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OSCILLOSCOPE and METER ORIENTATION

Any person who completes a college level physics course should be expected to have acquired certain skills not only in the lecture classroom, but also in the laboratory. Among them should be the ability to use electronic test and measurement equipment with some facility. Most of the labs this semester require the use of such equipment, but rather than spend time each lab to explain and practice with each device, we take this session to concentrate on the nature of such equipment, how each basically works, and how to use them in the labs, not only the teaching labs, but also in research and commercial work settings. One will find that there are some common features that the various kinds of equipment have. There are also some very different features that must be understood. It is expected that the student will be able to use the equipment to make measurements properly.

We concentrate on two basic kinds of equipment, the oscilloscope and the Volt-Ohm Meter. We look at the Volt-Ohm Meter first.

VOLT-OHM METERS

These meters are found to be available in two basic types, the ANALOG and the DIGITAL meters. Both accomplish essentially the same thing in that they allow the user to measure AC and DC voltages, DC current, and Resistance with accuracy and ease. Dependent upon the kind of measurement required, one kind or the other may be preferred. We examine how to use both. The situation determines which you may desire to use.

You will have the opportunity to use the several different kinds of analog and digital meters in the labs this semester. You will probably not be able to use the same one each time. That is not only okay, it is better so that you get acquainted with different ones. They all have the same basic features, but some are more precise, some have different knobs and switches, etc. We concentrate on the basic and common features:

PROCEDURE:

1. Voltage Measurements - You have two kinds of analog and one kind of digital meter at each lab station. Use these to measure some of the voltages of the batteries and power supplies available. These instructions lack specificity since they will vary with each lab station. Your instructor will explain how to do this. You must be sure that you have the correct polarity for DC measurements or you may damage the meters. Your instructor will demonstrate this to you. Basically, the RED lead is the POSITIVE terminal and the BLACK lead is connected to the GROUND or NEGATIVE terminal of the batteries. BE CAREFUL!
   
   
2. The major difference between the two analog meters is the price and precision. The inexpensive meters are capable of measuring a few voltage ranges with less precision than the moderately priced ones. Make several voltage measurements. Record these values and comment on the ease and precision of the measurements.
   
   
3. You will note that the digital meters are capable of far greater decimal precision, but are less ease of operation. Make several voltage measurements and comment on precision and ease of use. Discuss situations when the digital meter may be preferred over the analog meter and vice versa.
   
   

   Figure 1: Basic Features of Analog and Digital Volt-Ohm Meters.

4. Resistance Measurements - Measure some of the given resistive devices and record the values. Measure the resistance of your body. You may have to wash your fingers, perhaps with alcohol. Sometimes medical facilities even use abrasive devices such as sandpaper to get through the very highly resistive skin oils (which naturally protect the body.) Record and discuss you personal resistance (in ohms.)

OSCILLOSCOPE ORIENTATION:

The Oscilloscopes used in this course are general purpose, dual trace instruments. They are versatile tools for the lab and are particularly useful in studying circuits. The heart of the oscilloscope is the Cathode Ray Tube.

A beam of electrons comprises the cathode rays, formed by the "electron-gun". The electrons are thermally (heat) emitted, accelerated, and focussed to a small spot on the fluorescent screen. Figure 1 shows the basic electrical structure of a cathode-ray tube. The potential of the accelerating electrode is usually fixed at several thousand volts while the other two may be adjusted to increase the intensity or adjust the focus.

The beam may be deflected horizontally or vertically by pairs of plates. A beam is typically deflected horizontally causing it to sweep across the tube at some controlled rate and then a signal is imposed on this trace to cause some vertical deflection. The electronic apparatus to do this is not of importance at this point.

Figure 2: Dual Trace Oscilloscope, Ballantine 1010A

Our oscilloscopes provide two traces instead of one. The same theory applies for both traces. You may use either CH1 or CH2 as single trace instruments or combine them in certain ways. The benefits of each will become apparent as you become more sophisticated with their use. Initially we will be concerned with the elementary, safe use of the instrument.

Your objective will be to learn how to operate a cathode-ray oscilloscope in its various modes.

PROCEDURE:

1. Examine the oscilloscope and compare the control knobs as shown in Figure 3. Your instrument should be similar in design.
   
   
2. Set the carrying handle to the operating position, up. This is accomplished by pulling both detent knobs outward and raising the handle to some preset position not obscuring the screen.
   
   
3. Be sure the two lower channel control knobs are turned to OFF. They are the bottom right hand knobs with vertical arrows alongside them. Turn the Power/Intensity knob, lower left hand corner, clockwise to just beyond the OFF position. What happens?
   
   
4. Now, let us use one of the channels, say CH1. Turn its knob clockwise until the trace is in the middle of the screen. At this point note the position of the top center knob labeled TIME/CM. This controls the speed at which the sweep crosses a cm which is the scale on the screen. Time increments are in seconds, milliseconds and microseconds. Adjust the sweep speed and observe the various rates. The very fast rates appear to be a straight line. The <­­> knob to the right of the sweep rate control varies the left-right placement of the sweep. Set the sweep in the center of the screen by adjusting both of the knobs for the appropriate channel.
   
   
5. You should have a coaxial input signal cable. If we input a signal, i.e., a voltage, it will cause the sweep to be deflected up or down. Thus we can use the oscilloscope as a fancy voltmeter. Since this has an AC/DC switch (find it next to the VOLTS/CM switch for each channel) we can set it for the appropriate VOLTS and adjust the VOLTS/CM switch to the maximum for the signal we are measuring.
   
   
6. You have before you an audio generator. This supplies an oscillating signal, either a smooth waveform or a step square wave. Set the CH1 selector to GND. This will allow you to adjust the baseline signal to the middle of the screen.
   
   
7. Set the audio generator to about 25, connect the output leads from the left side (~) to the BNC connector at the oscilloscope. Adjust the amplitude of the audio generator about 90 clockwise from the stop. Turn it on. Set the VOLTS/CM knob to about .5 V and the TIME/CM to about 50 ms. You should observe a signal. Draw below what it looks like. Determine the period and amplitude. Measure the same with a voltmeter. Why is there a difference?
   
   
8. Report your results. You will be using the oscilloscope in future experiments.

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