EE 442. Lab Experiment No. 1 1/12/2007. Introduction to the Function Generator and the Oscilloscope

Transcription

1 EE 442 Lab Experiment No. 1 1/12/2007 Introduction to the Function Generator and the Oscilloscope 1

2 I. INTRODUCTION EE 442 Laboratory Experiment 1 The purpose of this lab is to learn the basic operation of a function generator and an oscilloscope. II. THE AGILENT 33220A FUNCTION GENERATOR (Image courtesy Figure 1 Agilent 33220A function generator PRELAB READING: A function generator is a two terminal piece of test equipment that produces a time varying voltage signal at its output terminals. The Agilent (Figure 1) is capable of producing standard waveforms, as well as a number of arbitrary user defined waveforms. Only the square wave, ramp, and sinusoidal waveforms will be used in this class. When the function generator is energized, it powers up in a default state. This default state is defined by a sinusoidal function at a frequency of 1 khz with a voltage amplitude of 100mV peak to peak (shorthand notation is pp, so this voltage would be 100mV pp.) The generator powers up with its output off. 2

3 PRELAB QUESTIONS: 1. What is the Agilent function generator capable of producing? 2. What is the default frequency and output voltage when the function generator is powered up? 3. Based on Figure 1, what is the frequency range of the Agilent 33220? EXPERIMENT: Upon power-up, the default frequency will appear on the display panel as 1.000,000,0 khz (an 8-digit number.) Press the button under the Ampl menu and the 100mV pp output voltage value will be displayed on the panel. Now press the Freq button to return to the frequency display. The purpose of this exercise is to learn to perform the following necessary operations: 1. Change the frequency of the waveform from 1 khz to whatever value is desired. 2. Change the voltage amplitude from 100 mv pp to whatever value is desired. The user has four options for displaying the voltage: Volts peak-to-peak (V pp), milli-volts peak-to-peak (mv pp), Volts rms (V rms) and milli-volts rms (mv rms). In this class, V pp will be used most of the time. 3. Change the time-varying function from a sinusoid to a square or triangular wave. To change the frequency, the knob may be used in conjunction with the arrow buttons. The arrow buttons change the highlighted digit position of the frequency value. The value of that highlighted digit is controlled by the knob. The arrow buttons move the blinking digit one space to the right or left. EXPERIMENT: 1. From the default frequency, use the knob to obtain a frequency of 3.2 khz. 2. Use the arrow buttons and the knob to set a frequency of khz. 3. Use the arrow buttons and the knob to set a frequency of Hz. Perhaps the easiest way to set a particular frequency (e.g., 1.234,567,8 khz) is to punch it in directly. 3

4 EXPERIMENT: 1. Press the ten buttons and the button under khz, in that order. The unit (button under the khz unit) button is the last button pressed when setting a new frequency value. Pressing this button also causes the generator to take in this new information and output a waveform at the new frequency. Another useful button is the left arrow button. This button allows you to correct a mistake or change your mind in the middle of entering a number. 2. Try another frequency: set the frequency to Hz. The next task to explore is the method of changing the value of the output voltage. The same knob and buttons are used to set the voltage as were used to set the frequency. Also, as was true for the frequency case, when establishing a new voltage value with the direct punch-in method, the last button to be pressed is one of the unit buttons. When a unit button is pressed, the generator will output a waveform with the (new) displayed voltage value. The options are: 1. milli-volts peak-to-peak (mv pp) 2. Volts peak-to-peak (V pp) 3. milli-volts rms (mv rms) 4. Volts rms (V rms) The voltage units can be changed, without changing the voltage value, by pressing a number on the keypad, using the arrow button to delete the number, and then pressing the button under the appropriate unit. Another method of establishing a voltage value is to use the arrow buttons to define the highlighted digit. Then set the value of that highlighted digit using the knob. Note that when the unit position is highlighted, the knob allows adjustment of both the units and the decimal point (they re linked together). EXPERIMENT: 1. Press the button under the Ampl menu. The voltage amplitude of the sinusoidal waveform should now be displayed. (The waveform should still be at its default value of 100 mv pp). 2. Change the voltage of the sinusoidal waveform to V pp using the knob and the arrow buttons. 3. Change the voltage to read an equivalent value in Volts rms. (Answer = mv rms.) 4. Change the voltage in step 3 to V pp by pressing the right arrow button to cause the mv pp to be highlighted. Then rotate the 4

