Chapter 28

Performing FFT Spectrum Analysis

Spectrum analysis is the process of determining the frequency domain representation of a time domain signal and most commonly employs the Fourier transform. The Discrete Fourier Transform (DFT) is used to determine the frequency content of analog signals encountered in circuit simulation, which deals with sequences of time values. The Fast Fourier Transform (FFT) is an efficient method for calculating the DFT, and Star-Hspice uses it to provide a highly accurate spectrum analysis tool.

The .FFT statement in Star-Hspice uses the internal time point values and, by default, through a second order interpolation, obtains waveform samples based on the user-specified number of points.

NOTE: New accuracy improvement feature added in the Hspice 99.4 release. The .option fft_accurate or .option accurate (which internally turns on the fft_accurate option) will force Hspice to dynamically adjust the time step so that each FFT point will be a real simulation point. This eliminates the interpolation error and provides the highest FFT accuracy with minimal overhead in simulation time.

Windowing functions, can be used to reduce the effects of truncation of the waveform on the spectral content. The .FFT command also allows you to specify the desired output format, to specify a frequency of interest, and to obtain any number of harmonics, as well as the total harmonic distortion (THD).

This chapter covers the following topics:

Star-Hspice Manual - Release 2001.2 - June 2001