Experiment

The Oscilloscope

The oscilloscope is used in various experiments in this course and a good understanding of its operation is required. Besides observing the changes that occur in an electrical signal when various control knobs are turned, measurements of peat-to-peak voltages, frequencies, and DC voltages will be made.

Diagram

The main component of the oscilloscope is the cathode ray tube (CRT) on whose screen the externally applied electrical signal is detected. In its barest essentials the operation of the CRT can be understood as an application of the basic principle governing the forces between charges.

Figure 1. A simplified diagram of the CRT.

The fundamental parts of the CRT diagrammed above:

A)filament – source of electrons

B)positively charged anode – accelerates the electrons

C)focusing electrodes – focuses the beam of electrons

D)vertically oriented plates used to move the beam of electrons back and forth

E)horizontally oriented plates that deflect the beam of electrons vertically

F)the CRT screen the beam of electrons hits

The type of oscilloscope we will be suing is called a dual trace oscilloscope. The oscilloscope allows two signals to be applied to two separate vertical input terminals, and the two signals can then be observed simultaneously on the screen of the CRT. We will be using only one of the inputs.

The following information describes the primary front panel features, the turn-on procedure, and the signal application procedure for the oscilloscope.

Figure 2. Front panel diagram.

Primary front panel features:

1)POWERon-off switch

2)INTENcontrols brightness of display

3)FOCUScontrols sharpness and clarity of display

4)Xcontrols horizontal position of display; when pulled outward, it expands the display by a factor of 5

5)TIME/DIVcontrols sweep time

6)VARIABLEvaries sweep speed; in CAL’D position, sweep speed is as selected

7)Ycontrols vertical position of display

8)INPUT Yvertical signal input

9)VOLTS/DIVselects vertical deflection sensitivity measured in volts per division

10)VARIABLEadjusts vertical deflection sensitivity between ranges; in CAL’D position, vertical sensitivity is as selected

11)VERT MODEselects which y-input is displayed

General procedure for turning on the oscilloscope

1)Set INTEN control to low intensity (counterclockwise)

2)Set VERTMODE to channel 1 or 2 (CH-1 or CH-2)

3)Set horizontal (X) and vertical (Y) position controls to midrange

4)Set horizontal and vertical VARIABLE controls to calibrated (CAL’D) position

5)Set TIME/DIV control to one millisecond/cm (1 ms) position

6)Set SOURCE to INT

7)Be sure all other pushbutton switches are disengaged

8)Set POWER switch to ON and allow a 30 second warm-up period

9)Turn the intensity (INTEN) control clockwise until a trace is visible on the CRT

10)Set channel 1 (or 2) DC-AC switch to AC

General procedure for connecting a signal to the oscilloscope

1)Set channel 1 (or 2) DC-AC switch to appropriate position for signal

2)Set channel 1 (or 2) VOLTS/DIV control to 1 volt/div

3)Connect signal to channel 1 (or 2) INPUT Y connector

4)Position display on CRT using the horizontal (X) and vertical (Y) position controls

5)Adjust TRIGGER control, if necessary, for a stable display

The oscilloscope is a versatile instrument from which quantitative measurements of voltage and time can be made. Advantage is taken of this capability in order to measure the peak-to-peak voltage and frequency for alternating current (AC) signals. In addition, the voltage across a dry cell (DC, direct current) can be measured.

The vertical distance from a crest to a trough for a sinusoidal signal represents the peak-to-peak value of the voltage. (Refer to Figure 3.) Count the number of vertical divisions on the CRT and multiply this value by the value on the VOLTS/DIV setting. For example, if the number of division is 3.7 and the VOLTS/DIV setting is 0.2, then the peak-to-peak voltage is (3.7 div) x (0.2 volts/div) = 0.74 volts. (Remember that the calibration knob (CAL’D) must be turned all the way in the clockwise direction until it clicks.)

The frequency of the sinusoidal signal is found by first determining the period of the oscillation, the calculating the reciprocal (f = 1/T). The period is the time for one complete oscillation and is represented on the oscilloscope by the horizontal distance from a crest to an adjacent crest. (Refer to Figure 3.) The value of the period is found by counting the number of horizontal divisions from one crest to the next adjacent crest, then multiplying this number by the TIME/DIV setting. For, example, if the number of division is 6.3 and the TIME/DIV setting is 0.5 ms/div, then the period is (6.3 div) x (0.5 ms/div) = 3.15 ms = 3.15 x 10-3 sec. The corresponding frequency is 1/3.25 x 10-3 = 317 Hz. (Remember that the calibration (CAL’D) knob must be turned clockwise until it clicks.)

The other instrument used in the experiment is a variable frequency generator that is capable of producing square, triangular, and sinusoidal waves of varying frequencies and amplitudes.

Apparatus: Oscilloscope, signal generator, leads, dry cell

Procedure:

1)Connect the leads of the oscilloscope to the signal generator – red to red and black to black. Turn on the oscilloscope and the signal generator.

2)Adjust the frequency knob on the signal generator until it is generating a sinusoidal signal of 1200 Hz frequency and arbitrary amplitude.

3)Adjust the horizontal (X) and vertical (Y) position knobs until the pattern is centered on the screen of the CRT.

4)Change the setting of the VOLTS/DIV knob until the signal fills vertically as much of the screen as possible.

5)Change the setting of the TIME/DIV knob until two or three full cycles are seen of the CRT

6)Record the number of divisions and the VOLTS/DIV setting for the peak-to-peak voltage.

7)Record the number of divisions and the TIME/DIV setting for the period.

8)Repeat steps (3) through (7) for two additional frequencies (7400 Hz and 12,400 Hz)

9)Have someone from another group set the frequency to an unknown value (have them COVER the sig. gen. display). Calculate the mystery frequency THEN read it off the sig. gen.

10)Disconnect the oscilloscope leads from the signal generator and adjust the oscilloscope to read DC voltages. Adjust the vertical position of the horizontal line seen on the CRT to a convenient position. Set the VOLTS/DIV setting to 1 volt per division. Take the leads from the oscilloscope and connect them to the dry cell. Notice that the horizontal line jumps to a new position indicating that the voltage has changed. Record the number of divisions and the VOLTS/DIV setting.

Results/Analysis

Show one calculation for Peak-to-Peak voltage and uncertainty:

Note:

Frequency (Hz)

/ Signal Generator Voltage Data and Results
Scale setting (volts/div) / Number of divisions / Peak-to-peak voltage (V)
1200
7400
12400
Dry Cell Data and Results
Scale setting (volts/div) / Number of divisions / Voltage (V)

Show one calculation for frequency and uncertainty:

Note:

Frequency (Hz)

/ Frequency data and Results
Scale setting (time/div) / Number of divisions / Period (s) / Calculated Frequency (Hz)
1200
7400
12400
mystery

Calculate the percentage error in your calculations of the frequency.

Actual frequency / Calculated frequency / Percentage error
1200
7400
12400
mystery -

What is the most important way to reduce the uncertainty in your measurements of either voltage or frequency?

Comment on how the calculated frequencies and their uncertainties compare to the true frequencies.

Conclusion