5 knob clockwise. Note the relationship between the units and the decimal point. 5. Set the voltage to V pp by punching-in the value in directly 6. Also remember that to check on the frequency of the voltage being generated just press the button under Freq. To return to the voltage mode, press the button under Ampl. To select the function to be generated, merely press the button associated with the square wave, ramp, or the noise function. Finally, in order to insure that the same voltage value appears at the output terminals of the function generator that appears on the display panel, perform the following sequence: 1. Press the Utility button. 2. Press the button under Output Setup. 3. Press the button under Load. High Z should be highlighted. 4. Press the button under Done. This procedure must be performed every time the function generator is turned on. A failure to complete this sequence may cause the voltage appearing at the terminals of the function generator (for certain load conditions) to be different from the value shown on the display panel. One of the consequences of this procedure is that the default voltage amplitude changes from 100 mv pp to 200 mv pp. (Checking these values is a handy test to see if this task was performed.) III. INTRODUCTION TO THE AGILENT DSO3202A OSCILLOSCOPE (Image courtesy Figure 2 Agilent DSO3202A oscilloscope 5

6 PRELAB READING: In its basic (and normal) mode of operation, an oscilloscope (Figure 2) is nothing more than a very sophisticated voltmeter. It is basically a two input-terminal instrument providing a two dimensional visual display of a time dependent signal voltage waveform. This display, which appears on the screen, makes possible the observation and measurement of voltage signals which can be very complex functions of time. The following paragraphs will describe how an analog oscilloscope works (As far as the user is concerned, the operation of a digital scope is the same but has some extra features). The primary component of an analog oscilloscope is a cathode ray tube (Figure 3). Inside the cathode ray tube (CRT) electrons are generated from a heated cathode and are accelerated towards the front of the screen. When these electrons strike the screen they cause the phosphorescent coating covering the screen to glow, thus creating a white dot at their point of impact. (Image courtesy Figure 3 Basic operation of an analog oscilloscope Also inside the CRT are two sets of plates, a horizontal set and a vertical set. An electric field is created between the horizontal plates and is proportional to the magnitude and polarity of the input voltage signal. This electric field causes the electron beam to be deflected proportionally to the intensity and polarity of the electric field. In this manner the beam is swept between the top and bottom of the screen. Another electric field is created between the vertical set of plates which sweeps the beam from left to right (facing the screen). 6

7 Before the input voltage can be used to deflect the electron beam, it must be scaled to an appropriate level to create an electric field between the horizontal plates of sufficient strength to deflect the electron beam a desired amount. The user can control this scaling with the volts/division knob. This knob is usually adjusted so that the waveform takes up as much of the screen as possible without running into another waveform (multiple trace operation) and without running off the screen. The voltage signal which creates the electric field that sweeps the signal from left to right can come from one of two places: internal or external to the oscilloscope. If the horizontal deflection voltage is derived internally, it will come from a sweep generator. The sweep generator creates a saw tooth shaped voltage waveform that draws the trace across the screen. The rate of the sweep generator is set by the user and is chosen to display the appropriate amount of the input waveform. This control is the seconds/division knob on the front panel of the scope. Each cycle of the sweep generator is initiated by a trigger signal which will be discussed in a later laboratory experiment. The horizontal deflection voltage can also come from an external input signal. Basically another set of input voltage terminals is connected to the horizontal deflection plates. In this manner, the user can graph a y voltage as a function of an x voltage rather than a y voltage as a function of time. EXPERIMENT: Turn the oscilloscope on and set the above controls to the following settings (The scope will take a minute or two to boot up). Horizontal System Controls 500ms/div To change this setting, turn the big knob on the Horizontal section of the front panel (Figure 4). The sweep rate (s/) is displayed at the bottom right corner of the screen next to the sampling rate (Sa/s). (Image courtesy Figure 4 Horizontal system controls 7

