Objectives

(a) To gain experience with the operation of an oscilloscope for the observation and measurement of transient and periodic electrical phenomena.

(b) To be able to use it to measure voltages.

(c) To study simple AC circuitry.

Equipment

Oscilloscope, sine and square wave generator, terminal board with 0.0047 mF capacitor (ceramic), 47 mH inductance, a 1.5 volt battery, a supply of coaxial cables, 3-cycle log-log and rectilinear graph paper (student-provided). Note on determining the value of a resistor using its color bands; Black=0, Brown=1, Red=2, Orange=3, Yellow=4, Green=5, Blue=6, Violet=7, Gray=8, and White=9; Silver=10% and Gold=5%.

Preliminary Discussion

The cathode-ray oscilloscope is an instrument which can be used to display the magnitudes of rapidly changing electric currents, potentials, or pulses as a function of time. The information is displayed on the face end of a "cathode-ray tube" (CRT). This face appears as a circular or rectangular window usually with a centimeter graph superimposed on it. (The picture tube in your TV set and the display terminal of most computers are cathode ray tubes).

The cathode-ray tube consists essentially of an "electron gun" for producing a beam of rapidly moving electrons called cathode rays, a fluorescent screen upon which a luminous spot is produced by the impact of the cathode rays, and a means for displacing the spot from its quiescent position as the result of current or voltage applied to the deflecting mechanism. Although the electron beam may be focused by means of magnetic fields, electrostatic focusing is usually used. Figure 5.2 shows the electrode structure of a typical cathode-ray tube having an electron gun with electrostatic focusing.

The electron gun consists of an electron source (i.e., an electrically heated cathode which "boils off" electrons), a grid G for controlling the electron intensity of the beam and hence, the brightness of the luminous spot, and two anodes A₁ and A₂. The final velocity with which the electrons leave the gun is determined by the potential of A₂, which is normally maintained constant. The electrostatic field between G and A₁ and between A_l and A₂ focuses the stream of electrons in a manner somewhat analogous to the focusing of light rays by lenses. Usually the focus control on the oscilloscope adjusts the potential of A₁.

After leaving the electron gun, the electron beam passes between a pair of horizontal plates. A potential difference applied between these plates deflects the beam in a vertical plane in direct proportion to the instantaneous voltage applied between the deflecting plates. This pair of plates provides the Y-axis or vertical movement of the spot on the screen. A pair of vertical plates provides the X-axis or horizontal movement of the spot on the screen.

The screen of a cathode-ray tube consists of a thin layer of a phosphor, which is a material that luminesces as the result of bombardment by rapidly moving electrons. The bombardment gives rise to both fluorescence and emission of light after bombardment. The phosphor is applied to the inside of the end of the tube by spraying, dusting, or precipitation from a liquid. Slow decay of phosphorescence makes possible the visual observation of non-repeating transients and prevents flicker in the visual observation of periodic voltages of low frequency. However, if it is too slow it causes blurring whenever an image on the screen changes form.

The main circuits of an oscilloscope and their functions are described below in (a) through (f). Figure A.1 of Appendix A shows the control functions for these circuits for the Hitachi V-222 oscilloscope.

(a) The time based generator. In order that the image plotted on the scope screen shall show the unknown Y-axis voltage as a function of time, it is necessary that the spot shall periodically sweep across the screen horizontally (along x-axis) with uniform velocity up to a certain point and then return instantaneously to its zero position. If the time taken for one timing sweep is equal to the period of the voltage applied to the Y plates, the pattern will consist of one cycle of the Y voltage. If the sweep frequency is equal to f_y/n, the image will show n waves of the Y voltage. The required horizontal movement of the fluorescent spot can be produced by means of an X voltage that periodically increases uniformly with time and falls to zero instantaneously upon reaching a given value. The wave form of such a linear sweep voltage is shown in Figure 5.3. Because of its shape, this signal is called a "sawtooth" voltage.

(b) The horizontal and vertical signal amplifiers. The oscilloscope amplifiers serve the dual purpose of providing sufficient voltage to give any desired deflection of the electron beam, and making possible the variation of deflection voltages without drawing appreciable current from the source under observation or from the sweep generator.

(c) The z-axis modulation controls the intensity and focus of the electron beam.

(d) and (e) The synchronization circuit ties the sweep of the horizontal time-base in with the applied vertical signal so that any harmonic signal can be displayed as a stationary pattern. The horizontal sweep is begun by a signal or condition called the "trigger". A variety of triggers are available as well as a variety of display selections following the trigger.

(f) The power supply supplies the various potential differences and currents for complete operation of the instrument.

The individual controls for the above circuits are identified in Figure A.1 of the Hitachi V-222, Appendix A. The corresponding list of controls and descriptions of their function are also given in Appendix A. Study this list and try to understand the function of as many of the controls as you can before going to the lab. In this lab you will experimentally study the functions of most of them.

Experimental Work

Part I: Prepare the oscilloscope for operation by following the instructions for "First Time Operation" in Appendix A. These procedures will avoid damage to the various oscilloscope circuits at turn-on. At the conclusion of these procedures you will have observed the horizontal line (called a trace). Be sure the trace is not too bright or the electron beam might burn the phosphor on the screen.

Now you should analyze and become familiar with the operation and functions of as many of the various controls as you can. Design and perform simple tests for the controls. Since most of these are fairly obvious, you need not record them in your report unless something unusual happens. If it does, of course, you should seek out the reason. Performing the work below will help you become acquainted with the various controls. Refer to Figure A.1, and "Controls and Connectors" in Appendix A.

