SI Insights: Five Rules of Thumb You Need for High-Speed Serial Link Analysis
February 13, 2008 |Estimated reading time: 1 minute
The earlier in the design cycle you can make the right decisions about a technology choice, a set of design features or an interconnect architecture, the shorter will be the development cycle. While a rule of thumb should never be used to sign off on a design, it can be used making a first pass analysis of design choices.
A rule of thumb is the tool to use when an OK answer NOW! is better than a good answer late.
In a high-speed serial link, the frequency dependent losses and reflections from vias stubs and other discontinuities dominate the performance of a channel. A simple, parameterized model that enables prediction of the differential insertion loss of the link can be a tool to quickly explore design trade-offs. Five simple approximations can help create this simple estimate. You can download a simple spreadsheet containing these approximations with the full version of this note from www.BeTheSignal.com.
- The attenuation from the dielectric is given by:
- Attenuation from the conductor (stripline)
- Attenuation in a coax cable, due to the conductor
- What frequency should be used for the loss in a simple loss budget analysis?
- Impact from the first order reflections from impedance discontinuities such as DC blocking capacitor pads, vias and component attach pads:
For example, if the return loss from a discontinuity is -10 dB, the insertion loss is reduced by -0.5 dB.
Note: this is the short form of SI Insights. You can download a PDF of the full version from http://www.bethesignal.com/.
Eric Bogatin is president of Bogatin Enterprises LLC. He has a B.S. in physics from MIT and a master's and Ph.D. in physics from the University of Arizona. He has held senior engineering and management positions at Bell Labs, Raychem, Sun Microsystems, Ansoft and Interconnect Devices
Register for Eric's next SI training classes at www.BeTheSignal.com:
Dallas, TX, March 3-7, 2008Essential Principles of Signal Integrity SI Characterization Techniques Hacking Any Interconnect
San Jose, April 7-11, 2008Essential Principles of Signal Integrity Best Board Design Practices Designing Transparent Vias