An examination of spectral analysis controls in a Tektronix 3000 Series Oscilloscope along with a Spectrogram demonstration.
Hi and welcome to our 60th Test and Measurement video. Today we’ll look at some very interesting Spectral Analysis controls that pertain to the RF input in the amazing Tektronix MDO3104 oscilloscope.
First of all, we need a signal at the input. There are a variety of possibilities, but we just have to make sure we don’t overload the instrument. The limit, printed on the front panel to the left of the RF port is 40 volts DC at 50 ohms. A good signal source is the internal arbitrary function generator, so we have run a BNC cable from the AFG output port on the back panel to the RF port on the front panel, using an RF adapter.
Turning on RF, all that is seen is the noise floor of the instrument, comprised of random electrical activity that is a consequence of the thermal state of the internal circuitry. The reason it is so prominent is that the sensitivity of this auto-ranging instrument is high with no signal applied in a lineal fashion as in the time domain.
To see a signal, press AFG. Currently, the default sine wave is being shown in the frequency domain. With the fundamental crammed up against the edge of the display, it is rather difficult to see what is going on. To bring the fundamental to the center of the display, first press waveform settings. Look at the frequency and make note of it. Right now it is the default 100 kHz. Then, under frequency in the RF section, press Frequency/Span, bringing up the corresponding menu. This looks a lot like the initial screen that is seen in a spectrum analyzer, which is actually the oscilloscope that is now active. Center frequency is highlighted, and we want to make it the same as the AFW waveform setting as previously noted.
You can crank this value up using Multipurpose Knob a, but even with Fine disabled, it is easier to use the keyboard. Type in 100 and then press kHz under Units. Now the fundamental migrates to the center of the screen where it may be more easily seen. Notice that there are no harmonics, because we are viewing a pure sine wave.
Let’s see the frequency spectrum associated with another waveform. Pressing AFG, we go to waveforms and using Multipurpose Knob a, we scroll to Square Wave. There is the fundamental. With the high-frequency square wave, we should see lots of harmonics. Where are they? For an answer, once again push Frequency/Span in the Frequency controls. The problem is that the harmonics are outside the limited span. So, highlight Span. Then, using the keyboard, we’ll set the span at 20 mHz. Now we see the harmonics, diminishing in amplitude as the frequency gets farther from the fundamental. Eventually, they will disappear below the noise floor.
A frequency domain display can also be shown as a spectrogram. To see what it looks like for the square wave, once again press RF, which brings up the RF Menu below the display. Then press Spectrogram. To the right, we see that the Spectrogram display can be turned on. Then press Menu Off. Characteristically, the Spectrogram rises slowly from the X-axis. You might think it is loading, but actually it is rising at a uniform rate because time is shown along the Y-axis. If you look closely, you will see that it continues to rise, a new signal continuously generated at the bottom and disappearing at the top as the signal goes off-scale. In this view, frequency is displayed across the bottom and amplitude is represented by color, the warmer colors corresponding to higher amplitude and the cooler colors corresponding to lower amplitude to the right as we move farther in frequency from the fundamental. Similarly, the background has a speckled appearance because it depicts the noise floor.
If we press AFG followed by Waveforms, it is possible to scroll through various waveforms and see how they are shown in the spectrogram. For example, here is what Sine Wave looks like. There are no harmonics. This is noise. Now there is no fundamental. And this is DC.
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