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Module 6: Multi-Gb/s Signaling Topic 4: Modulation

Module 6: Multi-Gb/s Signaling Topic 4: Modulation. OGI EE564 Howard Heck. Where Are We? . Introduction Transmission Line Basics Analysis Tools Metrics & Methodology Advanced Transmission Lines Multi-Gb/s Signaling Projections, Limits, & Barriers Differential Signaling Clocking Issues

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Module 6: Multi-Gb/s Signaling Topic 4: Modulation

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  1. Module 6: Multi-Gb/s SignalingTopic 4: Modulation OGI EE564 Howard Heck Section 6.4

  2. Where Are We? • Introduction • Transmission Line Basics • Analysis Tools • Metrics & Methodology • Advanced Transmission Lines • Multi-Gb/s Signaling • Projections, Limits, & Barriers • Differential Signaling • Clocking Issues • Equalization Techniques • Modulation Techniques • Special Topics Section 6.4

  3. Contents • Modulation/Encoding • Pulse Amplitude Modulation • Pulse Width Modulation • Multi-Carrier Modulation • Simultaneous Bi-directional Signaling • Summary • References Section 6.4

  4. Amplitude Modulation Intro Simplest form is binary signaling: • Maps all of the possible voltage levels into one of two values (“0” or “1”), as shown here: • Vmin and Vmax denote the minimum and maximum voltage levels. • V0 and V1 represent the “nominal” voltages for the “0” and “1” symbols. • Vthresh is the threshold that separates the two binary values. • Note: the levels could have any arbitrary value. • Also known as pulse amplitude modulation (PAM) or multi-level signaling Section 6.4

  5. N-PAM Signaling • We can encode a set of N symbols on a continuous variable, V, where . • Assign each symbol a nominal value . [9.2.1] • Define N-1 thresholds . [9.2.2] • Example: 4 PAM Section 6.4

  6. PAM Transfer Rate • The effective transfer rate (bits/sec) is [9.2.3] TR = transfer rate (bits/sec) m = # of signal levels f = operating frequency [Hz] Section 6.4

  7. Why Use PAM? PAM may be beneficial when: • Channel has severe bandwidth roll off (D>6) • Channel has high SNR in passband • Circuits are symbol rate limited Requirements & trade-offs: • Reduced swings (SNR) • More complex circuitry (cost) • Increased latency (encoding and decoding) Section 6.4

  8. Pulse Width Modulation (PWM) Intro • To use PWM, we must encode the data into pulse widths. To code m data bits, we need n edges: [9.2.4] T = Minimum pulse width. Limited by interconnect bandwidth. t = Edge placement capability. Limited by interconnect & circuit jitter. • Transfer rate: [9.2.5] Section 6.4

  9. PWM Performance Trends Section 6.4

  10. PWM Spectral Comparison for a 10 Gb/s Example NRZ • 1 bit/symbol PWM • 100 ps symbol width Example Waveforms PWM • 4 bits/symbol • 20 ps edge separation • 16 edge positions • 400 ps symbol width Power Spectrum Cumulative Power Spectrum Section 6.4

  11. Simultaneous Bi-Directional Signaling • Also known as Full Duplex Signaling. • Goal: double the effective transfer rate by sending bits simultaneously in both directions. • What happens when one agent drives high while the other drives low? • Assuming the drivers are of equal strength, the line gets pulled to ½VCC. We must have a way to handle this situation. • We’ll do it by using our knowledge of the state of the driver to adjust the switching threshold of the receiver. Section 6.4

  12. Simultaneous Bi-Directional Signaling #2 • The reference generator circuits use the output signal to dynamically adjust the (-) input to the receiver, as shown in the table below: Section 6.4

  13. SBD Trade-offs Requirements: • Near perfect termination • Minimized reference voltage noise • Added I/O complexity • consumes more area on the silicon (cost) • more difficult to test (cost) Section 6.4

  14. Summary Section 6.4

  15. References General • W. Dally and J. Poulton, Digital Systems Engineering, Chapters 4.3 & 11, Cambridge University Press, 1998. • S. Dabral and T. Maloney, Basic ESD and I/O Design, Wiley Interscience, New York, 1998, ISBN 0-471-25359-6. Section 6.4

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