I/O Interfaces & Buses

What is the role of equalizers (such as CTLE and DFE) in high-speed serial receivers?

  1. ATo adjust treble and bass to taste in music
  2. BTo equalize the numbers of 0s and 1s in the data
  3. CTo compensate for the channel attenuating high frequencies more, which smears the signal, and reduce inter-symbol interference
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Answer: C. To compensate for the channel attenuating high frequencies more, which smears the signal, and reduce inter-symbol interference

Board traces and cables attenuate higher frequencies more, so a fast bit stream loses its sharp edges and remnants of earlier bits overlap later ones (inter-symbol interference). A CTLE boosts high frequencies to offset the loss, and a DFE estimates the overlap from bits already decided and subtracts it. The transmitter adds emphasis right after transitions (de-emphasis) as well.

  • A. The name is shared with audio equipment, but the use differs.
  • B. That is the job of coding or scrambling.
  • C. Correct. It restores the shape of the smeared signal.
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More I/O Interfaces & Buses questions

  1. Which statement about the eye diagram used to assess high-speed serial signal quality is correct?
  2. Why does raising serial link speed make it harder to send signals over board traces of the same length, sometimes requiring retimers along the way?
  3. What does a requirement of "bit error rate (BER) at most 10^-12" for a high-speed serial link mean?
  4. Why are high-speed signal traces designed so that their characteristic impedance matches the transmit and receive circuits?
  5. High-speed links such as 100-gigabit Ethernet are sometimes built by bundling, for example, four 25 Gb/s lanes. Which describes this approach?