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The Baseband Subset VIs enables you to compute a subset of the baseband FFT measurement. Subset analysis uses the Discrete Zak Transform to compute a subset of the baseband FFT. The frequency resolution for spectral measurements computed with the Baseband Subset VIs equals the frequency resolution for measurements made with the Baseband FFT VIs. The acquisition time determines the frequency resolution. The only way to achieve a finer frequency resolution is to increase the length of the time record. In the case of baseband or subset analysis, a longer time record implies a larger block size.

The algorithm that the Baseband Subset VIs use computes only the desired spectral lines. The only programming difference between the Baseband Subset VIs and the Baseband FFT VIs is the additional parameter frequency range. The frequency range parameter specifies which spectral lines the Baseband Subset VIs compute. The computed spectral lines are always inclusive of the start frequency and the stop frequency.

Note  Setting the start frequency to 0 Hz and the stop frequency to fmax yields the same spectrum as the corresponding Baseband FFT VIs. If you set the stop frequency to –1, the Baseband Subset VIs return the Nyquist frequency as the highest frequency in the computed spectrum.

The following consideration can help you decide when to use the Baseband Subset VIs instead of the Baseband FFT VIs:

• The required block size yields an acceptable frequency resolution.
• The analysis of a narrow subset of the baseband span offers better processing performance than the Baseband FFT VIs can provide.