EE 442 Laboratory Experiment 1

Introduction to the Function Generator and the Oscilloscope

EE 442

Lab Experiment No. 1

1/12/2007

Introduction to the Function Generator and the Oscilloscope

I.INTRODUCTION

The purpose of this lab is to learn the basic operation of a function generator and an oscilloscope.

II.THE AGILENT 33220A FUNCTION GENERATOR

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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 33220 (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.

PRELAB QUESTIONS:

  1. What is the Agilent 33220 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 1.0362850 kHz.
  3. Use the arrow buttons and the knob to set a frequency of 23.56 Hz.

Perhaps the easiest way to set a particular frequency (e.g., 1.234,567,8 kHz) is to punch it in directly.

EXPERIMENT:

  1. Press the ten buttons 1 . 2 3 4 5 6 7 8 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 8.7654321 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 1.234 V pp using the knob and the arrow buttons.
  3. Change the voltage to read an equivalent value in Volts rms. (Answer = 436.4 mV rms.)
  4. Change the voltage in step 3 to 4.364 V pp by pressing the right arrow button to cause the mV pp to be highlighted. Then rotate the knob clockwise. Note the relationship between the units and the decimal point.
  5. Set the voltage to 3.723 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

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Figure 2 Agilent DSO3202A oscilloscope

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.

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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).

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 orexternal 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).

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Figure 4 Horizontal system controls

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.

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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.

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 Figure6, 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.

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?

Figure 7 Input circuitry between the probe and channel 1 amplifier

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.
  1. 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).
  1. 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.
  1. 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.
  1. 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 4.
  1. 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.)
  1. 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 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.
  1. 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.
  1. 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.