8 Vertical System Controls Channel 1 coupling to ground 2 V/div (1X probe) To change the coupling, push the 1 button to bring up the menu for channel 1 (Figure 5). This menu will allow you to change the coupling. While in this menu change the probe attenuation factor from 10X to 1X. This change will make the voltage scales match for the activities we are going to do later. To change the volts per division setting, rotate the large knob above the 1 button. The volts per division (V/) setting is displayed in the lower left hand corner of the scope s screen. (Image courtesy Figure 5 Vertical system controls The sweep rate was set to 500ms/div and there are 10 divisions on the screen, so it should take about 5 s for the dot to move from the left corner of the screen to the right corner of the screen. What you are seeing is the sweep generator voltage being applied to the horizontal deflection plates. This voltage is a piecewise linear function with respect to time as shown in the Figure 6. 8

9 Figure 6 Simulated sweep and trigger Waveforms Waveform 1 is a representation of the voltage from the sweep generator. Waveform 2 is a representation of the trigger waveform. Note how both the trigger pulse and the sweep start at the same time. PRELAB READING: Since the sweep portion of the saw-tooth voltage waveform is linear in time, when it is applied to the horizontal deflection plates, it produces a corresponding horizontal (x) deflection of the electron beam which is also linear in time. The beam (spot) therefore travels across the screen with constant velocity. Combining this linear time-vs.-x dependence, with linear voltage-vs.-y dependence, results in a time dependant voltage waveform that can be visually displayed, as a two dimensional graph, where the vertical axis is defined by a voltage scale, and the horizontal axis is defined by a time scale. There are a couple of other features present in the saw-tooth wave, shown in Figure 6, that should be mentioned. That portion of the waveform with a negative slope defines the retrace period during which the voltage of the saw-tooth wave returns to zero to prepare for another sweep. The reset (standby) period (where the sweep is flat-lined) is when the sweep generator and trigger circuits reset themselves in preparation for the next sweep cycle. During the retrace and reset period the electron beam is turned off. A trigger pulse waveform corresponding to the saw-tooth waveform is also shown in the figure. The relationship between these two waveforms will be discussed in the triggering section of this experiment. 9

10 PRELAB QUESTION: 1. How many channels does the oscilloscope have? Where is the auto-scale button? EXPERIMENT: Change the sweep rate from 500ms/div to 500μs/div. Rotate the knob one click at a time while noting the change of velocity of the trace on the screen. Notice that after a few clicks of the knob, the sweep generator is sweeping so fast that the dot appears to change into a line. The dot is moving so fast that the phosphorescent coating is still glowing when the dot returns. This persistence of the phosphorescent coating is what allows us to see a voltage signal waveform that repeats many tens to millions of times per second. IV. DC VOLTAGE MEASUREMENTS PRELAB READING: To perform this part of the experiment, keep the controls as they were in the previous procedure, with the CH1 vertical scale factor at 2 V/div and the sweep rate at 500μs/div. Note that the input coupling was set to the ground position. The grounded input setting and the CH1 vertical position control (little knob under the 1 button), provide a means for the user to establish a zero voltage reference trace location anywhere on the screen. Convenience and/or practical reasons can suggest the best location. The internal implementation of the coupling setting can be understood by referring to Figure 7. With the input coupling set to ground, any signal voltage connected to the CH1 input terminal would be disconnected (open circuited) from the oscilloscope. At the same time, zero volts (by a short circuit to ground) is applied to the input to the vertical attenuator/amplifier circuits. Note that if a voltage signal were connected to the CH1 input terminal, it would NOT be short circuited to ground. It is good experimental practice to always check (and reposition if necessary) the location (on the screen aka. graticule) of the zero volt trace prior to making a voltage related measurement. Note that if the coupling feature were not available, the alternative procedure of establishing a zero volt trace would be to disconnect the probe from the circuit being measured and then connect it to a ground terminal (to simulate zero volts). In time this method could become a very inconvenient process. PRELAB QUESTION: 1. How many coupling modes does the oscilloscope have? What are they? 10

