National Instruments NI Spectral Measurements Toolkit - Manual (Page 14)

NI Spectral Measurements Toolkit User Guide 14 ni.com
Block Zoom FFT
Use a block zoom FFT in situations when you cannot access data until the
data acquisition is complete. The block zoom FFT is a nondestructive zoom
FFT because it stores data before processing, so the data is available in its
original form if you need it for other operations. The block zoom FFT is an
algorithm that calculates a portion of a large FFT. The block zoom FFT also
improves the frequency resolution, df, by increasing the number of points
that the FFT processes.
A block zoom FFT uses only the part of a large FFT that represents the
frequency range you analyze. For example, if the input data has a length
L
× M, an FFT on the original data results in L × M points of FFT spectrum.
To analyze only 1/M of the whole spectrum, or L frequency bins, use a
block zoom FFT. The block zoom FFT computes L points of the original
L
× M point spectrum faster and with fewer calculations than if you
perform a large FFT on the entire data set and remove the unwanted
portion.
Perform the block zoom FFT technique by using the SMT Zoom FFT VI.
This VI is located on the Zoom FFT palette, which is a subpalette of the
SMT Advanced palette.
Determining When to Use Continuous or Block Zoom FFT
Choosing which zoom FFT to use for a particular application depends
on many factors, including system speed, memory, acquisition rate, and
application requirements.
An advantage of the continuous zoom FFT is that you can update the results
continuously to give a smooth display and minimize the time it takes for
transients to appear in the displayed spectrum. You can control the update
time with the %overlap parameter of the advanced settings cluster of the
SMT Config Cont Zoom FFT VI. This VI is located on the SMT Advanced
palette. A setting of
0% updates like a block zoom FFT and waits for the VI
to process an entire new data set before returning a result. A setting of 50%
updates twice as fast as a setting of 0% by reusing the last half of the
previous data block to return an updated result after the VI acquires and
processes half of the new data set.
You cannot predict whether the continuous zoom FFT can sustain a certain
acquisition rate in real time, so the best option is to try running the
application using the SMT Cont Zoom FFT VI. If you receive buffer
overflow errors from the acquisition VI, either reduce the acquisition
rate or use the block zoom technique.