Massachusetts Institute of Technology Department of Electrical Engineering and Computer Science 6 002 Circuits and Electronics Spring 2007 Lab Equipment Handout Handout S07 08 Prepared by Iahn Cajigas Gonz lez EECS 02 Updated by Ben Walker EECS 03 in September 2003 This handout is intended to provide a brief technical overview of the lab instruments which we will be using in 6 002 the oscilloscope multimeter function generator and the protoboard It incorporates much of the material found in the individual instrument manuals while including some background information as to how each of the instruments work The goal of this handout is to serve as a reference of common lab procedures and terminology while trying to build technical intuition about each instrument s functionality and familiarizing students with their use Students with previous lab experience might find it helpful to simply skim over the handout and focus only on unfamiliar sections and terminology THE OSCILLOSCOPE The oscilloscope is an electronic instrument based on the cathode ray tube CRT not unlike the picture tube of a television set which is capable of generating a graph of an input signal versus a second variable In most applications the vertical Y axis represents voltage and the horizontal X axis represents time although other configurations are possible Essentially the oscilloscope consists of four main parts an electron gun a time base generator that serves as a clock two sets of deflection plates used to steer the electron beam and a phosphorescent screen which lights up when struck by electrons The electron gun deflection plates and the phosphorescent screen are all enclosed by a glass envelope which has been sealed and evacuated The visible part of the CRT tube the screen is the outside of the glass wall on whose inner surface lies the phosphorescent film The screen is inscribed with a set of axes enclosed by a grid As 1 Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY the oscilloscope operates the electron beam traces a graph of the input voltage versus time on the phosphorescent film behind these axes The horizontal axis is the time axis and the vertical axis is the voltage axis PRINCIPLES OF OPERATION Courtesy of Iahn Cajigas Gonz lez Used with permission Figure 1 The internal parts of an oscilloscope The main component of an oscilloscope is an electron gun which ejects a constant beam of electrons into the surrounding vacuum see Figure 1 As the beam emerges from the gun it passes through a set of parallel plates the Vertical deflection plates oriented horizontally The voltage to be displayed is amplified and applied across these plates producing an electric field which deflects the path of the electrons vertically The polarity of the signal of interest determines whether the deflections will be up or down and the magnitude of the signal determines the amount of vertical displacement of the electrons Courtesy of Iahn Cajigas Gonz lez Used with permission Figure 2 Deflection of an electron in a uniform electric field After the beam has passed through the Vertical deflection plates it passes through a second set of similar plates that are oriented vertically A potential difference applied to these plates produces an electric field which deflects the electrons left or right Under most configurations one common exception is XY mode to be discussed later these Horizontal deflection plates make the 2 Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY beam move sideways at a constant speed By adjusting this speed with the Sweep knob the resulting trace on the screen can be spread out or compressed If the two deflecting voltages were held constant the electron beam would strike a fixed point on the phosphorescent film and a stationary point would be visible on the screen However most voltages of interest are time varying and so the voltage applied to the Horizontal deflection plates is varied with time in such a way that the spot moves from left to right on the screen as time passes Since the phosphorescent material has the property of emitting light for several milliseconds after the electrons have passed the total effect is for the electrons to leave behind a visible trail a timevarying waveform The horizontal deflection voltage or sweep voltage is also varied in such a way that when the beam reaches the right hand edge of the screen it starts over at the left hand side If the signal to be displayed varies periodically in time it is possible to synchronize the sweep voltage with the signal so that the curve appears motionless on the screen This is done with the Trigger Level control which sets the oscilloscope to begin a trace when the voltage it measures reaches a certain value The or button lets you choose whether you want the oscilloscope to trigger on a positive or negative voltage A trace that is running across the screen can usually be stabilized by adjusting the trigger level as long as the waveform is periodic USING THE SCOPE Now that we have acquired a basic understanding of how the oscilloscope works we can proceed to explore some of the most common tasks which can be accomplished with it The oscilloscope which we will be using in 6 002 is the Tektronix 2445 a four channel input scope Since the oscilloscope has four channels we could simultaneously graph four distinct input voltages on the CRT although we won t get that complicated in 6 002 Aside from the display itself the oscilloscope consists of various control panels which give the user complete control of how the scope processes and displays its inputs In the section that follows we will describe each of the control panels and outline the functionality of each of the settings found therein THE DISPLAY 3 Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY The oscilloscope s display can be adjusted in various ways in order to make the displayed signals brighter and more defined on the screen Under the display screen you will find a series of controls which will allow you to adjust the
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