Experiment 7
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 box, signal generator, breadboard, 2­ BNC to twin-banana cables, BNC to single-banana cable, double-BNC cable.

RELEVANT EQUATIONS

 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. 7-1. 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.

Figure 7-1: The Cathode Ray Tube (CRT)

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. 7-2.

a. Plug in the oscilloscope to the electrical outlet (110 V) and press the POWER button to the ON position.

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.

c. Focus.  Rotate this control to get the clearest, sharpest trace. If the intensity is too bright the focus will not be as sharp.

The trigger circuit synchronizes the vertical and horizontal motions of the electron beam. Usually the electrical signal to be displayed would be applied to channel A or channel B, and the trigger circuit would monitor this signal. When the signal reaches certain conditions the trigger circuit allows the time base circuit to start sweeping horizontally.

d. Trigger Source.  Depress the TRIG or X SOURCE button until the display reads A.

e. Rotate the HOLDOFF knob clockwise to the MIN position.

f. SEC/DIV (TB rocker switch).  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 the VAR knob is set to CAL.

g. X-POSITION.  This should horizontally center the trace. It should be approximately at mid position.

The VERTICAL section of the 'scope controls control the vertical motions of the electron beam. Signals are connected to the CHA and/or CHB inputs and are either displayed one at a time or simultaneously.

h. VERTICAL DISPLAY (A/B button).  This switch chooses which channel (one or both) is displayed. For now, place the switch in CH A position.

i. AC/DC, GND buttons.  These switches connect the CH A or CH B signals to the vertical amplifiers: with the AC/DC button 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. With the GND button depressed, the signal is disconnected from the amplifier, and zero volts is applied to the input. Place both switches in the DC position to begin.

j. VOLTS/DIV (A or B rocker switch).  This selects the sensitivity of the oscilloscope vertical amplifier. You might set it at 1 volt/div to start. Be sure that the VAR knob is set to CAL.

k. Y-POSITION.  Adjust the vertical position of the trace with this knob until the trace is centered on the screen.

(There are no prelab questions for this experiment.)