11 DC Blocking Capacitor 2 Selector Switch Channel 1 Input 3 1 Channel 1 Amplif ier 4 Figure 7 Input circuitry between the probe and channel 1 amplifier 0 EXPERIMENT: To make a DC voltage measurement, proceed as follows: 1. With the CH 1 input coupling still set to ground, position the zero volt trace at the vertical center of the graticule. 2. Locate the Agilent E3630A DC supply on the lab bench. Make sure it is turned off. Turn the +6V knob fully counterclockwise (This sets the output voltage to 0). 3. Connect the oscilloscope to the 6 Volt terminals of the DC supply by connecting a BNC to banana clip adapter to channel 1 and using banana clip leads to connect to the DC supply. Observe proper polarity. 4. Set the CH1 coupling selector to DC, turn on the DC supply, and increase the output voltage until about two major divisions of vertical displacement are seen. 5. Leaving the DC voltage output as it is, reverse the leads connecting the oscilloscope to the DC supply and observe the effect on the graticule. Now reconnect the leads as they were for step Turn on the digital multimeter located at you lab bench and set it to read DC volts (It boots up to this function). Connect it in parallel to both the scope and the DC supply. Compare the reading of the multimeter to that of the oscilloscope and to that of the indicator panel on the power supply (Make sure the +6V meter button is pushed in). If there are any large discrepancies, consult with your lab instructor. (Hint: to read the scope, count the number of major divisions and multiply the result by the setting of the V/div knob.) 7. Try reading DC voltages with the scope set at different scale factors. For example, change to 1 V/div and read the voltage from step 6. Is the trace almost off the screen? Remember that you re at liberty to relocate your zero volt reference if need arises. Make other random 11

12 voltage measurements, at various scale factor settings, and verify with the digital multimeter. When you are finished, disconnect the meter from the supply and turn it off. 8. Reduce the DC supply voltage to zero and set the sweep rate to 0.1 s/div. Set the vertical scale to 2 V/div and zero the trace one division from the bottom of the graticule. Now increase the voltage output of the DC supply in such a manner that you can control the vertical displacement of the beam as it moves horizontally across the screen. Vary this voltage up and down rapidly and slowly, and note the relationship between voltage, beam (spot) displacement, and time. 9. Reduce the DC supply voltage to zero and turn it off. Disconnect the scope from the supply and turn it and the meter off. V. AC VOLTAGE MEASUREMENTS PRELAB READING: In order to use the scope to make AC voltage measurements, which means the observation and measurement of time varying voltage signals, we first need to define some of the properties of periodic waveforms. This task is done in Figure 8 for a sinusoid. One defining property of a sinusoid is its peak amplitude (value). This value can be obtained on the scope by measuring the vertical (voltage) distance from the (user set) zero voltage (horizontal) axis to either the positive peak or the negative peak. In the case of a sinusoid, these two peaks are equal in amplitude. Another method, which is often quicker since the zero volt reference does not have to be used, is to measure the peak to peak amplitude and then divide by two. 12

13 Figure 8 Components of a sinusoid PRELAB QUESTION: 1. What is the peak voltage, and peak-to-peak voltage of the waveform shown in Figure 8? EXPERIMENT: Set the scope to measure a 2 V pp, 1 khz (2 V/div and 200 µs/div) signal and set the zero volt reference level. Make sure that the input coupling is set to DC before you proceed. Connect the scope to the signal generator with a BNC to BNC cable. Turn the signal generator on and set its output impedance to HIGH Z. If you connect the scope to the signal generator using any other method, be sure that you observe polarity vary carefully as both the scope and the signal generator are earth grounded. A mistake in polarity will mean a short circuit and probably a blown fuse in the signal generator. With the signal generator powered up and connected, set it to output a 2 V pp, 1 khz sinusoid and turn the output on (push the Output button). When everything is set up correctly you should see about 2 sinusoidal 13

