Simplifies Frequency Analysis

An oscilloscope provides powerful capability to debug frequency-related effects. Fast Fourier Transforms (FFTs) have long been part of oscilloscope toolboxes. Now, a spectrum analyzer like capability is available to simplify setup and use of the oscilloscope for analyzing frequency-dependent effects. It allows users who are familiar with RF spectrum analyzers to start using the FFT with little or no concern about the details of setting up an FFT. Setup is simple—select a center frequency, span, and resolution bandwidth and the necessary sample rate and time-domain acquisition length are automatically determined. Then, select an operational mode (Normal, Average, or Max Hold) for the spectrum display. Other settings for reference level and scale may also be made. With the Spectrum Analyzer and Advanced FFT option, the user is freed from the need to translate scope sample rate, memory, and acquisition length settings into frequency domain relevant units.

“Show Peaks” labels and tabulates spectrum frequency peaks, in an onscreen table, and lists them with amplitude values ordered from highest to lowest. Simply touch one of the peaks to zoom the spectrum display for a closer look. The table also makes apparent frequency peaks that you may not have otherwise noticed.

Displays shown in the tutorial are based on the following initial setup on a WaveRunner 6 Zi scope:

  1. Connect a coaxial cable from channel 1 to the Aux connector on the front panel.
  2. Recall the default setup: File pull down > Recall Setup> Recall Default.
  3. Turn off channel 2.
  4. Set the input coupling on Channel 1 to be 50 Ohms: Touch or click the channel 1 annotation box>touch or click on the coupling field >select DC 50 Ω.
  5. et up the Aux output to be the Fast Edge signal. Utilities pull down > Utilities Setup >Aux Output Tab>touch or click on Fast Edge. The Fast Edge signal is a 5 MHz, 450 mVp-p, square wave.
  6. Set up the Aux output to be the Fast Edge signal. Utilities pull down > Utilities Setup >Aux Output Tab>touch or click on Fast Edge. The Fast Edge signal is a 5 MHz, 450 mVp-p, square wave.
  7. Auto Setup the scope: Press Scope Setup then select Auto Setup from the fly-out menu.
  8. Set the channel 1 signal amplitude using the C1 dialog box. Set the vertical scale to variable gain by checking the Var Gain checkbox. Adjust the vertical scale of channel 1 to maximize the C1 signal amplitude on the display. It should be 90% of full scale
  9. This completes the initial setup. The scope display should be similar to Figure 1.

Figure 1:

The oscilloscope initial setup showing 10 cycles of the FastEdge signal

Start the Spectrum Analyzer option, using the Analysis pull-down menu and selecting Spectrum Analyzer.

Figure 2:

Use the Analysis pull down menu and select Spectrum Analyzer to display the Spectrum Analyzer dialog box

The Spectrum Analyzer dialog box, shown in Figure 3, contains all the controls for this option.

Figure 3:

The Spectrum Analyzer dialog box showing the principal controls of the Spectrum Analyzer

The user can select the source trace, in our example we are using the default trace, C1.

Window allows the user to specify the weighting window to be used for the FFT. The choices are Von Hann (Hanning), Hamming, Flat Top, and Blackman Harris.

The main controls for the Spectrum Analyzer option are the center frequency and span, just like an RF Spectrum Analyzer. Center and Span are similar to adjusting the position of the FFT zoom trace. You enter the Center Frequency and the Frequency Span. Frequency Span is similar to adjusting the zoom scale of the FFT. This does not change the sample rate or memory. The Spectrum Analyzer reports the maximum frequency that can be observed, which is one half of the sampling rate of the scope.

Alternately, Start Stop provides another way to adjust the position of the FFT zoom trace. You can specify the Starting and Stopping Frequencies.

Resolution Bandwidth is equivalent to changing the Timebase setting to increase or decrease memory in FFT mode. Reducing the Resolution Bandwidth equals more memory. The Spectrum Analyzer reports back adapted values for the resolution bandwidth if the value entered is not achievable. The default is to set the resolution bandwidth automatically as confirmed by the Auto check box being checked.

The main vertical scale controls are Scale and Reference Level. Reference Level sets the amplitude of the top of the screen in dBm. Scale is the same as adjusting the Vertical Gain knob in FFT mode and sets the scale in dB per division.

There are three operating modes Normal, Average, and Max Hold. Normal mode displays the power spectrum of the source trace. In Averaging mode, you can enter the number of spectra to be averaged. Averaging is effective in reducing the noise of the signal to see more of the harmonic or carrier detail. Max (Peak) Hold mode is useful for swept frequency measurements where it shows the history of peak values across the frequency axis. Max Hold shows the maximum level the signal reaches. it is also useful for finding infrequent spurs.

The Markers allows the user to set a vertical marker at the desired reference frequency. The Marker>Center Freq button moves the marker to the current center frequency.

Show Peaks lets you label and tabulate peaks. When Show Peaks is checked, the significant peaks are marked with a frequency and gain stamp and a table listing the peaks, ordered by amplitude from highest to lowest is displayed. If you select a peak in the table and push the SuperKnob in it will force the center frequency to the selected value from the table.

