Lab 1 - The Oscilloscope

You can get a pdf version of this manual here.

The aim of this experiment is to familiarize you with the use of a cathode-ray oscilloscope. This instrument will be employed in some of the later experiments, as well as in many electronics and research laboratories, so it is important to clearly understand its principles and its operation.

The oscilloscope is somewhat similar to a traditional TV, where you have an electron beam in an evacuated tube hit a phosphor-coated glass screen to make light where it hits. The electron beam is deflected up/down and left/right by voltage applied to some plates.


External voltages can be input to the y-axis or vertical amplifiers (typically on the left side of the scope) which control the vertical motion of the electron beam and thereby the vertical position of the spot on the phosphor-coated screen. The voltage on the x-axis amplifiers, controlling the horizontal motion (“time base”) of the cathode beam, is generated internally. The internal horizontal voltage has a saw-tooth shape, moving the spot with constant speed from left to right, and then back almost instantaneously. The horizontal speed (i.e. the frequency of the saw-tooth) can be controlled by the “time-base” unit on the right. Because the time base voltage has a calibrated “sweep” time, the displayed signal can be read off as voltage (vertical) versus time (horizontal).

The vertical signal can be amplified (calibrated amplification factors) and displaced vertically (“offset”) over the screen. The intensity and focus of the spot on the display can be varied. The vertical input can be put to ground (GND or 0 V), or coupled to the input signal connector directly (DC; direct current) or via a capacitor (AC; alternating current) which prevent any constant voltage level to pass, and only the time-varying part of the signal to pass (so that for example one can study a 100 mV AC ripple on top of a 5 V DC signal).

A second vertical input is provided, which allows the “simultaneous” display of two signals. This is not a truly simultaneous display, because the spot switches very fast between the two input signals for the display (CHOP), or displays them one after the other alternatively on the screen (ALT). The two signals can also be added together (ADD).

In some cases it is of interest to display the voltage of one signal versus that of a second signal using the X-Y mode of the scope. In that case the second vertical input connects to the horizontal amplifier (the internal saw-tooth generator is disconnected).

The TRIGGER is an important asset of the oscilloscope: it tells the instrument to start a horizontal sweep ONLY if the vertical signal (the signal of interest) exceeds a preset threshold. The threshold voltage is set by the (TRIGGER level) and the direction can also be chosen (+/- for direction of change, ie. going positive or going negative—trigger polarity). Thus, the horizontal sweep becomes synchronized with a repetitive vertical input signal: such an input signal is written many times over on the screen and thereby appears bright and stable in (horizontal) position. The trigger for the horizontal sweep can also be taken from an external signal (TRIGGER input), or from the 60 Hz power line frequency (LINE).

Note: both the vertical gain, and the time base functions, have both click settings and a variable gain knob. Usually you want the variable gain set to its calibrated setting; that is, a fixed rather than variable gain! Find these knobs and play with them. When done playing, return them to their calibrated positions, which usually corresponds to them being turned all the way to the right.

The function generator lets you put out either a square, sine, or triangle wave, with frequencies adjusted within a certain range. Equipment

  • Oscilloscope,
  • Dry cell battery,
  • 2 function generators.


This first lab serves as a familiarization session for making electrical measurements. Your lab notebook will be somewhat minimal: you will show a simple electrical schematic of each experiment you do, and record some numbers. You should feel free to play around and try things out, with just one point to note: Do not have the intensity control set too high, and never allow a small very bright spot to remain stationary on the cathode-ray tube face. The screen can easily be damaged if the intensity is too high.

Follow the following steps to explore the capabilities of the oscilloscope:

  1. Adjust the focus, intensity, the vertical and horizontal positions and the sweep time with no external input connected. You are looking for a straight horizontal line running through the center of the screen. Set the input mode for CH 1 to DC.
  2. Measure the voltage of a battery. To keep the scope trace sweeping across the screen, trigger off the 60 Hz AC power line by using LINE triggering mode. Remember to correctly take into account the scale you choose for the input and include an estimate of the uncertainty of your measurement (for example, 2.5±0.2 divisions multiplied by 1 volt per division gives 2.5±0.2 volts).
  3. Set the function generator to make a 1 kHz sine wave. Set the scope to trigger on it in DC coupling mode. Record the precise frequency from the readout of the generator (make sure you are using the generator with a digital readout for this part!). Measure the period of the sine wave (the time from peak to peak) in at (for example) 5.6±0.2 divisions, times 0.2 msec per division, giving 1.12±0.04 msec. Do the two frequencies agree within uncertainty? (They may not if either the scope or the function generator have a calibration error - an example of the difference between precision and accuracy). Measure the amplitude, and the peak-to-peak value, of the voltage of the sine wave.
  4. Change triggering from + to -, by pulling out the trigger level knob, and note what happens. Change the triggering voltage, and note what happens in a sketch. Change from sine to triangle, and repeat. Change from triangle to square, and repeat. (Why do things not change much with the square wave?).
  5. Now connect a second function generator to the second input. You might set the first input to a 500 Hz sine wave, and the second to a 400 Hz sine wave. Trigger off input 1. Trigger off input 2. Note the appearance of one input when you trigger off of the other. Can you explain what you see?
  6. Now generate Lissajous figures! Set the scope to X-Y input mode. Try and get both function generators to about the same frequency and amplitude. Try to fine-tune one function generator to get as near a circle or straight line as you can get. Look at the patterns that result as you detune the frequencies. Can you explain what you see?
  7. Now for the final test. Your T.A. will come by and mess up all the knobs for you. Get back to viewing a stable sine wave.

As mentioned before your lab notebook for this first experiment will be pretty simple, basically consisting of some simple sketches and schematics. (Note: Don't choose this lab for one of your lab reports!).

142/labs/lab1.txt · Last modified: 2011/02/22 21:41 by mdawber
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