INTRODUCTION TO THE
OSCILLOSCOPE
PRELAB
PURPOSE To become familiar with the oscilloscope and to use it to: measure the voltage and period of an AC signal, measure the phase difference between two AC signals, and verify Ohm's Law for AC circuits.
EQUIPMENT Dual trace-dual channel oscilloscope, step-down transformer board, signal generator, breadboard, 2 BNC to twin-banana cables, BNC to single-banana cable, double-BNC cable.
RELEVANT FORMULAS
Amplitude Measurement:
Period Measurement:
Frequency:
Phase Difference Measurement:
Ohms’ Law for AC Circuits:
DISCUSSION
The oscilloscope makes it possible to study the behavior of a voltage that varies rapidly with time. The oscilloscope actually traces out the time-dependent waveform of the voltage being measured. The face of the oscilloscope is the screen of a cathode-ray tube, which is similar to the picture tube in a television set or computer monitor. The input signal is amplified and used to control the vertical position of a beam of electrons that produce the trace observed on the screen. In normal use, the beam is swept horizontally across the screen at a uniform rate, so the horizontal position is proportional to time. The oscilloscope used in this lab is capable of displaying two input signals simultaneously.
The construction of the cathode ray tube (CRT) is shown schematically in Fig. 2.07-1. Near one end of the tube is a cathode for the emission of electrons, an anode that is held at a potential of two thousand volts or so for accelerating these electrons toward a fluorescent screen at the other end of the tube, and grids (not shown) for focusing the beam of electrons and controlling the intensity of the beam. The anode and the grids are metal cylinders; as the beam of electrons from the cathode passes through these cylinders it is accelerated and controlled by the electric fields formed by the anode and the grids.
Between the assembly of electrodes described above and the fluorescent screen are two pairs of metal plates, as shown in the diagram. The beam of electrons may be deflected by an electric field between the plates. In a typical application the tube is placed horizontally and is so arranged that one pair of plates controls the horizontal position of the electron beam on the screen and the other pair of plates controls the vertical position of the beam. Where the beam of electrons strikes the screen, it produces a bright spot, the brightness persisting for a short time. The position of the bright spot may be controlled by the potentials applied to the deflecting plates, or the potentials applied to the deflecting plates may be inferred from the position of the bright spot on the fluorescent screen.
Take a few minutes to familiarize
yourself with the instrument and its controls before proceeding to the
experimental procedure section. Starting from the left hand side of the
instrument let's get the oscilloscope running in a step by step manner.
Letters below refer to arrows indicating controls in Fig. 2.07-2.
b. Intensity
- rotate this knob fully clockwise and then as the trace is displayed reduce
the intensity. You may not see a trace at this time. The intensity should
not be excessively high; otherwise permanent damage to the CRT may occur.
If you have questions about what the proper intensity is, please consult
your lab instructor.
Figure 2.07-1: The Cathode Ray Tube (CRT)
d. Beam Finder
- Pressing this brings the trace onto the screen. Try it.
f. Trigger Source - Place the SOURCE switch in the INT position (up), and place the INT switch (just to the left of SOURCE) in the CH1 position (up). This connects the CH1 signal to the trigger circuit.
g. Rotate the VAR HOLDOFF knob counterclockwise to the NORM position. Place the MODE switch in the AUTO position and the SLOPE switch in the up position.
h. SEC/DIV - This sets the time that the trace will take to travel horizontally 1 cm. Set it at 5 msec/div for now. Usually you adjust this control so that several cycles of the waveform appear across the screen. Be sure to note the red CAL knob that must be rotated fully clockwise until you feel a very gentle click. The SEC/DIV knob is calibrated when the CAL knob is in this position and is pushed in.
i. POSITION
- This should horizontally center the trace. It should be approximately
at mid position.
k. AC-GND-DC - These switches connect the CHA or CHB signals to the vertical amplifiers: in the AC position, any dc component is blocked from the input; in the DC position, the entire signal (ac part + dc part) is applied to the input. In the GND position, the signal is disconnected from the amplifier, and zero volts is applied to the input. Place both switches in the DC position to begin.
l. VOLTS/DIV - This selects the sensitivity of the oscilloscope vertical amplifier. You might set it at 1 volt/div to start. Concentric with this knob is a red CAL knob that must be rotated clockwise to the click for the VOLTS/DIV knob to be accurate.
m. POSITION
- Adjust the vertical position of the trace with this knob until the trace
is centered on the screen.