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26 DESIGN007 MAGAZINE I NOVEMBER 2024 end of each column, you can find the "points to remember." I have grouped these into catego- ries for easy reference. Transmission Lines 1. Always use controlled impedance trans- mission lines for digital circuits. 2. For a perfect transfer of energy, the trace impedance must equal the source. 3. Series termination is used to match the source to the trace. 4. Place the series terminator close to the source (< 200 mil). 5. Terminate both ends of a trace in a multi- drop (daisy chain) bus. 6. All drivers where length (in inches) is equal to or greater than the rise and fall time (in ns) must have provision for termination. 7. Mismatched impedance causes signals to reflect back and forth along the lines, which causes ringing. 8. e propagation speed of a signal is (~6 in/ns in FR-4). 9. Signal bandwidth is about five times the fundamental frequency (5th harmonic). 10. e skin depth of copper is 2 µm at 1 GHz. is increases with the square root of frequency. 11. Keep the mark-to-space ratio of the waveform equal as this eliminates all the even harmonics. 12. Flight times of the critical signals should be within specification. is may differ from matched lengths. Check data to strobes and address/command signals to clocks. 13. A transmission line looks very similar to a low-pass filter, which attenuates high frequencies. 14. A square wave is made up of several sinu- soid waveforms of different frequencies. However, only the lower frequency com- ponents can transverse the transmission line. 15. FR-4 has a negligible loss at frequencies below 1 GHz, but since the dielectric loss is frequency dependent, at higher frequen- cies, the dielectric loss of FR-4 increases. 16. e maximum bandwidth of a signal is not determined by the fundamental fre- quency but by the rise time of the signal— 5th harmonic. 17. At high frequencies (> 10 GHz), a non-uniform dielectric in the substrate can cause skew in differential signals. 18. e three common transmission line structures of a multilayer PCB are embed- Figure 1: Eye diagram is used to evaluate the transmission line.