The Oscilloscope. Part I. Trouble-Shooting Guide.

Transcription

1 The Oscilloscope Contents: Part I. Trouble-shooting guide. Part II. Description of adjustments. Part III. Physics 250, Lab #3 information. Part I. Trouble-Shooting Guide. The following are step-by-step instructions for the use of the Hitachi V-212 and V-222 Oscilloscopes. This document is useful if you are having troubles operating the oscilloscope at the fundamental level. You will need a signal source as well as the oscilloscope. We will assume that you will use an HP 3311A Function Generator. 1. Be sure the oscilloscope and function generator are turned on. Red power lights should appear. 2. Connect the function generator to the oscilloscope: You will need a banana to BNC cable. Connect the banana plugs to the 600 S OUTPUT (not PULSE OUTPUT) connectors of the function generator. Connect the BNC cable to the INPUT on the lower left (CH1 or X) input of the oscilloscope face. 3. Set up the HP 3311A as follows: Range Hz button: 100 Function button: Sine wave (middle button) Frequency dial: 5 DC Offset: 0 (centered) Amplitude: straight up (about ½ max) 4. Oscilloscope settings: To begin with, it is only necessary to adjust the indicated settings. (The general order is top to bottom then left to right.) Trace section: INTENSITY: all the way clockwise, unless there is a stationary intense spot on the screen. In that case, turn the INTENSITY down a little. Time section (top left): TIM/DIV: 1ms

2 POSITION: straight up CH1 ALT MAG: out (not pushed in) Trigger section (top right): MODE: AUTO LEVEL: straight up SOURCE: line CH1 or X axis section (lower left): VOLTS/DIV:.5 POSITION: straight up AC/GND/DC: DC Mode section (lower center): MODE: CH1 INT TRIG: CH1 CH2 or Y section (lower right): not needed. 5. At this point you should see a rapidly changing pattern on the screen. If so, go to step 6. IF YOU SEE NOTHING: a. Carefully check steps 1-4 b. Turn the INTENSITY setting all the way up. c. Undo the cable from the oscilloscope. d. Try a different oscilloscope. IF YOU SEE A FLAT LINE (or almost flat line): a. Carefully check steps 1-4 b. Be sure the Function Generator power light is on. c. Turn the AMPLITUDE setting on the Function Generator to MAX. d. Be sure the AC/GND/DC switch of the oscilloscope is on DC. Try AC also. e. Turn the VOLTS/DIV knob to.1 f. Check the cable connections. g. Replace the cable, the Function Generator, then the oscilloscope. 6. IF YOU SEE A CHANGING PATTERN -- everything's going as it should! Now we need to make adjustments to the trigger section: a. Move the SOURCE switch to INT. b. Adjust the trigger LEVEL knob until you see a sine wave. 7. Adjust the FOCUS knob (left center) to get a sharp image.

3 ***Part II is on the next page!

4 Part II. Description of Adjustments!Trace section: Controls appearance of the trace INTENSITY knob: adjusts brightness of trace FOCUS knob: adjusts focus of trace ILLUM knob: adjusts background scale illumination CAL.5 V: a calibrated source of 0.5 V which can be used to check the scope. Connect this by cable to CH1 or CH2 input.!time section: Controls sweep rate. The x-axis is usually, so this section controls the x-axis. TIME/DIV: Adjusts how rapidly the trace sweeps across the screen. Times are s for the trace to move one division (square) on the screen. X-Y must be selected if you wish to see the Y (CH2) voltage as a function of the X (CH1) voltage rather than a normal sweep. SWP VAR: This knob changes the sweep in a continuous fashion. Note that when this knob is turned on, the s on the TIME/DIV knob only roughly apply and the UNCAL light comes on as a warning. POSITION: adjusts the left-right position of the trace on the screen. If you pull the knob, it magnifies the trace ten s on the x-axis (to see details of the trace). CH1 ALT MAG: shows two traces for CH1: the upper trace is normal and the lower trace is magnified ten s along the x-axis.!trigger section: Controls when a sweep starts. Improper triggering results in a jumpy trace or no trace at all. The oscilloscope determines when to start a sweep by analyzing the height and slope of a trigger signal. SOURCE:

