Ryu,
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See the file STEP_Sine_FFT.zip that I uploaded to the Temp folder.? It shows what happens to your waveform when you .STEP the .TRAN simulation's stop time.? When the waveforms are separated into different plot panes, you can see that part of some waveforms are missing.? That causes the FFT to process a modulated signal, unless you restrict the time interval that is passed to the FFT.? Without that step, the FFT processes the waveform over the full extent of the simulated data, which includes time where some of the .STEPs were not simulated.
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Andy
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