User`s guide

Correcting Y-axis Data
Y-axis data is stored in the instrument’s internal unit (usually volts) with all calibration corrections
applied. Two other kinds of correction, however, are left for you to apply:
• Engineering unit (EU) correction.
• Window correction. (Necessary only for FREQ or ORDER domain data)
Note Do not apply a window correction factor if it has already been applied by the instrument.
A field in the Channel Header record — windowCorrMode — tells you whether or not
the instrument has applied a window correction factor.
Also, do not apply a window correction factor if you are using the SdfYdata function; it
will automatically apply these corrections to the data. See the chapter ‘’Using SDF Data
with C Programs’’ for more information on SdfYdata.
EU correction allows you to convert y-axis data from the instrument’s internal unit to some user-defined
unit (such as g — the acceleration of gravity). An EU correction factor is included in each Channel
Header record; the factor’s field name is int2engrUnit.
Window correction compensates for the shape of the resolution bandwidth filter in swept-tuned
measurements. It compensates for the frequency-domain shape of the windowing function in FFT-based
measurements. (FFT, which stands for Fast Fourier Transform, is an implementation of the Discrete
Fourier Transform. Both are math algorithms that transform data from the time domain to the frequency
domain.)
Two window correction factors are included in each Channel Header record’s SDF_WINDOW structure:
wideBandCorr: corrects for lost power when you are analyzing wide-band signals
(like random noise).
narrowBandCorr: corrects for lost amplitude when you are analyzing narrow-band signals
(like a sinesoidal wave).
Check the windowCorrMode field (also in SDF_WINDOW) to find out if your instrument has already
applied one of these factors to the Y-axis data. The field contains encoded values:
0: no window correction.
1: narrowBandCorr.
2: wideBandCorr.
Standard Data Format (SDF)
B-30