14 cycles on the screen. You may have to play with the trigger level knob to obtain a stable display. Change the amplitude of the output from the signal generator and note the effects on the scope. Set the signal generator to output 8 V pp. The waveform on the graticule of the scope should be just superimposed between the +4 and -4 volt lines. Change to several different scale factors and make sure the output voltage is still 8 V pp. You may loose the signal as you change scales, if this problem occurs you will need to re-adjust the trigger level. A point can be made here in regards to basic voltage and time measurement techniques. In general, if the particular situation permits, it is always a good practice to enlarge the waveform as much as possible. This practice allows a more accurate measurement to be made. VI. MEASUREMENT OF FREQUENCY PRELAB READING: To determine the frequency of a sinusoidal waveform, using a scope, it is first necessary to measure the period T of that waveform. Figure 9 suggests two of the most common methods of measuring T. The first method is performed by measuring the time between two alternate zero crossings. The second method is performed by measuring the time between two consecutive (equal polarity) peaks. Both measurements should give the same result, although the zero crossing technique is probably the easiest to use and the most accurate. If the zero crossing method is used, it is imperative that the zero voltage reference be accurately set. Once the period T is found, the frequency can be 1 determined from the relation f =. T 14

15 Figure 9 Frequency measurement techniques PRELAB QUESTION: 1. What are the two methods for measuring the period T? EXPERIMENT: 1 We will make use of this expression f = by examining the relationship T between the frequency of the signal to be displayed and the sweep rate of the scope. The signal generator should still be set at 8V pp, 1 khz. Set the vertical scale of the scope to 2 V/div and 200 µs/div. Reduce the frequency and note what happens on the graticule. The waveform begins to spread out and pretty soon there is only one cycle of the waveform instead of two. Increase the frequency of the signal generator past 1 khz and note how the waveform gets compressed and more cycles of the sinusoid appear on the screen. From this exercise you should see that the period of the waveform decreases with increasing frequency. The next task will be to take a simple frequency measurement with the scope. Make sure the signal generator is still set on 1 khz. Re-establish the zero volt reference on the scope. Once this step is completed, the 15

16 waveform from the signal generator should be seen on the graticule. Before proceeding answer the questions below. What should the period of the waveform be (from the signal generator)? T = Based on the above answer, about how many waveform periods should be seen on the graticule if the sweep rate is 200 µs/div? Number of Periods = How many periods do you see on the scope? Number of Periods = How many divisions (estimate to the nearest tenth of a division) on the graticule does one period of the waveform occupy? Number of Divisions = What is the period of the measured waveform (number of divisions multiplied by the sweep rate setting)? T = What is the frequency of the waveform? F = With a 1 khz signal still displayed on the scope, change the sweep rate and note the effects of this change. When you increase the sweep rate to 100 µs/div you should see only one period of the waveform. When you decrease the sweep rate to 500 µs/div you should see about 5 periods of the waveform displayed on the graticule. Note that for the above sweep rate changes the frequency of the input signal remained constant. Leave everything set up as it is for the next part of this experiment. 16

17 VII. TRIGGERING PHENOMENON PRELAB READING: A. General Discussion In its simplest sense, the primary role of the trigger circuit is to determine where the trace will begin on the waveform of the signal to be displayed. This definition assumes that were the trace starts is important to the user! To accomplish the task of triggering, the following series of events take place. When certain conditions are satisfied, the trigger circuit will send a turn-on message (trigger pulse) to the sweep generator. Upon receiving this message, the sweep generator will turn on, causing the electron beam to begin its horizontal sweep across the screen. The conditions mentioned above are the voltage level and the slope polarity of the signal being triggered off of, both of which are pre-selected by the user. Another condition for the trigger pulse to be accepted is that the sweep generator can t be in the middle of a sweep when the trigger pulse arrives, i.e., a sweep can t be initiated in the middle of another sweep. It should be mentioned that the signal being triggered off of (trigger signal), whose waveform provides the basis for a trigger pulse, does not have to be the same signal as connected to the vertical input. However the frequencies of the two signals should be integrally related (one frequency should be an integer multiple of the other) if a stable waveform display is desired. In general, trigger signals are derived from one of two basic source locations: external or internal. An external trigger signal comes from a voltage not being measured by the scope. If this type of signal is used, it is connected to the external input terminal of the scope. The trigger source should be set to external and the trigger signal input coupling should be set to whatever coupling fits the situation (usually DC). An internal trigger signal is derived from voltage signals being measured by the scope. Examples of measured voltages are the voltages applied to either the channel 1 (CH1) or channel 2 (CH2) input terminals. If this trigger option is chosen, the trigger source should be set to CH1 or CH2 which ever input you want to trigger off of. 17

