Experiment 9

Introduction to the Oscilloscope

Objective:

To learn how to operate an oscilloscope and use it for signal analysis of different parts of an electronic circuit

 

Equipment:

A laboratory oscilloscope, two function generators, a few regular connecting wires, and a few oscilloscope connecting wires           

 

Theory:

 

Traditionally, an oscilloscope is made of a cathode-ray tube (CRT) in which a heating element at the back end of it releases electrons.  These electrons are accelerated toward the front end of the tube due to a high positive voltage.  A collimator first makes a narrow beam of the accelerating electrons.  The front (screen) of the tube is coated with a phosphor and becomes luminous when high-speed electrons collide with it.   A narrow beam of electrons arriving at the screen leaves a bright spot at its center.  Between the negative end (cathode) and the positive end (anode), there are two pairs of deflection plates.  Each pair consists of two parallel plates similar to a parallel-plate capacitor.  One pair is oriented in a horizontal position, and the other in a vertical position (see Fig. 1).  When the horizontal pair is connected to a fixed voltage, it imposes a uniform and vertical electric field to the traveling electrons that are to pass between its plates, causing them to deflect in the vertical direction.  If the vertical pair of plates is connected to a fixed voltage, it imposes a uniform and horizontal electric field to the traveling electrons that are to pass between its plates, causing them to deflect in the horizontal direction.  Therefore, by adjusting the voltage across these two pairs of plates, it is possible to guide the electron beam in any desired direction, and therefore control the position of the bright spot on the screen.

 

 

 

Fig. 1: A schematic diagram of an oscilloscope

 

If the horizontal pair is given a sinusoidal voltage, the spot performs a sinusoidal motion on the screen in the vertical direction.  If the frequency of oscillations is a few cycles per second (Hz), the up-and-down motion can be followed by the eyes; otherwise, if the frequency is 20 Hz or more, only a vertical line will appear to the eyes, as shown in Fig. 2.

 

 

 

The same would be true in the horizontal direction if a sinusoidal voltage were applied across the vertical pair of plates of the oscilloscope (see Fig. 3).

 

Sweep:

 

In order to be able to show real sinusoidal signals or graphs on the screen, each oscilloscope has a built-in sweep function that causes the spot to move in the horizontal direction.  The role of the sweep function is to move the spot from left to right at a desired constant speed, and return it to the left almost immediately.  This is done by the sweep function in the oscilloscope.  There is a knob on every oscilloscope that allows users to select a sweeping speed.  If a sweep of 5 seconds is selected, for example, it takes the spot 5 seconds to travel horizontally from left to right.  If a sweep of 1 second is selected, the travel time will be 1 second.  If, at a sweep rate of 1 per second, a sinusoidal voltage of frequency 1 per second is given to the horizontal plates (vertical motion), the oscilloscope then draws one cycle of a sinusoid across the screen every second, as shown in Fig. 4.

 

 

 Fig. 4:  Sweep Frequency = 1/sec. Vert. Oscill. Frequency = 1/sec.  Fig. 5:  Sweep Frequency = 1/sec.  Horiz.. Oscill. Frequency = 3/sec.

 

If the frequency of the vertical oscillations is 3 per second, then 3 cycles appear in the width of the screen, as shown in Fig. 5.  At a low frequency (1/s or 3/s) of the vertical motion, only the drawing process of figures 4 and 5 appears on the screen in every sweep.

 

To see a non-vanishing waveform in each sweep, both the vertical and the sweep frequencies must be at least 20 per second (20 Hz).  That way, before a drawing vanishes, another one replaces it, and it appears continuous to our eyes.

 

Function Generator:

 

A function generator is a device that can generate a few types of functions at different frequencies.  Typical functions are: sinusoidal, rectangular, and saw-tooth.  These are shown below:

A function generator may be set at different frequencies by the self-explanatory knobs or buttons on its console.  Another knob is usually provided for fine adjustments.  In the absence of an equipment manual, you may address your questions to your lab instructor.

 

Procedure:

 

A. Features

 

  1. Make sure that the oscilloscope is connected to the power outlet.  Locate the power on/off switch on the oscilloscope and turn it on.  Wait a few seconds for the filament to warm up and cast a bright spot on the screen.

 

  1. Locate the knob that adjusts the brightness, and make the spot visible, but not too bright, by changing the intensity of the beam.

 

  1. Locate the focus knob, and focus the spot.

 

  1. Locate the knob that sets the sweep function rate.  Turn it step by step and observe how the sweep speed changes.  When the sweep speed is low enough, you can follow its left-to-right motion with your eyes.  You can also notice the quick return of the spot back to the right after each sweep.

 

  1. Set the sweep function to zero, causing the spot to be stationary.  Locate the knob that adjusts the horizontal position of the spot, and bring the spot to the middle.  Also locate the knob that adjusts the vertical position of the spot, and bring the spot to the origin of the existing coordinate system on the screen.

 

  1. Step by step, increase the sweep frequency.  Try all possible speeds.  Also decrease the sweep frequency step by step, and set the sweep at the frequency of 1/s.

 

  1. Obtain a function generator.  Examine its function types as well as possible frequencies.  Connect it to an electric outlet and turn it on.  Use the special cable that connects the function generator to the oscilloscope and make the connection.  Set the generator at the frequency of 1/s and select the sine function shape.

 

  1. On the oscilloscope locate the knob that adjusts the amplitude of the display, and bring the amplitude to within the limits of the screen.  Of course, oscilloscopes can be calibrated to read the correct voltage.  For now, a qualitative study of how oscilloscopes operate is the main purpose of this experiment.  After this adjustment, you should see that the spot draws a full sinusoidal cycle on the screen every second.

 

9.      If you change the frequency of the function generator to 2/s or 3/s, 2 or 3 cycles will be drawn on the screen every second.

 

  1. Set the sweep function at 100/s, and at the same time set the function generator at 100/s as well.  You will see that a single-cycle display appears almost stationary after some minor adjustments.

 

  1. Set the function generator at 300/s with the sweep function still at 100/s.  You should observe 3 full cycles (almost stationary), after some minor adjustments.

 

  1. Try other frequencies of both the sweep function and the function generator, and write down your observations.  Record these observations in your lab report.

 

B. Lissajous Figures

 

If the sweep function is disabled, and instead another sinusoidal voltage is connected to the horizontal motion, interesting curves appear that are called “Lissajous” figures.

 

  1. Disable the sweep function by setting it to a ZERO sweep. 

 

  1. Connect another function generator to the horizontal motion terminals.

 

  1. Set the frequencies of both function generators (vertical and horizontal) equal, at 100 Hz, for example.  Make sure that the oscillations in both directions are within the display.

 

  1. Make fine adjustments on the function generator, and write down (and draw) your observation.

 

  1. Double both frequencies, and after fine adjustments, write down (and draw) your observation.

 

  1. Set one frequency to twice the other, and after fine adjustments, write down (and draw) your observation.

 

  1. Swap the frequencies in the previous step and, after fine adjustments, write down (and draw) your observation.

 

  1. Set one frequency to 3 times the other, and after fine adjustments, write down (and draw) your observation.

 

  1. Swap the frequencies in the previous step, and after fine adjustments, write down (and draw) your observation.

 

  1. If you obtain shapes of the following nature, you must have completed the above steps correctly.

Data:

          Given:   Follow values given under procedure

     

             Measured:  Write down or draw the requested observations under procedure

 

Calculations:  N/A

           

Comparison of the Results: N/A

 

Conclusion:   To be explained by students.  

 

Discussion:     To be explained by students.