A Survey of items appearing in the Tektronix 3000 Series oscilloscope frequency domain display.
Greetings and welcome to our 62nd Test and Measurement video. We’ll be checking out an interesting topic: the items that appear in the frequency domain display of the amazing Tektronix MDO 3000 series oscilloscope. These items are separate and distinct from those that appear in the time domain and also items that appear in the arbitrary function generator display. Not all of these items are present simultaneously. They vary depending on which oscilloscope functions are turned on using on-screen menus and front panel buttons.
The signal we are using in the first part of this demonstration is from 120-volt, single-phase utility power accessed at a branch circuit receptacle.
Here is this basic 60 Hz sine wave – just the fundamental with no discernible harmonics, first shown in the time domain.
The Tektronix MDO 3000 instrument offers two options for looking at the frequency domain. The first of these is acquired through an analog input channel, in this case Channel One, into which we have plugged a Tektronix TPP 1000 10:1 attenuating probe that is connected to the utility signal. To see this waveform in the frequency domain, first press Math. Then, in the contextual menu below the display, locate FFT and press the associated soft key. What we are seeing are the time domain display and frequency domain display shown simultaneously in the same screen.
The frequency domain display is derived from the time domain display based on a mathematical process known as the Fourier Transform, developed by Joseph Fourier in connection with heat flow. He found that any waveform no matter how complex and non-sinusoidal can be resolved into the sum of two or more sine waves. It follows from this that the Fourier transform of time itself is a complex-valued function of frequency, and this is exactly what we are seeing here. It becomes a little clearer if we push the soft key associated with Horizontal in the FFT menu and using Multipurpose Knob a, we shift the trace to the right. Notice now that the big spike is in the center where it can be seen. It corresponds to the fundamental, and since we are looking at a sine wave, that is where all the power is, so there are no visible harmonics.
The transformation from the time domain to the frequency domain is accomplished not really through the original Fourier Transform, which is enormously complex, but rather by means of the Fast Fourier Transform, much simpler mathematically. FFT was developed in the 1960’s and is based on some very efficient algorithms that greatly facilitate this operation, which can now be done instantly in the oscilloscope by pressing a button.
Now we’ll look at a different frequency domain representation. Here again we’re looking at a sine wave, this time originating from the arbitrary function generator and fed into the RF input by means of a BNC cable. The trace can be centered as before, this time by changing the center frequency to 60 Hz. As before the power is all in the fundamental. Changing the AFG output to Noise, we see a frequency domain representation that is much richer in harmonics.
Items that are shown on the screen in the frequency domain representation when AFG is on include, at the upper left next to the TEK logo, an indication as to whether the instrument is in the Run or Stop mode. The amplitude in the frequency domain graph is shown according to a logarithmic decibel scale, which for the depiction of power is more realistic than the lineal scale that shows amplitude in volts for the time domain.
At the bottom, RF means we are currently seeing a radio frequency input. The scale is 10 decibels per division. An AFG is connected, the signal shown being noise. That signal is 500.00 mV peak to peak. The center frequency is 60 Hz, shown to an astonishing seven decimal places. Span, 3 GHz and RBW, 3 MHz as well as time and date are shown.
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