Displaying Nyquist Plots on the MFIA Impedance Analyzer

September 27, 2017 by Tim Ashworth

Nyquist plots are a key tool when displaying complex impedance data. They display the real part of the signal on the X-axis against the imaginary part on the Y-axis. Each point on the plot represents a different frequency.

The MFIA (and the MFLI with the MF-IA option) can display impedance data in real time as a Nyquist plot. This blog post is a step-by-step guide to displaying impedance data in the Sweeper module of LabOne®.

The sample circuit used in this post is shown in Figure 1, mounted onto an MFITF carrier. It was measured in 4-terminal configuration with a test signal of 300 mV. The frequency was swept logarithmically from 1 Hz to 5 MHz and 200 points were acquired.

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Figure 1: A compound RC circuit including a capacitor, a parallel resistor and a series resistor.

Follow these steps to produce a Nyquist plot:

  1. Open the Sweeper module in LabOne and select the desired start frequency and stop frequency in the Control tab.
  2. Within the Control tab, enable the XY mode from the drop-down menu circled (see step 2 in Figure 2). Select Real(Z) as the X signal from the selection tree. If ON-invert is selected, the Y-axis will be inverted to display the Nyquist plot in the conventional way.
  3. In the Vertical Axis Groups field, select the imaginary part of Z with IA Imag(Z) and add to the vertical axis group.
  4. Start the sweep by clicking the single or run/stop button.
  5. Scale the Y-axis using the manual or auto button. The X-axis can be fixed to match the scale of the Y-axis by using the track button. Fixing the scale enables you to see if the resulting plot deviates from a circle.
  6. Save the data in .txt, .csv, ZView or MATLAB format. Alternatively, save the Nyquist plot as vector graphics using the button identified as step 6 in Figure 2  .

Figure 2: Screenshot of LabOne showing the Impedance tab and the Sweeper tab. The orange ovals show the six steps to displaying impedance data as a Nyquist plot. (Click to zoom)

The frequency dependence of this circuit means the capacitance can be considered a short at high frequency, and open at low frequency. The Nyquist plot in Figure 3 is overlaid with a curved arrow showing the direction of the frequency progression. The value of Real(Z) at high frequency gives us the value of Rs (2.2 kΩ), and the value at low frequency gives us Rp + Rs (6.8 kΩ + 2.2 kΩ = 9 kΩ). For more information on interpreting Nyquist plots, see this blog post. 

Figure 3: Screenshot of LabOne showing the Impedance tab and the Sweeper tab. The orange arrow show the direction of increasing frequency of the Nyquist plot. The value of Rs and Rp can be read off the chart using the cursor functions, as highlighted in the orange oval. (Click to zoom)

Real-time Nyquist plots can be quickly and easily displayed on the MFIA thanks to the new XY mode function in the LabOne Sweeper module.

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