Spectrum Software
Industrial Strength Simulation




Spectrum Software has released Micro-Cap 11, the eleventh generation of our SPICE circuit simulator.

For users of previous Micro-Cap versions, check out the new features available in the latest version. For those of you who are new to Micro-Cap, take our features tour to see what Micro-Cap has to offer.




Using FFT Windows


The FFT Windows feature provides a simple method that can be used in order to plot the basic FFT functions. It helps to automate many of the choices required to produce a Fourier harmonic content plot. These windows can display the harmonics (in magnitude or in dB), the phase of the harmonics, and the real and imaginary components of the harmonics. In previous versions of Micro-Cap, if one of these functions was to be plotted, it had to be specified before the simulation was run. With the FFT Windows, any waveform that has been plotted in the simulation can have its harmonic contents derived after the simulation has finished. The FFT Windows are simple to use as can be seen in the following example.

The analysis below is a 500us transient simulation of an operational amplifier modelled at the transistor level. The input to the opamp is a 10mV, 10kHz sine wave. The opamp has been configured to have a gain of 100 although at its 10kHz operating frequency, the gain is reduced to 55. Both the input and output waveforms of the amplifier are being plotted.

Standard Transient analysis simulation

In order to invoke the FFT Windows, you can either right click on the name of the waveform in the analysis plot and select Add FFT Window, or you can go to the Transient menu, click on FFT Windows, and select Add FFT Window. When either one of these actions is done, a Properties dialog box will be invoked that controls the display of an FFT Window. The Plot page of the FFT Window Properties dialog box is displayed below. This page controls which waveforms will be plotted in the window. The default waveform is the harmonics of the selected curve which is V(Out) in this instance. If we also want to plot the phase of the harmonics in the same window, we would just need to add another expression. Click on the Add button to add a new expression. Change the What To Plot field so that Phase is selected. V(Out) will be the target expression for this function also. Next, change the Plot Group for the phase expression to 2 so it will be in a separate group from the harmonics expression. Click OK to create the FFT Window.

Plot page of the FFT Window Properties dialog box

The resultant plot is displayed below. The plot displays both the harmonics and the phase of the harmonics for the V(Out) waveform. The X scale for these plots has been manually set in the Format page of the Properties dialog box to display from DC to 100kHz.

FFT Window displaying the harmonics and phase of V(Out)

In the transient analysis plot, the output waveform doesn't settle down until the second cycle, so there is an initial transient that will have an effect on the harmonic results. This initial transient can be easily omitted from the harmonic results. Hitting F10 or double clicking in the FFT Window will invoke the Properties dialog box for that window. The time range that the harmonic functions operate in is defined in the FFT page of the Properties dialog box as shown below.

FFT page of the FFT Window Properties dialog box

The FFT page initially has its Upper Time Limit set to TMAX and its Lower Time Limit set to TMIN. These variables take their values from the maximum and minimum times specified in the Time Range field of the Transient Analysis Limits dialog box. As initially set, the entire simulation will be included when deriving the harmonic results. To omit a startup transient, change the time window that is being used. For our analysis, setting the Lower Time Limit to 400u will create a 100us window in which one full cycle of the output waveform is analyzed. The simulation information from 0s to 400us will be ignored. The resultant display is shown below.

Note that the frequency interval that the simulation is being sampled with has changed from 2kHz to 10kHz. The frequency interval is determined by the following equation:

1/(Upper Time Limit - Lower Time Limit)

FFT page of the FFT Window Properties dialog box

A right click on the waveform name in the plot will also display any FFT Windows that have already been associated with the specified waveform so that they can easily be recalled. The FFT Windows submenu under the Transient menu will show all FFT Windows associated with the analysis.

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