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 1 EE 442 Laboratory Experiment 1 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 (Image courtesy http://www.agilent.com) 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. 2 EE 442 Laboratory Experiment 1 Introduction to the Function Generator and the Oscilloscope 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. 3 EE 442 Laboratory Experiment 1 Introduction to the Function Generator and the Oscilloscope 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 4 EE 442 Laboratory Experiment 1 Introduction to the Function Generator and the Oscilloscope 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 (Image courtesy http://www.agilent.com) Figure 2 Agilent DSO3202A oscilloscope 5 EE 442 Laboratory Experiment 1 Introduction to the Function Generator and the 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. (Image courtesy http://www.hyperphysics.com) 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 EE 442 Laboratory Experiment 1 Introduction to the Function Generator and the Oscilloscope 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.