Making a 3-D Spectrogram
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Most instruments make measurements in either the time or frequency domains. For example, a broad range of equipment ranging from audio analyzers to spectrum analyzers is available for making frequency-domain measurements, while digitizers, oscilloscopes, and other instruments are available for making time-domain measurements. However, joint time-frequency analysis, often displayed using a 3-D spectrogram (where the three dimensions are time, frequency, and amplitude), is substantially more difficult.
The wide (20 MHz) vector capture capabilities of the PXI-5660 RF signal analyzer, combined with Spectral Measurements Toolset algorithms and the ability of LabVIEW to display 3-D objects using ActiveX controls, make this measurement possible. A 3-D spectrogram is an important diagnostic tool, especially when looking at modulated waveforms. When a signal is downconverted and digitized by the RF signal analyzer, the Spectral Measurements Toolset computes the spectrogram. To create the 3-D spectrogram, a short time Fourier transform (STFT) is calculated. A STFT is composed of an array of complex FFTs at fixed time intervals. The results of the STFT are manipulated using array functionality in LabVIEW, and a 3-D plot of the result is displayed using an ActiveX graph object called from LabVIEW.
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