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Nyquist Plot

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • A Nyquist plot, named after Swedish-American engineer and physicist Harry Nyquist who developed it, is used in control systems engineering and signal processing to analyze a system’s stability and frequency response.

  • A Nyquist plot supplies data on the system's gain margin and phase margin, which suggests how much a system can be amplified or delayed before becoming unstable.

  • While a Nyquist plot doesn't dictate specific PCB design decisions, it plays a role in ensuring that the PCB functions as part of a larger control system.

Nyquist plot for a sample function with s = jω

Nyquist plot for a sample function with s = jω.

A Nyquist plot is used in control systems engineering and signal processing to analyze a system’s stability and frequency response. Harry Nyquist was a Swedish-American engineer and physicist who developed the plot. Typically, it is used to analyze linear time-invariant systems, such as electronic circuits, control systems, and filters.

How to Construct a Nyquist Plot

Frequency Sweep

Sweep the frequency of the input signal across a range of values, often from zero to infinity or some finite upper limit. This is usually done by varying the frequency of a sinusoidal input signal.

Transfer Function

Analyze the system using its transfer function, which relates its output to the frequency domain input. The transfer function is represented as a complex function of frequency (s = jω, where s is the complex frequency variable, j is the imaginary unit, and ω is the angular frequency).

Polar Plot

Calculate the complex value of the system's transfer function and plot it in polar coordinates for each frequency value in the sweep. The magnitude of the transfer function (gain) is plotted as the distance from the origin (the pole) to the point, and the phase angle is plotted as the angle from the positive real axis.

Nyquist Plot

Create a Nyquist plot by repeating this process for various frequencies. This plot can analyze the system's stability and frequency response.

Insights from a Nyquist Plot

A Nyquist plot provides insight into a system's stability and frequency response characteristics. It supplies data on the system's gain margin and phase margin, which suggests how much a system can be amplified or delayed before becoming unstable. Moreover, the plot helps identify the system's resonance frequency and damping characteristics, which aid in the assessment and design of control systems. Key insights from a Nyquist plot include:

  • Stability Assessment: The system is stable if the Nyquist curve (the plot) encircles the point (-1, 0) in the complex plane. The number of encirclements also relates to the number of poles in the right-half plane, which can affect stability.
  • Gain and Phase Margins: The distance from the Nyquist curve to the point (-1, 0) on the real axis quantifies the gain margin. A larger gain margin implies greater system stability. The phase margin, determined by the phase angle at the crossover frequency, indicates how much phase lag can be introduced before the system becomes unstable.
  • Frequency Response: The shape of the Nyquist plot provides insights into resonance frequencies, bandwidth, and damping characteristics. Peaks and troughs in the plot correspond to resonances and antiresonances, respectively.
  • Control System Design: Engineers use Nyquist plots to design and tune control systems. By analyzing the plot, they can adjust system parameters to meet desired performance criteria while ensuring stability.

Nyquist plots offer a comprehensive view of a system's stability, frequency response, and potential for control system design and optimization, making them a valuable tool in control engineering. It is a valuable tool in engineering and is often used alongside other techniques like Bode plots and root locus plots for system analysis and design.

Nyquist Plot and PCB Design

While a Nyquist plot doesn't dictate specific PCB design decisions, it plays a role in ensuring that the PCB functions as part of a larger control system. By understanding the system's stability and frequency response characteristics, you can make informed decisions about component selection, control loop design, and signal integrity, ultimately leading to a more reliable and effective PCB design for control applications. A Nyquist plot indirectly affects PCB design in the following ways:

  • Filter Design: If your PCB includes filters for signal conditioning or noise reduction, the Nyquist plot can help you understand how these filters will affect the frequency response of your system. This knowledge can guide your choice of filter components, cutoff frequencies, and filter types to meet the desired performance specifications.
  • Feedback Control: PCBs in control systems often involve feedback loops for stability and precision control. A Nyquist analysis can help you design and tune these control loops by providing insights into system stability, phase margins, and gain margins. Properly designed control loops on your PCB can ensure stable and accurate control of your system.
  • Signal Integrity: In high-speed digital or analog PCB design, signal integrity is critical. Nyquist plots can be used indirectly to understand the frequency response of signal paths and transmission lines on the PCB. This information helps optimize trace layouts, match impedance, and mitigate signal reflections and distortion.
  • Component Selection: Understanding the Nyquist plot of your system can inform your choice of components, such as amplifiers, op-amps, or filters, to meet specific frequency response requirements. This can impact the selection of capacitors, resistors, and other passive components on the PCB.
  • Testing and Validation: Once the PCB is designed and manufactured, you may perform functional tests and validations. A Nyquist plot can be a reference to assess whether the PCB's actual frequency response matches the expected behavior. Any discrepancies can guide troubleshooting and adjustments.

Cadence Can Help

A Nyquist plot is primarily a tool used in control systems analysis, and its direct impact on PCB design is somewhat limited. However, it plays a role in ensuring that the PCB works as part of a larger system and needs to be considered in the design of the PCB.  Designers can use Cadence PCB Design and Analysis Software tools and  OrCAD PCB Designer to ensure a robust PCB design.

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