiation loss is important, when it comes to electromagnetic compliancy (EMC), in practice the amount of energy lost to radiation is very small compared to other loss processes, and this mechanism will have little impact on the received signal. As frequencies increase, engineers and PCB designers not only have to contend with tech- nical challenges such as system timing, cou- pling, impedance discontinuities and radiation, but must also deal with a significant number of frequency dependent variables. However, choosing the right stackup and the most appro- priate dielectric material, for your application, are a good start to reliable performance by in- creasing bandwidth and improving signal and power integrity. Points to Remember • The frequency dependence of transmis- sion lines causes rise time degradation and re- duces the upper bandwidth of the signal result- ing in reduced channel data transfer rate. • As signals propagate along a lossy transmis- sion line, the amplitude of the high-frequency components is reduced, in magnitude, whereas the low-frequency components are unaffected. • High-frequency components are reflected back to the source, creating ringing and over- shoot unless absorbed by a source termination. • The skin effect imposes a frequency de- pendency on conductors. • As the frequency increases, the current is forced into the outer surface of the copper, due to the skin effect, dramatically increasing loss. • The AC resistance will remain approxi- mately equal to the DC resistance until the frequency increases to a point where the skin depth is smaller than the conductor thickness. • Routing in symmetric stripline structure essentially doubles the current carrying capac- ity, of conductors, at high-frequencies. • AC dielectric loss is the dissipation of en- ergy, through the movement of charges, in an alternating electromagnetic field as polariza- tion switches direction. At high frequency, the conductivity increases due to the increased mo- tion of the dipoles. • A low Dk is desirable for high-frequency design. Also, dielectric loss tends to be lower in materials with lower dielectric constants–which is also beneficial. • Selection of the most appropriate dielec- tric material is in important consideration for product performance. • The amount of energy lost to radiation is very small compared to other loss processes. PCBDESIGN References 1. Barry Olney's Beyond Design columns: Transmission Lines–from Barbed Wire to High- Speed Interconnects, Material Selection for SERDES Design. 2. Transmission Line Losses—Microwaves 101. 3. Signal and Signal Integrity Simplified, by Eric Bogatin. 4. High-Speed Digital System Design, by Ste- phen Hall. Barry Olney is managing director of In-Circuit Design Pty Ltd (iCD), Australia. The company is a PCB design service bureau specializing in board-level simulation, and developed the iCD Design Integrity software incorporating the iCD Stackup, PDN and CPW Planner. The software can be downloaded from www.icd.com.au.