Using the .FFT Statement

The general form of the .FFT statement is shown below. The parameters are described in .FFT Statement Parameters.

Syntax

.FFT <output_var> <START=value> <STOP=value> <NP=value>
+ <FORMAT=keyword> <WINDOW=keyword> <ALFA=value> <FREQ=value>
+ <FMIN=value> <FMAX=value>

Table 28-2: .FFT Statement Parameters
Parameter	Default	Description
output_var		Can be any valid output variable, such as voltage, current, or power
START	see Description	Specifies the beginning of the output variable waveform to be analyzed - Defaults to the START value in the .TRAN statement, which defaults to 0 s.
FROM	see START	An alias for START in .FFT statements
STOP	see Description	Specifies the end of the output variable waveform to be analyzed. Defaults to the TSTOP value in the .TRAN statement.
TO	see STOP	An alias for STOP in .FFT statements
NP	1024	Specifies the number of points used in the FFT analysis. NP must be a power of 2; if NP is not a power of 2, Star-Hspice automatically adjusts it to the closest higher number that is a power of 2.
FORMAT	NORM	Specifies the output format: NORM= normalized magnitude UNORM=unnormalized magnitude
WINDOW	RECT	Specifies the window type to be used: RECT=simple rectangular truncation window BART=Bartlett (triangular) window HANN=Hanning window HAMM=Hamming window BLACK=Blackman window HARRIS=Blackman-Harris window GAUSS=Gaussian window KAISER=Kaiser-Bessel window
ALFA	3.0	Specifies the parameter used in GAUSS and KAISER windows to control the highest side-lobe level, bandwidth, and so on 1.0 <= ALFA <= 20.0
FREQ	0.0 (Hz)	Specifies a frequency of interest. If FREQ is nonzero, the output listing is limited to the harmonics of this frequency, based on FMIN and FMAX. The THD for these harmonics also is printed.
FMIN	1.0/T (Hz)	Specifies the minimum frequency for which FFT output is printed in the listing file or which is used in THD calculations. T = (STOP-START)
FMAX	0.5NPFMIN (Hz)	Specifies the maximum frequency for which FFT output is printed in the listing file or which is used in THD calculations.

Example

Below are four examples of valid .FFT statements.

.fft v(1)

.fft v(1,2) np=1024 start=0.3m stop=0.5m freq=5.0k window=kaiser alfa=2.5

.fft I(rload) start=0m to=2.0m fmin=100k fmax=120k format=unorm

.fft par(`v(1) + v(2)') from=0.2u stop=1.2u window=harris

Only one output variable is allowed in an .FFT command. The following is an incorrect use of the command.

.fft v(1) v(2) np=1024

The correct use of the command is shown in the example below. In this case, an .ft0 and an .ft1 file are generated for the FFT of v(1) and v(2), respectively.

.fft v(1) np=1024

.fft v(2) np=1024

Star-Hspice Manual - Release 2001.2 - June 2001

Using the .FFT Statement

Syntax

Table 28-2: .FFT Statement Parameters

Example