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Introduction to the Oscilloscope, Function Generator, Digital

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Introduction to the Oscilloscope, Function Generator, Digital Physics 401 Experiment 1 Physics Department, UIUC Physics 401 Classical Physics Laboratory Experiment 1 Introduction to the Oscilloscope, the Signal Generator, the Digital Multimeter Table of Contents Subject Page I. Introduction----------------------------------------------------------------------------------------- 2 II. The Oscilloscope----------------------------------------------------------------------------------- 3 A. Inputs--------------------------------------------------------------------------------------- 3 B. Horizontal Time base--------------------------------------------------------------------- 4 C. Triggering---------------------------------------------------------------------------------- 5 III. Experiments----------------------------------------------------------------------------------------- 5 A. Calibration of AC Mode of DMM------------------------------------------------------ 6 B. Effect of DC Offset----------------------------------------------------------------------- 11 C. Input and Output Resistance------------------------------------------------------------ 13 IV. Appendix Oscilloscope, Wavetek, DMM specifications and front panels----------- 16 Rev. [Spring 2012] Page 1 Physics 401 Experiment 1 Physics Department, UIUC I. Introduction The aim of this first lab is to introduce the basic operations of several laboratory instruments by examining the response of various electrical circuit elements to steady (DC) and alternating (AC) applied voltages. These laboratory instruments are: the two-channel digital oscilloscope (Tektronix 3012B), the function generator (Wavetek 81) and the digital multimeter (Agilent 34401A). The first step is to become familiar with the operation of a very versatile source of DC and AC voltages – the function generator. Later on in the course the function generator and oscilloscope will be used to examine the behavior of various electrical components, and the characteristics of AC and DC circuits. As a review some relationships are summarized below for a pure sine wave and a complex waveform. (1) For a pure sine wave, Vt()= Vo sin( 2π tT) , with amplitude in volts Vo (2Vo is the peak-to- peak amplitude in volts), and period in seconds T : Quantity Formula Unit 1 frequency f = Hertz (Hz) T 2π angular frequency ω ==2π f radians per second T (rad/s) root mean square voltage 1 T 2 Volts (V) VVtdt==⎡⎤() 0.707 V rms T ∫0 ⎣⎦ o 1 T full-wave average VV= ()tdt= 0 Volts (V) full− wave average ∫0 voltage T (2) For a complex periodic waveform with period T = 2π ωo , and with Fourier expansion ∞∞ Vt()=+ aon∑∑ acos () nωω o t+ b nsin () no t, nn==11 where n is an integer, ao , an and bn are constants with units of volts, the root mean square voltage is ∞∞ 11T 2 2221 VVtd==++⎡⎤() taab() () (). rms ∫0 ⎣⎦ o ∑∑n n T 22nn==11 Rev. [Spring 2012] Page 2 Physics 401 Experiment 1 Physics Department, UIUC Note that the integration is over one period. Here is a physical interpretation of the rms voltage. Let the voltage source, V(t), drive a current through a resistor, R. Also let a battery (a DC source) with voltage Vrms power a resistor of the same resistance, R. The power dissipated in the resistor is the same in the two circuits. Vrms R V(t) R Figure 1. Two equivalent circuits which illustrate the meaning of an rms voltage. II. The oscilloscope An oscilloscope is used to provide a visual display of a time dependent voltage, i.e. a graph of voltage versus time. The oscilloscope is arguably the most important laboratory instrument. There are basically two different kinds of oscilloscopes: analog and digital. In this laboratory we will use the Tektronix model TDS3012B digital oscilloscope. The basic specifications of the Tektronix 3012B dual trace oscilloscope are summarized in part IV (Appendix) along with other instruments. A. Inputs The primary function of the oscilloscope is discussed below. This oscilloscope has the ability to display two different waveforms simultaneously, for example, the input and the output of an amplifier. The signals are fed into the oscilloscope inputs, Channel 1 and Channel 2, through BNC connectors. The chassis of the oscilloscope represents ground potential. The two Rev. [Spring 2012] Page 3 Physics 401 Experiment 1 Physics Department, UIUC signals may be amplified differently using the Volts/Div knobs to achieve the desired display. The two input signals may also be summed or subtracted (±). A digital oscilloscope measures a waveform by taking a sample of the waveform during a period called sampling time. By displaying the samples after signal processing, the accurate waveform can be viewed on the screen. The digital oscilloscope also stores the samples in its memory. This feature enables us to save the waveform and measure other characteristics of the waveform later. See Appendix for how to access various functions of the oscilloscope. Note that the basic voltage measurements and time measurements