Fourier and Inverse Fourier Transforms FFT/IFFT

Fourier (FFT) and Inverse Fourier (IFFT) Transforms operate and display like any other graph type (e.g. DC, AC, TRAN etc). Once the FFT/IFFT has been calculated select those graph types and plot signals. After each run, calculate FFT data from a TRANSIENT run and an inverse IFFT by pressing the "ff" button (with red bits on top and bottom), or from the Waveforms/Calculate FFT/IFFT menu item. The FFT/IFFT can be automatically calculated on each load of new output data by selecting the check box in the Options/General tab (blue "PO" button). To save calculation time, restrict the calculation to only signals that have been already selected for display. Note: if the schematic configuration is changed between runs, the selected run may not be the same. FFT data is only performed on existing Transient data. IFFT data is only performed on existing AC data.

IFFT AC data points must be equal distance to each next point, so make sure the sweep is in linear mode, not as decades as usually done with AC plots.

What is a Fourier Transform?. Well Fourier transforms on the time domain transient data will produce a graph of the its frequency spectrum. This can be most useful in examining the level of distortion components from an amplifier when clipping.

FFT Notes:
1 Set the graph to the type of data needed to be display. e.g. "FFT" or IFFT. Use the green buttons or double click on the graph and set via the graph tap of its dialog setup window, or the view menu. SuperSpice does not know what graph type to display unless set.
2 Select signals, either by alt-click on the wire/voltage or pin/current, place or move the test point, or double click the graph and use the signals tab and select individually. It does not default to the last displayed signals for other graph types.
3 Make sure the FFT/IFFT has been calculated first. To calculate the FFT, press the green "ff" with red bits on top and bottom. (Not the "FF" button, which is fantastic filter). Then press the green "fft" or "ift" buttons (first click on the correct graph to activate that specific graph) this will set the graph type to FFT from trans or the inverse IFFT from AC data. BUT, as each graph type has its OWN set of signals to be displayed, to alt-click on the wire pin to display the signal in FFT form as required. It don't know yet what was previously displaying that signal via the TRAN graph. Note all signal states for all graph types are saved between runs. SuperSpice can automatically calculate the fft on each load, so new data gets updated immediately by setting the option via the blue "PO" button dialog.

4 The "Linearize Transient data" check must be enabled for FFT data. By default, transient run data step size is variable.

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