FFT/THD Module

PN302Posted on September 12, 2025 Updated on July 14, 2026

Table of Contents

This article describes how to use the Cockpit FFT/THD module to interact with the user code running on imperix controllers. For users seeking getting-started assistance with Cockpit itself, a separate article is available in PN300.

FFT/THD module basics

The FFT/THD module allows the user to examine variables Scope module data in the frequency domain. User variable and oversampled variable data acquired by the Scope, as well as math variable data calculated by the Scope, can be viewed in this way.

To transfer variables already in the Scope module to the FFT/THD module, right click on a variable or plot area and select the ‘Plot in FFT/THD module’ option in the context menu that appears. A variable doesn’t have to be displayed in the Scope to be acquired and displayed by the FFT/THD module.

FFT/THD module interface

This area shows the time domain waveform of the variables displayed in the module. The preview can also be used to visually adjust the size and position of the window in time over which the Fourier transform is performed.

This is the area where the spectra of the scoped signals are displayed. User variables can be dragged and dropped from the project pane into a plot, which will add them to the Scope module acquisition and display the corresponding spectrum in the area.
The x-axis range (when no zoom is applied) is defined by the Sample rate [Hz] set by the user code (and displayed in the right bar). If multiple plots are created, their x-axis will always remain synchronized, including when zooming.
The user can remove a variable from a plot in the FFT/THD tab of the bottom bar or through the variable plot context menu accessed by right-clicking on the plotted signal.

When switched on, clicking on the plot shows an annotation box for the closest plotted point. The annotation contains the x- and y-axis position of the point, as well as the name of its variable.

Starts or stops the acquisition of the Scope. The button in the FFT/THD module and the start/stop button in the Scope module always have the same state. The same holds for the Acquisition state and the Acquisition loading bar.

Variables added to the module are displayed in the FFT/THD tab of the bottom bar. The bottom bar FFT/THD tab menus allow the user to:

  • Modify each variable’s display style and measurement unit
  • Toggle a variable’s visibility without removing it
  • Remove a variable from the module (and Scope acquisition if it’s not displayed in the Scope module)
  • Move variables between plot areas

In addition to this, the THD and WTHD metrics calculated in the Harmonics mode are displayed here.

Creates an empty plot area at the bottom of the FFT/THD module.

Besides the fields that allow the precise definition of the displayed y-axis range, the FFT/THD module offers the option to switch between different Scale settings for the y-axis:

  • ‘Absolute, linear’, which displays the magnitudes of the calculated Fourier transform, without any visual or mathematical rescaling
  • ‘Absolute, log10’, which arranges the magnitude values in a log-scale
  • ‘Relative, dB’, which transforms the magnitude values according to the Amplitude dB formula: \(20\text{log}_{10}\big(\frac{|X|}{X_{ref}}\big)\), where \(|X|\) is the magnitude of the Fourier transform and \(X_{ref}\) is set in the Ref. val. input field below the Scale menu
  • ‘Relative, log10’, which divides all of the magnitude values with \(X_{ref}\) and arranges the resulting values in a log-scale

Besides the fields that allow the precise definition of the displayed x-axis range, the FFT/THD module offers the option to switch between linear and log-scaling for the frequency axis.

The precise window size can be set through this menu, either directly or by setting the desired frequency resolution. The typical windowing functions (Rectangular, Hann, Hamming, Blackman, Blackman-Harris and Flat top) are provided and can be applied to the samples within the window before transform.

Allows switching between two aspects of viewing the same spectral content:

  • Spectrum mode, where the spectra are plotted as continuous lines
  • Harmonics mode, where the spectral content is plotted as bars

Calculating and displaying spectral content

In the FFT/THD module, the signal spectra are calculated using a variation of the Fast Fourier Transform (FFT) algorithm. Similarly to math variables in the Scope module, the calculations are performed on the PC side, after the acquisition of a scope window.

Spectrum mode

The Spectrum mode gives the best visual overview of the spectral content of the scoped signals. Since the widest window results in the highest frequency resolution, the default FFT window in this mode is the same as the acquisition window.