Part II: Measurement of D.C. and A.C. Voltages using the oscilloscope.

Do the following and describe and explain all results in your report:

Using Channel 1 (Ch.1 or X) controls only:

(1) Set the "DC-GND-AC" Switch (11) on GND.

(2) Adjust the vertical "position", knob (19) so the horizontal trace is vertically centered on the screen.

(3) Set the "DC-GND-AC" switch to DC.

(4) Turn the variable, "VAR" red knob (15), voltage control fully clockwise. The surrounding black knob (13), "VOLTS/DIV", can be adjusted to change the vertical scale calibration in volts per centimeter above and below the zero (central) or ground position. The calibration is correct ONLY when the red knob is fully clockwise.

(5) Measure the voltage of a 1.5 volt battery connected to the input of Channel 1 (9). Voltages are measured on an oscilloscope by first counting the number of divisions of vertical deflection and then multiplying by the VOLTS/DIV setting (knob (13)).

Measure the battery voltage for several settings of the VOLTS/DIV knob (13) such as 2.0 V/DIV, 1.0 V/DIV and 0.5 V/DIV. What is the best setting of the V/DIV knob if you want to measure the voltage most accurately? Note what happens to your voltage measurement if the variable red knob (15) is not fully clockwise.

Reverse the polarity of your battery and again measure the voltage. If you do not get the same value as before, your trace was probably not vertically centered when the DC-GND-AC switch was in the ground position. If such is the case, you can still get an accurate result by taking the average of the results for the two polarities. Why?

(6) Set the DC-GND-AC switch to AC with the battery still connected. Explain what happens.

(7) Attach the function generator to the horizontal input (9), set the DC-GND-AC switch to A.C., set "TRIGGER SOURCE" (31) to internal trigger and "MODE INT TRIG" (32) to CH.1. You are now ready to measure an A.C. voltage from the function generator. See Appendix B for the operation of this generator. Connect to the output of the generator and set the function selector to the sine wave position. Turn on the generator and set the frequency to 60 Hz. Adjust the 20 V peak-to-trough max control on the generator until you measure 4 Volts peak-to-trough on your oscilloscope. Ask your instructor to verify that you are observing this correctly. Note that the setting of the horizontal time scale, knob (26) TIME/DIV affects how many cycles of the 60 Hz sine wave is seen on the screen at one time. A setting of (26) at 10 ms/DIV (10 milliseconds/division) will cause several cycles of the 60 Hz wave to be displayed. (You will learn why this is so in the next section). Note also that by making peak-to-trough measurements of an A.C. voltage, you do not have to worry about whether the trace is perfectly centered vertically. Thus, this mode gives you the freedom to move the pattern up or down (23) or left or right (29) so that you can place a convenient part of the pattern such as one of the maxima or minima exactly on a scale division.

Part III: Measurement of the Period and Frequency of A.C. signals using the oscilloscope.

Take the pattern you just observed in step (7), part II, for the 60 Hz, 4 V peak-to-trough sine wave and adjust the TIME/DIV knob (26) until only 1 or 2 cycles of the wave are displayed. Measure on the horizontal scale the time period, T, of 1 cycle of the voltage oscillation. This means determining the number of divisions constituting 1 cycle on the screen and multiplying by the setting of the TIME/DIV knob (26). Compare your result with f = 1/T = 60 Hz.

Repeat the above for several frequencies from the function generator such as 100 Hz, 1000 Hz and 10,000 Hz. In each case adjust the TIME/DIV (26) so that only 1 or 2 cycles are displayed.

Compare your results for the generator frequency as given by the dial setting with what you measure with the oscilloscope. Neither the generator nor the oscilloscope have been calibrated to better than several percent so you cannot expect perfect agreement. What is the largest percent error you observe?

(8) Disconnect the function generator. Set the horizontal sweep (i.e., "TIME/DIV") for 5 ms/cm, turn the vertical gain (i.e., Ch.1. "VOLTS/DIV") up high, and touch your finger to the positive input lead. Adjust the vertical gain until you see a wave pattern with a few centimeters amplitude. Measure the period of the wave and deduce its frequency. Identify the source of the signal for which your body is serving as an "Antenna". What is the AC output voltage of your body antenna?

Part IV: Lissajous Figures.

Now set the horizontal sweep (i.e., the "TIME/DIV" knob) at the position "X-Y". Then connect the Channel 2 input (10) to the transformer (small yellow box) which is plugged into the AC line power of laboratory and serves as a 60 Hz reference standard. The electron beam of the oscilloscope will be displaced vertically by a sinusoidal potential at a frequency of 60 Hz. Connect the "Sine-Wave" output of the Function Generator to the input of Channel 1. With the time/div knob at x-y, the electron beam will now also be displaced horizontally at the frequency of the function generator. Observe the various stationary "Lissajous patterns" which occur as the frequency of the signal generator is varied.

These Lissajous figures can be used to calibrate the function generator at frequencies between about 20 and 200 Hz because the 60 Hz power line frequency is a very accurate reference.

Whenever a stationary pattern is achieved as the frequency is varied, the frequency ratio of the vertical input to the horizontal input is equal to the ratio of the number of horizontal to vertical points of tangency to a rectangle that encloses the pattern. Compare the frequency of the Function Generator as measured from the Lissajous patterns with that indicated by the dial setting for 30, 60 and 120 Hz input to Channel 1.

An example of a pattern for which the vertical input frequency is twice the horizontal input frequency is shown in Figure 5.4.