Introduction

Modern oscilloscopes routinely include the ability to perform fast Fourier transforms (FFTs) to aid debug of frequency-related design issues. Setting up an oscilloscope to perform an FFT isn’t always the easiest thing to do. On the other hand, many users are far more comfortable using a spectrum analyzer for spectral measurements. Thus, oscilloscopes such as Teledyne LeCroy’s WaveSurfer 10 oscilloscope offer a spectrum-analysis software option. The software gives users a spectrum analyzer-like interface, allowing them to quickly perform FFTs with the oscilloscope without having to be concerned with the details of setting up the measurement. Furthermore, it eliminates the need for a discrete spectrum analyzer on the bench, thereby maximizing the real-estate benefits of the already compact WaveSurfer 10.

The Spectrum Analysis User Interface

The Spectrum Analysis feature is invoked in the Analysis pulldown menu. The initial screen, after adjustments to suit the 1-kHz CAL input signal, will appear as in Figure 1.

Figure 1:

Upon invoking the Spectrum Analysis option in the Analysis pulldown menu, users are presented with the controls dialog at the bottom of the screen

A closer look at the Spectrum Analyzer dialog box is shown in Figure 2. Within that interface, the Spectrum Analyzer tab presents a familiar set of parameters for setting up the instrument for viewing and measurement of waveforms in the frequency domain. By selecting center span control, one may easily set the center frequency and the total frequency range of the FFT. Or, the user may choose to select the Start and Stop frequencies. Of note, the Windowing control near at bottom right allows for selection of the weighting window to be used for the FFT. Options include Von Hann (Hanning), Hamming, Flat Top, and Blackman Harris.

Figure 2:

A closer look at the Spectrum Analysis controls dialog at the bottom of the screen

Resolution bandwidth in this implementation has been automatically set up to strike a balance between the requested frequency span, the memory, and sample rate usage. If the user prefers manual controls, the automatic function may be disabled. The desired resolution bandwidth may be defined by the user.

In the Mode section, users may select from Normal, Average, or Max Hold modes. Normal mode displays the power spectrum of the source trace with a choice of persistence on or off. In Averaging mode, users may enter the number of spectra to be averaged. This mode is effective in reducing signal noise to see more of the harmonic or carrier detail. Max (Peak) Hold mode aids in swept-spectrum measurements by showing the history of peak values across the frequency axis. Max Hold shows the maximum level that the signal attains. It is also useful in finding infrequent spurs.

Touching the Spectrogram / Scale tab of the dialog displays settings for Scale, Spectrogram, and Time Domain. A variety of output scales are available, depending on the signal of interest. Options include dBm, dBV, dBmV, dBμV, V rms, and A rms.

The Spectrogram section of the interface gives users a visual indication of how the spectrum changes over time and accumulates 256 time spectra in the display. Spectrograms may be viewed in either two or three dimensions and in monochrome or color gradients.

Figure 3:

The Spectrogram display, seen at the top of this screen capture, showshow the spectrum changes over time. In this example, the spectrogram is in 3Dmode and in full color

Peaks and Markers

In the Spectrum Analysis UI, selecting the Peaks/Markers tab leads to the software’s important ability to automatically find and label peaks in the spectrum. It also opens the door to the software’s powerful markers function to either automatically mark harmonics or peaks, or manually move multiple markers to make measurements of delta frequencies. As shown in Figure 4, the software makes short work of this task.

Figure 4:

This image hints at the power of the Peaks/Markers tab, which automaticallyfinds and continuously displays a specified number of peaks in the FFT plot. In this example, the Peaks table is shown with the peak frequency display turned on.

The first section on the left of the Peaks/Markers tab is for Tables, where users may choose to display an interactive table of either peaks or markers. Enabling of table display is accomplished by checking Show Table in the View section. Figure 4 shows an example of a peaks table at top left. Rows in the table are interactive. Any of the listed frequencies can be assigned as the center frequency by clicking on the row and then tapping Apply under Action. The number of peaks found is adjustable under the View section up to a maximum of 100 peaks. Notably, peaks will be automatically found and continuously updated as the analyzer is running. Peaks may also be sorted in amplitude or frequency order. Display of the peaks’ frequencies in the FFT view is optional.

Changing the Table designator in the Peaks/Markers tab to Markers brings the user into the Markers UI shown in Figure 5.

Figure 5:

Switching to the Markers table opens up a range of options and controls for marker setup as well as marker measurements

Note that the table at upper left is now of markers, with absolute frequencies and amplitudes listed. Once again, the table is interactive in the same manner as is the Peaks table. Data may be displayed as absolute values, or as delta values from the reference marker. In the View section, various marker options appear as well as the option to display frequencies in the FFT grid. Markers can be set on peaks or harmonics, or five default markers may be displayed. The Marker Control section provides a number of ways in which to position each marker.

Conclusion

The Spectrum Analysis software option for the WaveSurfer 10 oscilloscope offers users a full-featured means of setting up and examining signals in the frequency domain. With a spectrum analyzer-like interface and advanced peaks and markers, it makes a suitably equipped WaveSurfer 10 oscilloscope an even more powerful instrument.