At this point you should see the screen image shown in Figure 4.

Figure 4:

The starting state for the Spectrum Analyzer tutorial

Turn off channel 1 by pressing the button marked “1” in the vertical control group on the front panel or by unchecking the Trace On check box in the channel 1 dialog box.

The Fast Edge signal that is being analyzed is a 5 MHz square wave. Let’s adjust the frequency span and centered frequency to observe the first 10 significant harmonics. Since the square wave has only odd harmonics that should be a span of 100 MHz. We will change the center frequency to be 50 MHz so we see all components from near DC to 100 MHz. Alternatively you could press the Start/Stop button on the spectrum analyzer dialog box and set the Start Freq to 0 kHz and the Stop to 100 MHz. Both possibilities are shown in figure 5 and 6.

Change the vertical scale from 20 dB/division to 10 dB/division using the Scale field on the spectrum analyzer dialog box. Touch or click on the field to change it using the WavePilot SuperKnob. Touching or clicking on the Scale field twice will open an on-screen numeric keypad. Similarly, adjust the reference level to 10 dB. The screen should now appear as shown in Figure 7.

Figure 7:

Result of changing the span, center frequency, vertical scale and reference level

These four controls are the principal controls used on an RF spectrum analyzer. Note that while changing the span the scopes time/division setting changed automatically from 200 ns/division to 2 µs/division. The Spectrum Analyzer’s user interface is intended to make these changes automatically.

The Resolution bandwidth is currently being set automatically as indicated by the Auto check box in the resolution bandwidth section of the spectrum analyzer dialog box being checked. If you uncheck this box you can set the resolution bandwidth manually. Uncheck the box and change the resolution bandwidth from the default 100 kHz to 1 MHz. Note that the spectrum display is now much coarser and that the scope’s time/division setting has changed from 2µs/division to 200 ns/division, this happens because resolution bandwidth is related to the scope acquisition record length or capture time. Check the Auto box again to restore the default settings. Again all these changes do not require the user to know these relationships.

Change the reference level to 0 dB. This will bring the spectrum baseline onto the screen. Change the Mode from Normal to Average. Note that the baseline is smoothed as the average of spectral values is computed. Increase the number of average from 10 to 200. Note that increasing the number of averages reduces the amplitude of the baseline noise. Averaging is used to improve the signal to noise ratio of the spectrum display. Restore the number of averages to it default value of 10.

Select the Max (Peak) Hold Mode. The baseline will move upward as the peak values are recorded. Remove the input connector to channel 1. Note that the spectrum values remain. Max Hold is used to collect and store the maximum values within each frequency bin. Press the clear sweeps button on the front panel of the scope. The spectrum should disappear and the noise baseline should move upward. Restore the input connection the spectrum should return. Restore the mode setting to Normal.

Double click or touch the Reference Freq field in the Markers section of the spectrum analyzer dialog box. Change the Reference Frequency to 15 MHz. The vertical dashed line Marker indicator will move to the spectral line at 15 MHz (third harmonic of the square wave). Press the Marker to center frequency button in the markers section. Note that the spectrum is now centered at 15 MHz. Change the Freq Span to 10 MHz. You are now looking at the spectrum from 10 to 20 MHz centered at 15 MHz as shown in Figure 8.

Figure 8:

Using the Marker to re-center the spectrum display

Restore the Center Freq to 50 MHz and the Freq Span to 100 MHz. Set the Reference Freq to 0 Hz.

Check the Show Peaks check box in the Peak Search section of the spectrum analyzer dialog box. Each of the spectral peaks should now be marked with a circled “X” and the frequency and amplitude listed is a table to the left of the display. Touch or click on the Max Peaks field. Rotate the super knob counter clockwise to change the number of peaks to 5. Note that the table now has 5 entries. Restore the Max Peaks to 10. Press the Stop button in the trigger section of the front panel to stop updating the scope display. Touch or click on table entry 5, it will be highlighted. Press the SuperKnob in. Note that the center frequency is changed to 45 MHz as shown in Figure 9. The table entries are hyperlinked to the center frequency setting and will center the spectral display on the chosen table entry for closer examination.

Figure 9:

Using Show Peaks and the Peaks Table to re-center the spectrum display

Change the Freq Span to 10 MHz. The spectral line a 45 MHz will be expanded horizontally as shown in Figure 10.

Figure 10:

Looking at an expanded view of the spectral line at 45 MHz

Touch or click on the Window field on the spectrum analyzer dialog box. Rotate the SuperKnob clockwise to scroll through all four weighting function types. Weighting functions are used to help compensate the FFT for errors caused by the finite record lengths used in oscilloscopes. They affect the width of the spectral response, as you have just witnessed. They also suppress close in spurious responses which can be seen as the ‘skirt’ width at the base of the spectral line. Von Hann (Hanning) weighting is the default and represents a good general purpose weighting function. Flat Top and Blackman Harris have broader responses that are generally used in swept spectrum response measurements to produce more accurate amplitude measurement. Figure 11 shows a comparison of the spectral shape of each of the four weighting functions.

Figure 11:

A comparison of all four weighting function responses

This completes the Tutorial.