VIDEO: Frequency Menu in the MDO3000 Tektronix Oscilloscope

Bench-type spectrum analyzers are generally more costly than comparable oscilloscopes, but the Tektronix RSA306B makes use of the owner-supplied PC to reduce the cost substantially while maintaining a full range of features and specifications.

Greetings once more and welcome to our 89th Test and Measurement Video. Today’s topic is the Frequency Menu in the Tektronix MDO3000 oscilloscope.

First we’ll take note of the Fast Fourier Transform that is available when a signal is applied to one of the analog channels. If you push Math and then FFT, the standard time domain signal is displayed along with the FFT frequency domain signal.

Both are shown in a single display, and this is why the instrument is known as a mixed domain oscilloscope, abbreviated MDO. That is in contrast to the mixed signal oscilloscope, MSO, in which two different signals are displayed in the same screen.

That, however, is not exactly what we’re looking at today. We want to leave the time domain altogether and move over to the frequency domain. To do this, we’ve plugged the instrument’s internal AFG into the RF port using an RF adapter.

The first thing to notice is that except as noted previously when pressing Math and FFT, the time domain and the frequency domain are mutually exclusive. In other words, if you press an analog channel input button, RF is turned off and if you press the RF button, any and all analog channels are dectivated.

So let’s look at the RF menu. To make things interesting, I’ve switched the internal AFG from sine to square wave.

What you see across the screen is the noise floor of the oscilloscope. This is the spectral distribution of random electrical energy caused by thermal motion of particles within semiconductors and other components within the instrument. For most purposes, this noise floor can be disregarded. It is just there, and all lower amplitude signals are not displayed.

As mentioned, right now we are looking at a square wave, which, as you know, is very rich in harmonics due to the fast rise and fall times of the waveforms. These ultra-fast transitions are very high frequency components and as such they generate strong harmonics that are not present in a sine wave.

In this display nothing much other than the noise floor is visible for two reasons. The fundamental is hard to make out because it is mostly flush against the edge of the screen. And the harmonics are not visible because the frequency span is not suitable to display them. We can fix that.

First, directly below the RF button, press Frequency/Span. This opens the vertical Frequency and Span Menu at the right side of the screen. This particular menu is of great consequence in operating an oscilloscope in the frequency domain and also in operating a spectrum analyzer. Some users have problems getting a coherent frequency domain display. To get anywhere you have to understand the geometry of the frequency spectrum as it relates to the instrument’s display.

To start, with the Frequency and Span Menu open, make note of the square wave fundamental frequency. You’ll find it in the AFG menu bar at the very bottom of the screen. The default frequency is 100.00 kHz. At present the default Center Frequency is 1.50 GHz, which is not even close. Using the keyboard, type in 100 and hit the kHz units button. That brings the square wave fundamental to the center of the screen where we can see it. Still, we see no harmonics, and that is because the span, 3.00 GHz, is far too great.

Type in 20 MHz and hit Menu Off to reveal the entire screen. Now you see the one strong fundamental at the center of the display and an array of harmonics diminishing in amplitude as the spectral separation from the fundamental increases.

Notice also that when you change the Span, Start and Stop frequencies automatically fall into line.

If you press amplitude and turn Multipurpose Knob a, you can eliminate the clipping at the top and get a more user-friendly display.

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