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Signal Integrity - Fundamental Concepts

Signal Integrity (SI) refers to the quality of signals on a transmission line. Good signal integrity means that the signal's voltage or current on the transmission line reaches the expected value as intended, without excessive deviation.

The root cause of signal integrity issues is that in the real world, digital levels (0/1) are not ideal; signals are inherently analog. In low-speed circuits, signal integrity issues are not very noticeable because interconnections are transparent to electrical signals, and analog effects can be ignored. However, as you move into high-speed territory (exceeding 100 MHz or with rise times less than 1 ns), the voltage or current waveforms of digital levels begin to distort, leading to significant discrepancies between the received and transmitted information, resulting in errors. Therefore, when designing high-speed circuits, signal integrity issues must be taken into consideration.

Broadly speaking, signal integrity encompasses three main categories:

  • Signal Integrity (SI): Concerns the distortion of signal waveforms.
  • Power Integrity (PI): Involves the interconnects of power distribution networks and noise on relevant components.
  • Electromagnetic Compatibility (EMC): Refers to the ability of electronic devices to operate properly in electromagnetic fields without being susceptible to electromagnetic interference or causing interference to other devices.

Basic Principles of Signal Integrity

  • Any signal interconnect consists of a signal path and a return path that together form a transmission line. At every step along the transmission line, the signal experiences transient impedance. To ensure optimal signal transmission quality, the transient impedance should remain constant, such as by ensuring the transmission line has a uniform cross-section.
  • Each signal has a return path, not just a ground. Analyzing the return path can help resolve issues.
  • For capacitors, fast edges result in low impedance. This includes both external capacitors and parasitic capacitance on the transmission line.
  • Inductors produce voltage when there is a rapid change in current magnitude or magnetic flux. This can lead to issues such as reflection noise, crosstalk, switch noise, ground bounce, rail collapse, and electromagnetic interference. When there is a rapid change in current flowing through the inductance in the ground return path, it results in ground bounce, which is the cause of switch noise and electromagnetic interference.
  • Signal bandwidth refers to the highest frequency of effective sinusoidal components (with reference to square waves of the same frequency). The bandwidth of an interconnect model represents its ability to accurately predict the actual performance of the interconnect at this highest sinusoidal frequency.
  • In most cases, the formulas for signal integrity are defined or approximate values.
  • Lossy transmission lines result in edge degradation. Signal loss increases with higher frequencies due to skin effect and dielectric loss.

Fundamental Issues of Signal Integrity

Broadly, signal integrity issues can be categorized into the following four types:

  • Signal distortion within a single network
    • Reflection (caused by transient impedance changes)
    • Signal quality issues (frequency-dependent losses leading to edge degradation in interconnects)
    • Timing errors (differences in electrical characteristics or length of interconnects causing misalignment of multiple signals, leading to differential signal distortion)
  • Crosstalk between two or more networks (including power and ground bounce, two special forms)
  • Rail collapse in power and ground distribution (PI)
  • Electromagnetic interference and radiation from the entire system (EMC)

References and Acknowledgments

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