Oscilloscope review: The oscilloscope has ‘buttons’, ‘soft keys’, and ‘knobs’. The ‘buttons’ are on the right half of the oscilloscope. The ‘soft keys’ are the buttons under the screen. The ‘knobs’ are what you turn on the right side of the oscilloscope.

Looking at signals in the frequency domain is so useful that the oscilloscope has a built in method for converting a time varying signal into the frequency domain. This calculation is called a Fast Fourier Transform or (FFT).

The objective of this section is to become familiar with how to measure the frequency domain representation of a signal using the oscilloscope.

1. Connect the function generator to the oscilloscope. The signal should be a sinusoidal signal with a frequency of around f=10 kHz and an amplitude of 1 Vpp. You should be able to see a good sinusoidal signal with a very small amount of noise.
2. Push the math ‘button’, located in the grey square on the right of the front of the oscilloscope as indicated in the red box in the following picture and select the FFT Operator using a ‘soft key’.
3. The following picture shows you approximately what your signal should look like. The yellow trace is the time varying signal and the pink trace is the frequency domain representation (Fourier domain).
4. FFT Overview
   1. With the FFT algorithm the mapping of the time varying signal to frequency depends on the total time band, the total number of samples, and the sampling rate.
   2. With the oscilloscope the total number of samples is fixed. So we only have one variable, which is the total time window. The sampling rate changes with the time window.
   3. The frequency plot is divided up into frequency bins. You can think of this as each pixel on the graph corresponds to a range of frequencies.
   4. There is an inverse relationship between the width of the frequency bin and the sampling period.
   5. If you want a better frequency resolution you need a wider time window.
5. Play around with the time window to see the effect on frequency resolution.
   1. Set the time/division to 0.02 ms. This time resolution produces about 2 periods of the sine wave. This time resolution is what the auto scale will typically change the time resolution to.
      1. We have an f=10 kHz signal so set the frequency span to 100 kHz and the Center to 50 kHz. You do this by pushing the Math ‘button’ and using the ‘soft keys’ where is says Span and Center.
      2. Use the cursor to pin point the f=10 kHz signal. You select the cursor using a ‘button’, change it to the FFT trace using ‘soft key’, and use a ‘knob’ to move the cursor.
      3. Notice how coarse the frequency signal is. The oscilloscope lists your FFT Resolution in the button right corner.
   2. Increase the time/division by a factor of 10 (time/division = 0.2 ms). Make sure that the frequency span is still 100 kHz and the center is 50 kHz.
   3. Increase the time/division by a factor of 10 (time/division = 2 ms). Make sure that the frequency span is still 100 kHz and the center is 50 kHz.
   4. Notice how the spike at f=10 kHz gets more narrow as the time/division increases.
   5. In summary, it takes a lot of periods of a signal to produce a good frequency domain representation. Therefore, the signal needs to be zoomed out to get a good frequency domain signal.
   6. You can adjust the amplitude and position of the purple signal (this is the FFT signal) by using the knobs in the gray math box. The red box in the following picture shows how you change the amplitude of the FFT signal and the following picture shows how to shift the signal up and down. You can push the shifting button to put the FFT signal in the middle of the screen.