SNLA236 September   2015 DS125BR111 , DS125BR401A , DS125BR820

 

  1.   Linear Repeaters Used in SAS/SATA Applications
    1.     Trademarks
    2. 1 Introduction
    3. 2 SAS-3 Transmit Waveforms
      1. 2.1 Default Transmit Equalization Coefficients
      2. 2.2 Using Linear Equalization to Recover Tx FIR Information
      3. 2.3 Linear Equalization Effects on Jitter Due to Attenuation
    4. 3 Equalizer Placement within a SAS-3 Channel
    5. 4 System Results Using Linear Equalization
      1. 4.1 Linear Equalization Effects on Jitter Due to Attenuation
      2. 4.2 Channel Extension
    6. 5 Passing Out Of Band (OOB) Signals
    7. 6 Conclusions
    8. 7 Appendix A: Layout Considerations
  2.   Revision History

2.3 Linear Equalization Effects on Jitter Due to Attenuation

A key benefit of linear equalization is how it can have a positive impact further down the channel after significant energy has been attenuated away from the initial transmit signal. By moving the point of linear equalization 14” from the SAS-3 transmitter it is easy to see just how much jitter accumulates after only ~ 14 dB of attenuation at 6 GHz. When combined with a typical Reference 1 waveform, the DS125BR820 significantly improves the eye opening and reduces jitter. This is shown in Figure 4.

waveforms_before_after_linear_equalizer_snla236.gifFigure 4. Waveforms Before and After a Linear Equalizer

The combination of Expander package, transmission line traces, and vias can easily add up to a total attenuation of 10-12 dB when connecting a SAS Expander to a set of external miniSAS-HD connectors.