Language Reference

FFT

210

OUTPUT 723;"FFT TRA,TRB,TRC;"

Weight trace B with trace C data, then

 

 

perform F ourier transform. Store re-

 

 

sults in trace A.

220

OUTPUT 723;"BLANK TRB;"

Blank trace B .

230

OUTPUT 723;"VIEW TRA;"

Store and view trace A.

240

OUTPUT 723;"MKN;"

Place a marker at the center of trace A.

250

OUTPUT 723;"HD;"

Blank active function readout.

Description

What to Remember When You Execute FFT

The FFT command performs a discrete Fourier-series transform on the source-trace array, then stores the logs of the magnitudes of the results in the destination array as follows:

1.The source trace is lengthened with zero-amplitude elements if it is shorter than the destination trace.

2.The amplitude of the source trace is weighted according to the window algorithm.

3.The Fourier transform is performed.

Only magnitude components are stored. No phase or sign information is preserved in the results. Because of aliasing, the FFT command computes only the values of the even points of the destination trace. The odd values are obtained by interpolation. The source trace is sent unchanged. Four prede￿ned window algorithms are available with the TWNDOW command. In addition, you can create your own with the TRA, TRB, or TRC command. For maximum precision, the peak values of user-created windows should approach 32,767. The window trace may contain negative values (to

032,768) but they do not increase weighting resolution. The values in the window trace are treated as fractional numbers. The average window value is computed and used to correct (normalize) the results in absolute units.

￿Be sure trace data re￿ects one complete sweep measured with a 0 nm span. SNGLS and TS suggested.

￿TWNDOW (Hanning, Hamming, uniform, ￿at-top) required when using TRA, TRB, or TRC. Use TWNDOW to select a weighting function (trace window) from one of the four available algorithms, or create your own with the TRA, TRB, and TRC commands.

￿The lengths of the source and window traces must be equal and greater than four position units.

￿Transform traces that have been measured during zero span mode.

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