MAE 334_09 Lab 2 Page 1 of 9 Laboratory 2 - Basics of A to D Conversion Objective To become more familiar with the process of data acquisition by analog to digital conversion and to explore the capabilities and limitations of A to D conversion. We will again use LabView’s Virtual Bench in this experiment – selecting the virtual bench oscilloscope for capturing data. Through out the semester, you will find the VirtualScope to be a very basic tool for monitoring instrumentation signals. Background Data acquisition by analog to digital conversion has become the most common way (by far) to record and process laboratory measurements. In this experiment we will use a function generator to produce time varying voltage. The voltage produced by the function generator simulates the voltages produced by transducers taking measurements during experiments or by the transducers that are part of modern machines. This experiment is divided into three parts – the first part will familiarize you with the process of performing data acquisition with the Virtual Scope. The second will consider “aliasing” which refers to the time domain limitations of the sampling process inherent in A to D conversion. Finally, in the third part, we will carefully consider the issue of “quantization”. This is related to precision limitations in the A to D conversion of voltage measurements. Figure 1. Setup Photo for Lab 2MAE 334_09 Lab 2 Page 2 of 9 Part 1 – Basic Operation of the A to D Process 1. From the desktop icon, click on the LabView Virtual Bench program. 2. When the Virtual Bench tool bar buttons appear, select the VirtualScope (the left most button). 3. When the VirtualScope window appears, resize the Scope window to properly position the graphic display and adjust the size or maximize as desired. Your display should look similar to Figure 2. Click on Edit and then General Settings to display the dialog window of Figure 3. Begin this experiment by setting the Virtual Bench Scope to have the lowest buffer size possible (500 pts, as shown). Doing this will cause any single run of the Virtual Bench Scope to plot and record 500 data points over the full scale time selected. 4. Set the oscilloscope time display and sampling speed by adjusting the time base (center knob to the right of the plot seen in Figure 2) to ten seconds full scale (one second per division). Look to the upper left just above the plot to verify your setting. It should show “1 Sec/div”. Over the full scale time of 10 seconds, the 500 samples correspond to a . Figure 2. The Standard Display for the Virtual Bench OscilloscopeMAE 334_09 Lab 2 Page 3 of 9 Figure 3. Virtual Bench Edit - General Settings dialog window. This is used to set the number of samples desired. The Device name may differ on some of the lab systems. sampling rate of 50 S/s (Figure 2). This indicates that the scope is sampling at 50 samples per second (i.e., the “sampling frequency” is 50 Hz). Time, of course, is displayed on the horizontal axis. 5. Next, set the oscilloscope voltage sensitivity (the vertical axis) by adjusting the knob on the upper right to obtain 2 volts per division (“2V” as shown in Figure 2). Since the full scale range of the ADC (analog to digital converter) is +/- 10 V, setting the volts/division to 2 will nominally display the full range of the ADC. 6. Verify that the other Scope settings are as pictured above in Figure 2. Only Ch 1 is enabled, the Measure is set to Ch 1, the Trigger Mode: Auto, Ch 1. 7. The function generator of Figure 4 can be used to produce waveforms of variable frequency and amplitude. Start, as in Figure 2, with a triangle wave, a signal frequency of 0.5 HZ and an amplitude of about +/-5 V (that is 10 volts or 5 vertical divisions from minimum peak to maximum peak). Check that the 0 dB/-30 dB push button is in, the FREQUENCY knob is at 5 and the RANGE Hz X.1 button is selected.MAE 334_09 Lab 2 Page 4 of 9 Figure 4. Simpson function generator 8. When you have a stable display, similar to Figure 2, become familiar with the overall arrangement. Change the triangle wave frequency. Adjust the voltage amplitude up and down. Try square waves and sine waves. Try a 5 volt/division scope setting. Adjust your amplitude to exceed +/- 10 volts. Activate the 0/-30 db push button. Use the scope’s zoom in/zoom out buttons on the lower right of the plot to enhance the horizontal (time) resolution of the plot. Try the “Run” and “Single” buttons. 9. Return to the nominal condition described by items 5, 6 and 7 above and indicated in Figure 2. We now want to save a single data record. While the scope is running, click the “Single” button, to store a single data record of the triangular waveform of 500 points. To save this exact data record to a file, click on File and then Save Waveforms on the drop down menu. You will obtain the dialog window shown in Figure 5. The User Name and Comments fields will appear at the beginning of the saved data file. The file created by the Virtual Scope is a text file readily compatible with Excel. After you click the OK button in the dialog window, you will be prompted for a data file name and storage location. You can store it in My Documents or c:\temp or on your USB flash disk. 10. Explore your saved file with Excel. You should see each of the 500 data points, the time at which each sample occurs and the voltage measured for each sample. Use Excel “x/y” scatter plotting to display a graph equivalent to your original Virtual Scope display. Obtain a “print screen” image of your Virtual Scope display to print in your report (this will be ReportFigure 1). Your report should also include a copy of your nicely scaled Excel plot (ReportFigure 2).MAE 334_09 Lab 2 Page 5 of 9 Figure 5. Select Waveforms window found under the File - Save Waveforms... pull down menu Part 2 – Aliasing In a complex periodic or a random signal, the “sampling theorem” states that a minimum of two points per cycle of the highest frequency component must be obtained in order to accurately reconstruct the signal’s frequency content. Of course, for a deterministic sinusoidal signal, more than