EXPERIMENT 4 The Oscilloscope Objectives 1) Explain the operation or effect of each control on a simple oscilloscope. 2) Display an unknown sinusoidal electrical signal on an oscilloscope and measure its amplitude and frequency. Introduction To measure an electrical voltage you would use a voltmeter. But what happens if the electrical voltage you want to measure is varying rapidly in time? The voltmeter display may oscillate rapidly preventing you making a good reading, or it may display some average of the time varying voltage. In this case, an oscilloscope can be used to observe, and measure, the entire time-varying voltage, or "signal". The oscilloscope places an image of the time-varying signal on the screen of a cathode ray tube (CRT) allowing us to observe the shape of the signal and measure the voltage at different times. If the signal is periodic (it repeats itself over and over) as is often the case, we can also measure the frequency, the rate of repeating, of the signal. What the Oscilloscope Does The oscilloscope plots voltage as a function of time. Figure 1There are two types of voltages AC and DC. AC (derived from ALTERNATING CURRENT) indicates a voltage, the magnitude of which varies as a function of time. An AC signal is shown in Figure 1. In contrast, DC (derived from DIRECT CURRENT) indicates a voltage whose magnitude is constant in time. The voltage is on the vertical (y) axis and the time is on the horizontal (x) axis. A constant voltage (DC) shows up as a flat horizontal line. The scope has controls to make the x and y scales larger or smaller. These act like the controls for magnification on a microscope. They don’t change the actual voltage any more than magnification makes a cell on the microscope slide bigger; they just let us see small details more easily. There are also controls to shift the center points of the voltage scales. These “offset” knobs are like the controls to move the stage of the microscope to look at different parts of a sample. You will learn about other adjustments in the course of the lab. The Oscilloscope Experiment In this experiment you will familiarize yourself with the use of an oscilloscope. Using a signal generator you will produce various time varying voltages (signals) which you will input into the oscilloscope for analysis. There are two main quantities which can be measured with the aid of an oscilloscope that characterize any periodic AC signal. The first is the peak-to-peak voltage (Vpp), which is defined as the voltage difference between the time-varying signal’s highest and lowest voltage (see the square wave shown in Figure 2). The second is the frequency of the time-varying signal (f), defined by f =1T f is the frequency in Hz and T is the period in seconds (the period is also shown in Figure 2). Sometimes the angular frequency ω in rad/sec is used instead of the frequency f in Hz. They are related by: fπω2= The form of an AC signal is: DCVftV+)2sin(0π where, VDC is a constant DC offset, that shifts the sine wave up or down. The form of an AC signal with no DC offset is: )2sin(0ftVπ The amplitude, V0, is related to the peak-to-peak voltage Vpp of the signal by: Vpp = 2V0Figure 2 How the Oscilloscope Works An oscilloscope contains a cathode ray tube (CRT), in which the deflection of an electron beam that falls onto a phosphor screen is directly proportional to the voltage applied across a pair of parallel deflection plates. A measurement of this deflection yields a measurement of the applied voltage. The oscilloscope can be used to display and measure rapidly varying electrical phenomena. The internal subsystems of the oscilloscope are shown in Figure 3a and the front panel of the oscilloscope is shown in Figure 3b. To see the front panel of the oscilloscope in more detail, open the pdf file for this lab on the course web page and use Adobe’s magnify option. Figure 3aFigure 3b A vertical amplifier is connected to the y-axis deflection plates. It serves to amplify the input signal to the y-plates so that the CRT can show an appreciable vertical displacement for a small signal. The horizontal amplifier serves the same purpose for the x-axis plates and the horizontal display. Although an external input signal can be applied to the x-axis input, this function of the oscilloscope is not used in this course. Instead, a sweep generator internal to the oscilloscope is used to control the horizontal display. The sweep generator makes a beam move in the x-direction at a constant, but adjustable speed. The beam’s speed is adjusted using the time base (TB) control knob. This allows the oscilloscope to display the external y-input signal as a function of time. The sweep generator functions as follows. A saw-tooth voltage is applied to the horizontal deflection plates. A saw-tooth voltage is a time-varying periodic voltage and is shown in figure 4a. The voltage first increases linearly with time and then abruptly drops to zero. As the voltage increases the beam is deflected more and more to the right of the CRT screen. When the voltage reaches its maximum value, the beam trace will be at the far right hand side of the screen (x = 10 cm). The voltage then abruptly retraces back to zero – during this phase the signal is not displayed on the scope. The result is the beam spot sweeps across the screen with the same frequency as the saw-tooth signal. The horizontal position of the beam spot is shown in figure 4b. Note: the time it takes the beam spot to move across the screen (sweep time) is equal to the period of the saw-tooth signal. The rate at which the beam spot sweeps across the screen is selected by using the time base (TB) selector knob and is calibrated in time/cm. Because both the phosphor screen and the human eye have some finite retention time, the beam spot looks like a continuous line at frequencies higher than about 15 Hz.The Signal Generator To investigate how the oscilloscope works in this first experiment, we will need to give it a test input signal. To accomplish this, we will be using a signal generator like the one pictured below. Figure 5: The Signal Generator It is important to understand the function of all of the dials and switches on the signal generator. • The digital read out (upper left) displays the frequency that the signal generator is currently set to. This readout is in Hertz (Hz). •