are displayed numerically on the screen along with the graph. B. Horizontal Time Base Both input waveforms are always displayed with the same time base, which of course may be adjusted over a wide range by the Time/Div knob. The horizontal sweep rate depends upon an internal oscillator (actually, a ramp generator of variable frequency) and the internal horizontal deflection amplifiers. The time base calibration may be checked by connecting a BNC cable with leads or an oscilloscope probe from the 1.0 kHz output (PROBE COMP ≈ 5V) on the front of the oscilloscope to an input; this output provides a stable square wave (5 Volt) with 1.00 ms period. The BNC connectors have cylindrical couplings on the end which twist and lock into position on the oscilloscope. Note that the coaxial cables have inner and an outer conductor. The outer conductor of the cable is grounded to the oscilloscope chassis. For most experiments the DELAY should be turned off. Pressing AUTOSET will set the controls for a default usable display. It is a time saver when you lost your display or you are not getting a stable display. A useful feature in comparing the phase of two waveforms is XY mode (DISPLAY > XY Display > Triggered XY). When set at the XY mode, the internal time base is disconnected. C. Triggering In order for waveforms displayed on the scope to appear stationary, it is necessary to synchronize the horizontal sweep with the input wave. This may be accomplished by setting the trigger source to Channel 1 (if Channel 1 is being used). Set the Trigger Menu >> Normal & Rev. [Spring 2012] Page 4 Physics 401 Experiment 1 Physics Department, UIUC Source >> Ch 1. The stability of the triggering is controlled by the Trigger Level knob. Adjusting this knob will change the trigger voltage required to start the sweep and therefore affects the phase of the waveform you see: Figure 2. Sinusoidal wave form displayed at various trigger levels If you move the trigger level greater than the peak or trough level, you will see the running waveform. It is similar to setting Auto mode in an analog scope. Digital scopes has special control to capture the signal if the signal is lost or unstable. When you press the AUTOSET button, the scope sets vertical, horizontal and trigger controls automatically. You can then manually adjust any of these controls if you need to optimize the display. III. Experiments By carrying out the exercises below you will become familiar with the Wavetek Model 81 Function Generator (Wavetek for short), the Hewlett Packard 34401A Digital multimeter (DMM), and the Tektronix TDS3012B Digital Oscilloscope (Scope). The DMM we use in the lab is a true RMS (root mean square) responding meter, which means it calculates the square root of the time-averaged voltage squared (see the expressions on pages 2 and 3). This is slightly different from an average responding meter, which is calibrated to read the same as a true RMS meter for sine wave only. For other waveforms, an average responding meter will exhibit substantial differences from an RMS meter. The AC V mode of the DMM measures the AC-coupled true RMS value. This mode is useful in situations where small AC signals are present in large DC offsets. An example of this situation is measuring AC ripple voltage present on DC power supplies. Rev. [Spring 2012] Page 5 Physics 401 Experiment 1 Physics Department, UIUC A. Calibration of AC Mode of the DMM To check the calibration of the AC V mode of the DMM the Scope can be used as a standard. To get started a detailed description of the set-up of all three instruments is provided. It would be a good idea to check off each step described below as you carry it out. Make certain all three instruments are plugged in, then turn them on. 1. Scope Turn on the oscilloscope. If it is already on, we suggest you turn it off and then back on again. The Scope goes through a set-up procedure when turned on. Wait until all tests have passed. The status report of the set-up procedure will stays on for a few seconds before it disappears itself. To remove it prematurely, you may press the MENU OFF key located at the right side of the screen display. The scope can be generally divided into 2 areas: display and the front panel control. Note the locations of the grey keys at the bottom and the right side of the display. They are also called screen buttons. The grey keys allow you to choose the soft key selections on the display. If a pop-up menu appears, continue pushing the key to select an item form the menu. Note also the knobs and buttons on the Scope front panel. Knobs are used to change the screen displays. The dark buttons bring up soft key menus on the display that allow you to access other features using the grey keys at the screen area. The regular buttons are instant action keys and changes generally appear on the screen instantly. There are 4 colored buttons which are self explanatory and are associated with the Vertical OFF button. Specifications and other documentations of the scope may be found in the Appendix. a) Turn the WAVEFORM INTENSITY) to viewable intensity level.
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