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In some applications, you need to obtain spectral information with a very fine frequency resolution over a limited portion of the baseband span. In other words, you must zoom in on a spectral region to observe the details of that spectral region. Use the zoom Fast Fourier Transform (FFT) to obtain spectral information over a limited portion of the baseband span and with greater resolution. Just as in baseband analysis, the acquisition time determines the frequency resolution of the computed spectrum. The number of samples used in the transform determines the number of lines computed in the spectrum.

Zoom FFT analysis achieves a finer frequency resolution than the baseband FFT. The Zoom FFT VIs acquire multiple blocks of data, modulates, and downsamples to obtain a lower sampling frequency. The block size is decoupled from the achievable frequency resolution because the Zoom FFT VIs accumulate the decimated data until you acquire the required number of points. Because the transform operates on a decimated set of data, you need to compute only a relatively small spectrum. The data is accumulated, so do not think of the acquisition time as the time required to acquire one block of samples. Instead, the acquisition time is the time required to accumulate the required set of decimated samples.

The Zoom FFT VIs complete the following steps to process the sampled data:

1. Modulate the acquired data to center the analysis band at 0 frequency.
2. Filter the modulated data in the time domain to isolate the analysis band and prevent aliasing when the data is resampled at a lower sampling frequency.
3. Decimate the filtered data to reduce the effective sampling frequency.
4. Accumulate the decimated data until sufficient samples are available to compute the spectrum.
5. Use the Discrete Zak Transform to efficiently compute the desired spectral lines.
6. Demodulate, or shift, the computed spectrum.

Frequency Resolution of the Zoom FFT VIs

Use the Zoom FFT VIs to compute the spectrum of a signal over a narrow frequency range with an arbitrarily fine frequency resolution. To get an approximation of the frequency resolution seen with the Zoom FFT VIs, use the following formula:

Note  The exact frequency resolution is returned as df in the spectrum that the Zoom FFT VIs compute.

Zoom Measurement

The following example demonstrates a zoom power spectrum measurement.

Acquire a sine wave at 1,390 Hz with a National Instruments dynamic signal acquisition (DSA) device and the VI displayed in the following block diagram.

Acquire the signal at 51.2 kHz. The VI reads the data in blocks of 2,048 samples. Compute the frequency resolution of this measurement using baseband analysis with the following equation:

Use the SVFA Zoom Power Spectrum VI to analyze a narrow band with a much finer frequency resolution. The following front panel shows the result of limiting the measurement to the frequency band between 1 kHz and 2 kHz and computing 400 lines.

Derive the frequency resolution of the computed spectrum with the following equation:

The following front panel shows the zoom settings control used to acquire the zoom measurement results displayed in the previous front panel.

Use the zoom settings control to specify the frequency range, window, number of lines, and percent overlap used in the zoom analysis.

You also can use the Zoom Power Spectrum Express VI to configure zoom measurements interactively.