18 EXPERIMENT: B. Trigger Level Control Make the following settings on the scope. Set the trigger level to 0V (use the double arrow knob in the Triggering section), slope to positive, trigger mode to auto, sweep rate to 0.2 ms/div (push the Mode/Coupling button to bring up the appropriate menu) and the CH1 vertical scale set to 2 V/div. Also set the scope to trigger off of CH1 (trigger source). Establish a zero volt trace and make sure the scope is connected to the signal generator. Adjust the signal generator output waveform to 10 V pp, 1 khz. The trace should be starting at 0 V on the left side of the screen and have positive slope where it starts. Once the scope and signal generator are set up correctly, rotate the trigger level control counterclockwise and watch what happens to the beginning of the trace (at the left side of the screen). The trace should be starting at a negative value. As the control is rotated further, the starting value of the trace should become more negative. If the trigger level is reduced below the negative peak of the signal, the waveform will become unstable. Rotate the trigger level control the other way and you will notice that the trace starts from a positive value. The trace will start from an increasingly positive value as the trigger level is made more positive. Again, if you exceed the peak voltage of the waveform the trace will cease to be stable. C. Trigger Slope Control Adjust the trigger level control so the trace is starting at about zero volts from the left edge of the screen. Watch what happens to the beginning of the trace when you change the slope from positive to negative. Wiggle the trigger level control button a little to note the effects of changing the slope. D. Trigger Modes We have been using the automatic triggering mode for the last experiments. In this mode of operation triggering will always occur and a trace will always exist even in the absence of a trigger signal. It is a very convenient feature since it allows a trace to exist even when zero volts is applied to the input that is being triggered off of. This condition happens every time you establish your zero reference trace. The problem with the automatic triggering mode is that it will not create a stable trace at frequencies less than about 20 Hz. For these frequencies the normal mode of triggering must be used. 18

19 The problems with the normal mode of triggering are that triggering will occur only when triggering off a non DC signal and triggering will not occur if the magnitude of the trigger level exceeds the amplitude of the signal being triggered off of. Both the normal and automatic triggering modes have their attributes and drawbacks, but the automatic mode will be the most versatile for our needs. (To change the trigger mode press the Mode/Coupling button and then press the button next to the sweep option.) VIII. MEASUREMENT OF SIGNALS WITH AC AND DC COMPONENTS EXERCISE (THIS SECTION IS OPTIONAL) The voltage signals studied so far in this experiment have been either AC or DC. Many signals in practical circuits have both an AC component and a DC (time average value) component. For example, the voltage v= 4+ 2sin( ωt) V has a DC component of 4 V and an AC component of 2sin( ω t) V. The AC portion has a peak value of 2 V and a frequency of ω f = Hz. The next task of this experiment is to connect the function 2π generator in series with the DC power supply to create the above expression. Before energizing the DC supply (Agilent E3630A, use the +6 V tap) turn it voltage control all the way down (fully ccw). Connect the two sources in series and then connect the scope across them as shown in Figure 10. It is imperative that the DC supply and the function generator be properly connected together. If, for instance, the two devices were connected parallel, damage would be done to the signal generator. Ask your lab instructor to check your wiring before energizing either source. 19