5 the 60 INT: the signal which controls triggering is an input source, either CH1 or CH2 as chosen by INT TRIG switch in the Mode section (bottom center). EXT: the trigger signal s supplied through the BNC connection labeled TRIG IN. LINE: the AC power provides the trigger signal, so the oscilloscope triggers at a fixed rate of Hz. MODE: AUTO: automatic trigger. Differs from NORM in that if there is no signal, you will see a flat line. NORM: normal trigger. Differs from AUTO in that if there is no signal, you will see nothing. TV-V and TV-H TV vertical and horizontal. Useful only for working on TVS. LEVEL: adjusts the level at which the trigger starts. Look at the left edge of the trace as you move the level knob. The trigger should occur on a positive slope of the trace unless the knob is pulled out, then it triggers on a negative slope.!ch1 or X section (and CH2 or Y section): Voltage usually appears along the Y axis, so this generally controls the y-axis functions. VOLTS/DIV knob: chooses the voltage range of the input signal. The number of volts per division (square) is selected by the knob. The red knob in the center does two things: 1) When pulled, it magnifies the y-axis by five s. 2)When turned, it allows you to change the height of the trace in a continuous fashion. In this case, the volts per division is NOT calibrated and the UNCAL light comes on as a warning. POSITION knob: adjusts the up-down position of the trace. AC/GND/DC switch: to choose AC coupling, DC coupling, or ground. GND causes the trace to be a flat line at zero volts. This is useful when you wish to measure a voltage off the screen. (Otherwise you don't know where to find zero.) AC coupling. This causes the average voltage of the trace to be shown on the screen as zero. This is useful, for example, when looking at small variations in a 10V DC signal. Only the variations will appear on the oscilloscope. DC coupling. Shows the trace with the actual voltage. (Compared AC coupling.) INPUT connector: BNC connector for the input signal. Ground connector: A banana connector to ground the oscilloscope to an external ground, if desired

6 (only on the CH2 side).!mode section: Controls the type of trace shown. MODE: CH1: Shows CH1 as the y-axis, as the X-axis. CH2: Shows CH2 as the y-axis, as the X-axis. ALT: First shows one trace with CH1 as the y-axis and as the X-axis and then a second trace with CH2 as the y-axis and as the X-axis. CHOP: Shows SIMULTANEOUSLY two traces. The first is CH1 as the y-axis and as the X-axis and the second is CH2 as the y-axis and as the X-axis. (The sweep actually shifts back and forth between the two traces, somes giving a "chopped" appearance to the traces.) ADD: The sum of CH1 and CH2 is shown on the y-axis and on the x-axis. INT TRIG: Selects which input controls the trigger, CH1 or CH2. VERT MODE is a mystery to me! **** Part III is on the next page!

7 Part III. Physics 250, Lab #3 Information. MEASUREMENTS OF PERIODIC TIME-VARYING ELECTRICAL SIGNALS THE OSCILLOSCOPE Remember it is important to understand the general operation of the oscilloscope and to know precisely what each adjustment accomplishes, but it is not important to know exactly how the instrument performs the function. The heart of the oscilloscope is a cathode-ray tube, the basic structures of which are illustrated in Fig. 1. Electrons are boiled off a hot cathode and are focused and accelerated by a series of electrodes to form a narrow beam. The beam strikes a fluorescent screen and produces a spot visible from outside the tube as a bright dot. The beam, being composed of electrons, can be deflected by means of charged plates located below and above and also on opposite sides of the beam. Since electrons have a very small mass, the deflections can be very rapid. The oscilloscopes in this laboratory can respond in s as short as 0.2 µs, and more expensive oscilloscopes can operate as fast as 1 ns. The beam deflection is caused and controlled by applying voltages to the horizontal or vertical deflection plates. The voltages produce electric fields that deflect the beam so the deflection is proportional to the voltage applied to the plates. focusing and accelerating electrodes horizontal deflection plates spot on screen electron gun vertical deflection plates fluorescent screen Fig. 1. The internal structure of a cathode-ray tube The oscilloscope is most often, though not always, used in a mode in which the beam sweeps horizontally across the tube face and then flips back to the starting position. This procedure is repeated continually. This sweep is accomplished by applying a sawtooth voltage variation to the

