To learn how to operate an oscilloscope and use it for signal analysis of different parts of an electronic circuit
A laboratory oscilloscope, two function generators, a few regular connecting wires, and a few oscilloscope connecting wires
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).
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.
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.
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.
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.
Given: Follow values given under procedure
Measured: Write down or draw the requested observations under procedure
Comparison of the Results: N/A
Conclusion: To be explained by students.
Discussion: To be explained by students.