Oscilloscope

OSCILLOSCOPE

Objective
This lab is designed to show how a measuring oscilloscope operates and how it is used. It will be used in the exercise to measure the amplitude and frequency of a sinusoidal voltage.

Background
The major component of an oscilloscope is the cathode ray tube. It's construction is similar to the picture tube in a television. Inside the vacuum tube, a beam of electrons is fired at high speed toward the screen. A bright spot on the screen shows where the electron beam hits the screen. A voltage applied to horizontal and vertical deflection plates causes the electron beam to move horizontally and vertically.

A circuit called the horizontal sweep system provides a linearly changing voltage to the horizontal deflection plates. This causes the beam to sweep from left to right across the screen at a constant speed. A dial, often labeled 'sweep time/cm' or 'time/div', is used to adjust the sweep speed to values indicated on the dial.

The voltage being measured with the oscilloscope is connected to the vertical deflection system, which causes the beam to move vertically a distance that is proportional to the applied voltage. A dial, often labeled 'volts/cm' or 'volts/div', allows the user to adjust the sensitivity of the oscilloscope. This capability allows the user to measure a very wide range of voltages.

The trigger system is a circuit that starts each new sweep of the beam at the same voltage point. This insures that successive sweeps will lie on top of one another, producing a display on the screen that appears stationary for higher sweep frequencies.

Procedure
1. Connect the output of the function generator to the vertical input on the oscilloscope.
2. Set the function generator to produce a sine wave with a frequency of about 1 Hz. (That is one cy/sec)
3. Study each control on the oscilloscope and try to determine its purpose. Locate the calibrated vertical and horizontal dials discussed above as these are the controls you will use most often when making adjustments and measurements. (Ask for help if you need it).
4. Adjust the oscilloscope so that the beam is moving horizontally at about 1 or 2 cm/sec, and oscillating vertically a distance almost the size of the screen. (You may need some help on this). Have the lab instructor take a look at the display.
5. Set the frequency of the function generator to approx. 1000 Hz. Adjust the oscilloscope to produce a display of two or three cycles of the sine wave. Now adjust the amplitude of the function generator to produce a voltage with an amplitude of about 4 volts. You will do this by measuring the amplitude of the waveform in cm. Given this measurement and the setting of the vertical sensitivity control, the amplitude of the input can be determined.
6. The frequency of the input voltage can be determined by measuring the horizontal distance in cm for one cycle of the wave. Given this measurement and the setting of the horizontal sweep speed control, the period of the wave can be determined. The frequency is then found by taking the reciprocal of the period.
f = 1/T Have the lab instructor take a look at the display.
Calculate the frequency shown on the oscilloscope and compare that to the oscillator frequency value.

Use this method to measure several different generator frequencies and compare your measured values with the values displayed on the generator.

7. Adjust the function generator to produce the following voltages and have the instructor view the waveform on the oscilloscope.
2000 mV at 40 kHz
2.5 V at 200 Hz
8. Have the lab instructor adjust the function generator to a particular oscillating voltage. Measure and record the amplitude and frequency of the voltage.