Technical details

The displayed frequency domain signal is the magnitude of the one-sided spectrum of the scoped variable, with the frequency range \(f \in [0, \frac{f_s}{2}]\). The Scope sampling rate, \(f_s\), corresponds to the control task frequency defined in the user code. The frequency resolution of the resulting spectra depends on the window in time over which the Fourier transform is performed and can be adjusted in the FFT Settings menu in the right bar.

Harmonics mode

In the Harmonics mode the spectral content is calculated with the same method as in the Spectrum mode, but is plotted in a way that makes it easier to visualize and compare the individual harmonics between of different signals. In addition to this, the Total Harmonic Distortion (THD) and Weighted THD (WTHD) metrics are calculated and updated in the bottom bar menu for the variables that are currently displayed in the plot. As is typical for the Power Electronics domain, the default FFT window is such that the frequency resolution is 50 Hz.

Technical details

THD is defined as ratio of the root-mean-square of the harmonic content and the fundamental frequency, typically taken to be 50 or 60 Hz. In the FFT/THD module, THD is calculated according to the following formula:
\(\text{THD}=\frac{1}{V_1}\sqrt{\overset{\infty}{\underset{i=2,3,\dots}{\sum}}V_i^2}\),
where \(V_1\) is the amplitude of the harmonic at the fundamental frequency and \(V_i, i\in\{2,3,\dots\}\) are the amplitudes of the harmonics at the \(i\)-th multiple of the fundamental frequency.

WTHD is calculated in a similar manner, except the harmonics are weighted to be inversely proportional to their order:
\(\text{WTHD}=\frac{1}{V_1}\sqrt{\overset{\infty}{\underset{i=2,3,\dots}{\sum}}(\frac{V_i}{i})^2}\)

Interacting with FFT/THD module graphs and plots

The FFT/THD module consists of one or more plot areas that display graphs based on variables defined by the user code. This section lists out the tips and tricks for manipulating plot variables and plot areas in the module.

Managing plot variables

To add a variable to a plot, select it in the USER VARIABLE, ANALOG INPUTS or DIGITAL I/O section in the project pane, drag and drop it over the plot. To add multiple variables to a plot, keep the Ctrl key pressed and click on the desired variables to select them. Alternatively, click on the first variable to select it, keep the Shift key pressed, and click on the last variable to select. Variables selected like this can then be dragged and dropped into a plot all at once.

Added variables will show up as rows in a table in the bottom bar tab corresponding to the module. Here, variables can be removed, their style and visibility and plotting order can be managed and they can be moved from plot to plot. In the Harmonics mode, the THD and WTHD metrics are shown as well.

All of these actions can also be accessed from context menus by right-clicking on a plotted variable or on the empty space in the plots.

FFT plot variable context menu and plot area context menu

Managing plot areas

New plot areas can be added by clicking on the + button in the bottom right of the module. The total module height can be freely distributed between the plots. To resize plot heights click on the border between the plot areas and drag.

  • To zoom in and out along the horizontal axis, place the mouse cursor over the area of interest. Then, use the mouse wheel to zoom in or out around the location of the mouse cursor.
  • To zoom in and out along the vertical axis, place the mouse cursor over the area of interest. Then press the Ctrl key and use the mouse wheel to zoom in or out around the location of the mouse cursor.
  • To zoom on a specific area, click and drag to draw a blue rectangle over the zoom area.
  • To achieve a horizontal auto-scale, right-click and drag horizontally. A light grey horizontal strip will appear. Release the mouse button to perform the horizontal auto-scale.
  • To achieve a vertical auto-scale, right-click and drag vertically. A light grey vertical strip will appear. Release the mouse button to perform the vertical auto-scale.
  • To enter panning mode press and hold the mouse middle button or Ctrl + Left mouse button and move the mouse.
  • To prevent a plot from changing the displayed range of values, hover over the y-axis and click the lock icon that shows up. The same can be done frome the y-axis pane menu in the right bar.

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