8 horizontal deflection plates as illustrated in Fig. 2. (The plane of the plates in the tube is vertical, but they control horizontal deflection of the beam.) The voltage begins negative and rises linearly; thus, the spot starts at the left and moves linearly in across the face of the scope. The voltage then suddenly drops back to the negative starting condition, and the spot flips back to the left and repeats its motion. The voltage generators that produce this sweep are built into the oscilloscope. You can select different sweep speeds by merely turning one dial, and by a second adjustment you determine the precise instant when the beam flips back to restart a trace. V(t) Sweep Voltage Sweep Voltage Path of Spot Path of Spot V (t) Sweep Voltage Sweep Voltage Fig, 2. Voltage applied to the horizontal deflection plate and its effect on the beam spot location First consider only one horizontal sweep of the spot across the scope. A -dependent voltage V(t) you desire to study must be applied to the vertical plates of the cathode ray tube at the same the sawtooth sweep voltage is applied to the horizontal plates. While moving across the screen at a constant speed, the spot will move up and down proportionally to the voltage V(t). A graph of the voltage as a function of (with on the horizontal axis) is displayed on the screen. The effect of the sawtooth voltage variation illustrated in Fig. 2 is simply to transfer the variation in V(t) to a distance variation on the face of the scope. The oscilloscope thus serves as a sophisticated graph plotter of V(t) and can plot a graph in a few millionths of a second. If the voltage V(t) is periodic, as illustrated in Fig. 3, and if the flipback of the sawtooth horizontal voltage can be synchronized with some periodic feature of V(t) [1, 2, 3,... n periods of V(t),] the spot will be returned to the left of the screen at precisely the right to retrace a section of V(t) produced in the previous sweep period. Since V(t) is periodic, the beam will thus retrace the identical pattern. The result on the screen is a stationary pattern that is continually being replaced but appears to the viewer to be standing still due to the persistence of your eyes and of the fluorescent material on the screen. This repeating sweep process is easily observed at Fig. 3. Illustration of the required synchronization between the period of V(t) and the restart of the sawtooth sweep voltage

