The spectrogram math function displays the frequency spectrum of a waveform as a vertical column time synchronous with the source waveform. It is an ideal analysis tool to study frequency agile waveform sources. Figure 1 shows an ultra wideband (UWB) signal. It shows a signal using frequency hopping with a repetitive 1,2,3, sequence. The upper trace is the time domain view of the source waveform. The lower trace is the spectrogram. Frequency is read vertically on the spectrogram and time is the horizontal axis. The Spectrogram dialog box shows the setup for this display. The frequency span is 2.11 GHz with a center frequency of 3.96 GHz. Axis labels on the lower trace show the vertical span is from 2.9 to 5.02 GHz. The time axis is synchronous with the source trace so you can see exactly when the frequency changed.

Figure 1:

: The lower trace is the spectrogram of a TFC1 UWB signal. Frequency is read from the vertical axis. The horizontal axis is time and it is synchronous with the upper source waveform

If we horizontally expand the view you can see the dynamics of the frequency change as shown in figure 2. Here each individual frequency ‘burst’ is clearly lined up with the spectrogram.

Figure 2:

A closer view of the spectrogram showing the time alignment with the source trace.

The Spectrum dynamic range control sets the amplitude mapping of the spectrum. In this screen image it is set to 32 dB and is showing only the spectral components within 32 dB of the highest peak. If the Spectrum dynamic range is increased lower amplitude components, possibly including noise, will be shown. In Figure 3 the spectrogram response to a sine wave with its frequency swept from 1 MHz to 80 MHz is shown with the Dynamic range control set to 48 dB. In addition to the fundamental we can see the tracks of the second and third harmonics. These have slopes that are two and three times those of the fundamental and are visible due to the larger dynamic range being mapped onto the spectrogram display.

In addition to the ability to plot a series of frequency spectra as a function of time, the spectrogram math function also allows the presentation of time waveforms as a vertical column. This is useful for viewing changes in a time waveforms dynamic timing. Figure 4 shows this mode of operation with a pulse width modulated signal.

The acquisition was done in sequence mode and the upper trace shows 500 sweeps of the source trace overlaid on the grid. You can see that there are 9 discrete pulse widths.

In the column view of the waveform you can see the variation of the waveform width as a function of time.

Figure 3:

Showing the effect of the dynamic range control

Figure 4:

An example of the “waveform to column” spectrogram mode.

The spectrogram math function is a useful tool for studying waveform dynamics in either the time or frequency domain. It is a feature unique to LeCroy oscilloscopes and another example of the tools these fine instruments make available to the user.