20 Red/+ DC Power Supply Red/+ Oscilloscope Black/- Black/- Black/- Red/+ Signal Generator Figure 10 Circuit diagram for connecting the power supply and the signal generator to the oscilloscope Set the scope to 200 μs/div, DC coupling, CH1 to 1 V/div, to trigger automatically, and trigger off of CH1. Turn on the signal generator and set its output to 0 V pp, 1 khz. Position the zero volt reference trace 2 major divisions from the bottom of the graticule. Turn on the DC supply and increase the voltage until the trace is at the 4 volt line on the graticule. Now use the knob to sweep the signal generator s output to 4 V pp, 1 khz. Sketch the resultant composite waveform on the attached graph paper. The average value of this composite waveform is 4 volts and should remain that value for any amplitude sinusoid. This characteristic should have been observed as the amplitude of the signal generator was being increased to 2 volts. It is time to examine AC coupling of the CH1 input. From Figure 11, note that when AC coupling is selected, a capacitor is placed in the path between the input terminal to the amplifier/attenuator circuits of the scope (the switch will connected to point 2). The purpose of the capacitor is to block the DC component while allowing the AC component to pass. In essence, the average value of the waveform will be reduced to zero, leaving only the time varying portion of the waveform. This capability allows accurate measurement of the AC component of a signal when it is dwarfed in magnitude by the DC component. 20

21 DC Blocking Capacitor 2 Selector Switch Channel 1 Input 3 1 Channel 1 Amplif ier 4 Figure 11 Input circuitry between the probe and channel 1 amplifier 0 Change the input coupling from DC to AC and sketch the new waveform on the graph paper, use the same coordinates as you used for your last sketch. Note that the waveform dropped by a level equal to the amount of the average value of the waveform. The ability to have both AC and DC coupling allows for an easy method to measure the average of a waveform. Turn off the function generator and DC supply and remove them from the circuit. IX. EXTERNAL HORIZONTAL INPUT (X-Y OPERATION) (THIS SECTION IS OPTIONAL) Thus far in this experiment we have been using the internal sweep system as our only means of generating a horizontal deflection voltage. To help in understanding the horizontal deflection system we will now perform an experiment employing an externally applied horizontal voltage, using the DC supply. The function generator will still be used to supply the vertical input voltage. First, establish the zero volt reference trace at the center of the graticule. Then connect the function generator with its output set to 4 V pp, 1 khz. Set the scope to read this 4 V pp, 1 khz signal. Once the signal is displayed on the scope, slow the sweep rate to 100 ms/div. Notice how the display on the scope changed from a couple of cycles of sinusoid to a bar across the screen 4 volts high. Reduce the signal generator frequency to 1 Hz. To maintain a stable waveform, switch the trigger settings from automatic to normal (see part V for instructions if needed). Adjust the trigger level until the trace starts at zero volts. You should see a 4 V pp, 1 Hz sine wave on the screen. Next, change the scope from Y-T mode to X-Y mode (push the Main/Delayed button and then push the button next to the Time Base block). This mode bypasses the internal sweep circuit and connects the horizontal deflection plates to the CH1 input terminal. With nothing applied to the CH1 input, the display on the scope should resemble a 4 volt high vertical line in the center of the screen. Use the horizontal position control to move this line to the left side of the screen. 21

22 Connect the DC power supply to the CH1 input of the scope. Set the CH1 scale factor to 500 mv/div. Turn the voltage control knob all the way down and then energize the power supply. Now, slowly increase the DC voltage on the power supply and watch what happens on the screen of the scope. Play with this knob, turning it back and forth. With a little practice, you should be able to generate a decent sine wave. The X input capability allows the scope to be used as an X-Y plotter. One example of this mode of operation is the observation of i-v terminal characteristics of electronic devices. Since the scope is a voltage sensitive instrument, the current axis values on the i-v plot are obtained by connecting the scope across a resistor R in series with the device under test. A 1/R scaling factor is then used to obtain the i axis values. This example is mentioned only to illustrate a point, and is not a performance part of this experiment. Turn off the function generator and the DC supply and disconnect the scope probes. To prepare for the next experiment, set the sweep rate of the scope to 20 μs/div. X. AN ADDITIONAL FEATURE OF THE OSCILLOSCOPE A final feature worth mentioning is the auto-scale feature. Pushing this button will automatically set the volts/div, s/div, and triggering for most of the waveforms you will need to measure. After you use this button, you can use the Measure menus to make the scope show the measurements on the screen, rather than reading them from the graticule. XI. CONCLUSION Now you should be able to make a variety of AC, DC, and combination of AC and DC measurements. Save your notes from this section as the oscilloscope will an invaluable tool in subsequent experiments. 22