Examining the FFT Output

Star-Hspice prints the results of the FFT analysis in a tabular format in the . lis file, based on the parameters in the .FFT statement. The normalized magnitude values are printed unless you specify FORMAT= UNORM, in which case unnormalized magnitude values are printed. The number of printed frequencies is half the number of points (NP) specified in the .FFT statement.

If you specify a minimum or a maximum frequency using FMIN or FMAX, the printed information is limited to the specified frequency range. Moreover, if you specify a frequency of interest using FREQ, then the output is limited to the harmonics of this frequency, along with the percent of total harmonic distortion.

In the sample output below, notice that all the parameters used in the FFT analysis are defined in the header.

****** Sample FFT output extracted from the .lis file
fft test ... sine
 ******  fft analysis                     tnom=  25.000 temp=  25.000
******
fft components of transient response v(1)
Window: Rectangular
First Harmonic:    1.0000k
Start Freq:    1.0000k
Stop  Freq:   10.0000k
dc component: mag(db)=  -1.132D+02  mag=   2.191D-06  phase=   1.800D+02
frequency    frequency    fft_mag     fft_mag    fft_phase
  index       	 (hz)        (db)                    (deg) 
    2         1.0000k      0.          1.0000      -3.8093m
    4         2.0000k   -125.5914    525.3264n     -5.2406
    6         3.0000k   -106.3740      4.8007u    -98.5448
    8         4.0000k   -113.5753      2.0952u     -5.5966
   10         5.0000k   -112.6689      2.3257u   -103.4041
   12         6.0000k   -118.3365      1.2111u    167.2651
   14         7.0000k   -109.8888      3.2030u   -100.7151
   16         8.0000k   -117.4413      1.3426u    161.1255
   18         9.0000k    -97.5293     13.2903u     70.0515
   20        10.0000k   -114.3693      1.9122u    -12.5492
	total harmonic distortion =     1.5065m  percent

The preceding example specifies a frequency of 1 kHz and THD up to 10 kHz, which corresponds to the first ten harmonics.

NOTE: The highest frequency shown in the Star-Hspice FFT output might not be exactly the same as the specified FMAX, due to adjustments made by Star-Hspice.

.FFT Output Description describes the output of the Star-Hspice FFT analysis.

 

Table 28-3: .FFT Output Description

Column Heading

Description

Frequency Index

Runs from 1 to NP/2, or the corresponding index for FMIN and FMAX. Note that the DC component corresponding to the index 0 is displayed independently.

Frequency

The actual frequency associated with the index

fft_mag (dB),
fft_mag

There are two FFT magnitude columns, the first in dB and the second in the units of the output variable. The magnitude is normalized unless UNORM format is specified.

fft_phase

The associated phase, in degrees

A .ft# file is generated, in addition to the listing file, for each FFT output variable. The .ft# file contains the graphical data needed to display the FFT analysis results in AvanWaves. The magnitude in dB and the phase in degrees are available for display.

Notes:

1. The following formula should be used as a guideline when specifying a frequency range for FFT output:

frequency increment = 1.0/(STOP - START)

Each frequency index corresponds to a multiple of this increment. Hence, to obtain a finer frequency resolution you should maximize the duration of the time window.

2. FMIN and FMAX have no effect on the .ft0 , . ft1 , ..., . ftn files.

Star-Hspice Manual - Release 2001.2 - June 2001