PC/CP320 Physical Computing

Function Generator and Oscilloscope

Objectives

A function generator or signal generator produces a voltage signal with a specified shape, frequency, amplitude, duty cycle, and DC offset. It's primary purpose is to produce a known accurate input signal to test a component's or circuit's response. One lead of the signal generator is attached to ground and the other(s) provide input to the device under test.

An oscilloscope, sometimes simply called a scope, is a device that measures the voltage difference between the positive and negative probes and displays the voltage difference as a function of voltage difference against time. The primary functions of an oscilloscope are to test circuit performance over time (i.e. meters are good for an instantaneous measurement) and to compare two signals (e.g. the input to the circuit against the output from a circuit).

Always make sure that your function generator, your oscilloscope, and your circuit are properly grounded. If ground is improperly setup, any results you may see are meaningless.

  1. To gain familiarity with a function generator.
    • how to properly connect a function generator to a circuit
    • how to select the signal shape, frequency, amplitude, duty cycle and DC offset
    • understand how the above signal characteristics are set on the function generator and how they impact the signal shape
  2. To gain familiarity with an oscilloscope.
    • how to properly connect an oscilloscope to a circuit
    • how to determine the signal's shape, frequency, amplitude, duty cycle and DC offset

Equipment

Procedure

Function Generator and Oscilloscope Introduction

  1. Connect the signal generator to an oscilloscope as shown below. (Use a scope probe connected to the scope, and a normal BNC connector for the function generator.)

    connecting scope and function generator
    Use the signal generator to produce a sine wave at 1 kHz and peak amplitude of 1 Volt (2 volts peak-to-peak). Verify the amplitude and frequency are correct using the scope.

  2. Play with the trigger settings on the scope to see what happens. For instance:
    • Set the trigger to the channel that's not connected.
    • Set the trigger to the channel that is connected, and then move the level around within the voltage range of the waveform, and then outside it.
    • Try any other trigger settings you want to see what they do.
    After playing around with the settings, use a setting that displays a stable waveform on the screen.

  3. With your signal on the screen, see what happens when you change the "Probe" setting for Channel 1 on the scope to 10X. Does the signal change? Does the voltage scale for the channel change?

    Now, with the "Probe" setting for Channel 1 still set to 10X, set the switch on the probe itself to 10X. What happened?

    Make a note of how the probe switch and the probe setting on the scope need to go together.

    Switch both the probe switch and the scope setting back to 1X.

  4. Now that you understand how the oscilloscope works, sketch a signal in your lab notebook. Include the scope settings and show how to calculate amplitude, DC offset and period from the trace.

  5. Notice how the rotary buttons along the bottom of the signal generator have two different labels; one of the labels applies when the button is pushed in, and the other applies when the button is pulled out. See what happens with the waveform as you test each button so that you understand what happens each way. After this go back to the required waveform.

  6. Wire up a speaker (two connections: one to ground and one to the input signal) so that you will be able to both hear the sound of the waveform produced by the signal generator and see the waveform produced by the signal generator. (The signal generator is the input to both the speaker and the scope.)
    • Did the voltage drop when you connected the speaker?
    • By how much?
    • What does this tell you?

  7. Make sure the signal generator is set to a sine wave output for this part.
    Infants can typically hear sinusoidal signals that range in frequency from 20 Hz to 20 kHz. Things go down hill as you age. The fine hairs in your ears that sense sound become brittle and break off. Usually the smallest hairs break first, making you lose your ability to hear high frequency sounds. You can speed this process up, significantly, by listening to loud noises like gunfire, jet aircraft, and music. Determine the highest and lowest frequency audio signal you can hear. Sometimes it is hard to tell if you can really hear a signal or are just imagining it. Have someone else turn the signal on/off while you are looking away. See if you can reliably tell if the signal is on or off. Do this for each person in the group.

    Please keep the volume to a reasonable level! How did you adjust the volume?

  8. Repeat the experiment above, but using square waves and triangle waves. Compare the sound of sine waves to square and triangular waves. Do they sound the same or different? Does your answer depend on the frequency of the signal? Can you hear the same range of frequencies for the different wave shapes?

    Demonstrate your procedure to the lab supervisor. Be prepared to summarize your findings.

  9. Select a square wave on the function generator. What happens when you vary the duty cycle? Does it affect the sound? How would you determine the duty cycle from the scope?

  10. What happens when you vary the DC offset? Does it affect the sound?; How do you determine the DC offset from the scope?

    Demonstrate your procedure to the lab supervisor. Be prepared to summarize your findings.

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