9 slower sweep speeds. A stationary pattern will appear if the period of the sawtooth sweep variation is equal to, two s, three s, or any integral multiple n of the period of V(t), and n periods of V(t) will appear. In this case, n periods of V(t) will appear on the screen. This concept is suggested by the drawings in Fig. 3 with n = 2. Since it is very difficult to study a voltage variation graph that is not stationary on the face of the scope, it is important to provide a means of synchronizing the return of the horizontal sawtooth voltage and the period of V(t). This synchronization implies taking some information relative to the period of the V(t) variation and using this information to restart the sawtooth voltage. The sweep trigger control allows the operator to provide this synchronization. Since this particular adjustment is critical and rather difficult to understand, an explanation follows: The trigger control simply sends a signal to the sawtooth voltage generator to tell the generator to return and start its sweep. The trigger circuit can function only if the period of the sawtooth variation (set by the sweep adjustment) is approximately a multiple of the period of V(t). The trigger circuit must get the information from somewhere, and most oscilloscopes have four possible sources or modes of sweep trigger operation. Specifically, the trigger starts the sweep under the following conditions, provided the inner (red) knob of the trigger control is set on automatic: (a) any the voltage V(t) crosses zero with a positive slope (int +), (b) any the voltage V(t) crosses zero with a negative slope (int -), (c) each the 110-volt power line voltage crosses zero with a positive slope (line), (d) each an externally applied signal crosses zero with a positive slope (ext). The inner (red) knob on the trigger mode switch controls the level (not zero) that V(t) must reach before the trigger signal is produced. You can often place this knob on automatic, but for refined work where you want to trigger on a weak signal or on a particular part of a signal, you may manually adjusted the trigger level. To make this adjustment well requires some personal experience. The oscilloscope is generally used to study -dependent voltage signals by applying the sawtooth sweep signal discussed above to the horizontal plates. You can, however, apply any signal you desire directly to the horizontal plates and plot a graph of V y (t) vs V x (t) on the face of the scope. In this case the pattern will stand still only if the two are periodic with frequencies that are integer ratios. Notice that all input electrical connections to the oscilloscopes involve two terminals, one of which is the common ground connection that is the framework of the instrument. This ground terminal is actually connected to earth ground through the third wire of the power plug. All voltages are measured relative to this ground reference. The horizontal and vertical input terminals send signal voltages through amplifiers before they are applied to the respective deflection plates that cause the electron beam to move. Thus, by using the amplifier you can change the size of the picture on the face of the scope for any specified signal.

10 At the conclusion of these introductory remarks on the oscilloscope, it must be emphasized that the oscilloscope is a quantitative measuring instrument. The scale (horizontal scale) is calibrated (when the inner knob on the sweep adjustment is in the cal position) so numerical values taken graphically from the face of the scope are accurate to a few percent. The vertical scale is calibrated in terms of volts applied to the vertical input terminals, and you can read numerical values from the scope face just like you read from any graph. The oscilloscope is a very sensitive but rugged instrument. You need not worry about doing damage to the instrument by making any adjustment except one: You must not turn the beam intensity adjustment on the on/off switch to maximum bright when the beam is not moving. You can also damage the instrument if you apply voltages in excess of 600 volts to any terminal or if you drop the instrument. With these minor precautions, feel free to experiment and play with the instrument. Today, you will consider relatively simple circuits involving resistors, capacitors, and inductances and will apply voltages that vary either sinusoidally, as a triangular sawtooth variation, or as a square (- e to + e, high-low) variation, as illustrated in Fig. 4. The sinusoidal voltage variation is generally referred to as a simple ac voltage, the sawtooth wave as a triangle wave, and the rapidly repeated switching of a dc voltage from a + to a - voltage level as a square wave. Actually no wave phenomenon is involved since the voltage varies only in. You will have available a socalled audio-signal generator that produces the voltage variations discussed. Using these applied voltages, it is possible to study experimentally the essential features of ac circuits, including LC resonant circuits, as well as RL and RC constants. Refer to your physics text for details of the associated theory. When you study an instrument such as the oscilloscope, it is significant to understand its limitations and its areas of optimal operation. The oscilloscope is an extremely versatile instrument and has a number of adjustments. As you study the instrument and apply it to a variety of situations, you should always be asking yourself whether the instrument is giving distorted graphs or reproducing faithfully the voltage variations you seek to study. When operating at the extreme conditions of weak signal or very fast or very slow -varying phenomena, you should be particularly wary. V V V Fig. 4. Signal shapes available from the audio generator It is easy to think of either a sinusoidal or square-wave voltage variation at any frequency between one cycle per second and one megacycle per second, but to produce and manipulate such voltages is more difficult than to think about them; in fact, it is easier to measure than to produce such voltage variations. Those who understand the concepts of Fourier analysis realize that square waves involve harmonics that are very high and thus difficult to produce. In all measurements below you will find that the oscilloscope is more precise and accurate than the signal

11 generator itself. In reality if used properly, the scope will account for only a small fraction of the nonideal nature of the measurements. As you study the square-wave voltage from the signal generator at very high frequencies, you will see the reasons for the above comments.