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15 March 2013

Spectrogram Display Is Another Tool in the SI Shed

An oscilloscope with spectrum-analysis capability marries the best that both instruments offer in one package. You get the traditional oscilloscope view of signals in the time domain, but you also get the spectrum analyzer’s ability to take the same signal and look at it from the frequency perspective.

A very cool twist on the spectrum analyzer aspect of such oscilloscopes is the Spectrogram display found in Teledyne LeCroy’s spectrum analysis software. This view of the input signal shows a history of spectral changes in the signal in a separate display grid. You can see as many as 256 spectrum captures in a vertically stacked display (Figure 1).

Figure 1: An example of a spectrogram display showing the frequency variation of an FM signal

The spectrogram display is a component of the Spectrum Analyzer option in Teledyne LeCroy oscilloscopes. The spectrogram display shows a history of spectral changes in a separate display grid. Up to 256 spectra are displayed in a vertically stacked display as shown in Figure 1.

The controls in the Spectrogram field of the Spectrum Analyzer dialog box are used to turn on and configure the spectrogram display. The View box turns the display on or off. The Type entry allows selection of either a two-dimensional (2D) or three-dimensional (3D) display. Figure 1 is the monochrome, 2D view in which the spectral amplitude is proportional to the display intensity. By unchecking the Monochrome check box, the spectrogram will indicate spectral amplitude by color. The slider control in the Spectrogram field controls the mapping of both the intensity and color to the amplitude of the spectrum. Figure 2 shows a 3D display with color-graded amplitude encoding.

The spectrogram display is ideal for viewing dynamic frequency changes in frequency-agile communications systems such as Bluetooth. It can also be used to study harmonic structures as seen in Figure 2. It is an extremely useful adjunct to the spectrum analyzer display.

Figure 2: An Example of a 3D spectrogram using color graded amplitude encoding