ELEC105L Fundamentals of Electrical Engineering Spring 2010 Lab 7 Waveforms and Oscilloscopes and Captures Oh My Why is this important So far this semester you have been learning how to use the DC power supply to produce constant voltages and the multi meter to measure voltages currents and resistances Some of the lab exercises coming up this semester will use voltages and currents that vary with time To prepare you for these experiments you will learn about two pieces of equipment the function generator and the oscilloscope that can be used to generate and measure time varying voltages Grading This lab is a familiarization exercise and therefore will be weighted 50 points To receive full credit turn in the data table described in Step 12 to your instructor as well as the two screen image files captured in Steps 21 and 22 All items are due at the end of the lab session Only one submission per lab group is required however each member of the group must contribute to its production Introduction Each bench in our lab is equipped with a function generator Agilent 33120A capable of producing a variety of periodic voltage waveforms Along with the standard waveforms sine square ramp pulse and noise this instrument produces other interesting signals such as a simulated cardiac waveform and exponentially decaying or rising voltages We will use this instrument to provide input signals for circuits such as amplifiers and filters that will be studied later in the semester Each bench is also equipped with an oscilloscope Agilent DSO7034A that is used to measure and display voltages as a function of time Multiple voltage waveforms can be displayed simultaneously on the oscilloscope s screen The oscilloscope can measure many different parameters of the waveform such as peak to peak voltage maximum and minimum voltage period and frequency of the waveform and average voltage Procedure 1 The Agilent 33120A function generator can be modeled as an ideal voltage source in series with a 50 internal resistance as shown in the circuit on the left in Figure 1 The circuit on the right shows a circuit model of the function generator with a load attached If the box labeled Load were replaced with a resistor R you should be able to predict the resulting value of VL assuming you knew the value of R Write an equation for VL in terms of VOC R and the 50 resistance 50 Voc 50 Voc Load VL Figure 1 Model of function generator alone and with load 1 2 Now if VOC 2 V predict VL when the load resistor R is an open circuit infinite resistance 50 10 and 1 k and fill in the table below Load Resistance Expected VL 50 10 1 k 3 Let s find out if your predictions agree with what you measure Find two cables with a BNC connector on one end and two alligator clips on the other end BNC stands for Bayonet Neill Concelman Bayonet refers to the little nubs on the female connectors that help keep the male connectors in place and Neill and Concelman were the two engineers credited with developing the connector series To find out more about the BNC connector and to see a picture of one visit this Wikipedia article http en wikipedia org wiki BNC connector 4 Connect the BNC plug on one of the cables to the jack receptacle labeled Output on the function generator 5 Connect the BNC plug on the other cable to the Channel 1 input connector on the oscilloscope This connector is labeled 1 and colored yellow 6 Connect the red alligator clips to each other and the black alligator clips to each other and then turn on the function generator and the oscilloscope 7 The default waveform for the function generator is a sine wave with a frequency of 1 kHz and an amplitude of 100 mV peak to peak mVpp Find the button either a regular button or a soft button immediately below the display on the oscilloscope labeled Freq and press it to display the signal frequency Knowing that the period of a signal is the inverse of its frequency calculate the corresponding period of this signal 8 Press the button on the function generator labeled Ampl Change the amplitude of the signal to 2 Vpp using the large round knob The knob changes the value of the digit that is highlighted in the display The two buttons labeled and are used to select the digit that is changed by the knob 9 Find and press the white button labeled Autoscale on the oscilloscope 10 A sine wave showing measured voltage as a function of time should appear on your oscilloscope screen Along the top of the display you should see CH1 1 00V div or something similar which indicates the vertical scale for voltage You should also see 500 0 s div which indicates the horizontal scale for time Determine the period of the waveform by inspecting the display Is this what you expected Select some other waveforms on the function generator and see what they look like on the oscilloscope 11 The measurement you are taking corresponds to the circuit in Figure 1 where the load is now the oscilloscope The input Th venin equivalent resistance of the oscilloscope is very large about 1 M for the DSO7034A Determine the peak to peak voltage for the sine wave How does your measurement compare with what you derived in Step 2 Do not be alarmed if the values do not agree it should make sense later 2 12 Obtain a 51 resistor Connect both red alligator clips to one end of the resistor and both black clips to the other end You can use a breadboard for this part if you wish Estimate the peak to peak amplitude and the period of the voltage signal by examining the oscilloscope display Repeat your measurements with a 10 resistor and then a 1 k resistor Enter all of your measurements in the table below and transfer your predicted load voltage values from the table you filled in earlier Load Resistance Expected VL Measured VL Measured Period 50 10 1 k 13 The manufacturer of the function generator assumed that a 50 load a common load used in the communications field would be connected to the output jack The voltage that you see displayed on the function generator is the same voltage that you would measure across the attached load if the load resistance were 50 If the load connected to the function generator has an equivalent resistance of some value other than 50 the displayed value will not be correct Based on this information do your measurements above make sense 14 Become acquainted with some of the controls on the oscilloscope a Turn the largest knob with the yellow dot What is its effect b Turn the smaller knob with the yellow dot