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LAB 4: DATA ACQUISITION AND ANALYSIS Acquisition Rates When acquiring and analyzing data, it is important to remember that the acquisition rate of your system limits your system’s accuracy. For example, if you are trying to capture an event that only lasts 1/100th of a second, you cannot use a sampling rate of 10 Hz (once every 1/10th of a second) or you may “miss” the event. This is especially important if you are sampling a waveform (such as a sine wave) because sampling at an inadequate frequency can cause an effect called aliasing , in which the waveform you seem to be capturing does not have the same frequency as the waveform actually being measured. Nyquists’s Theorem In order to avoid aliasing, Nyquist’s theorem states that you must use a sampling frequency of at least twice the frequency of the signal you are trying to capture. In other words, if the signal being acquired has a frequency of 15 Hz, you must sample at a signal of at least 30 Hz to be able to discern the correct frequency of the wave. Realistically, sampling at only twice the input rate will only tell you the correct frequency. In order to attain the correct frequency, shape, and amplitude, a sampling rate of 10 to 15 times the input frequency must be used. Digital Storage Oscilloscopes The digital storage oscilloscope (DSO) is a tool that is widely used to capture time-domain information from instrumentation. The DSO screen can be adjusted both horizontally (time domain) and vertically (voltage domain) to capture waveforms of any frequency and amplitude. Our DSOs have cursors that can be used to measure and analyze incoming waveforms. Data Acquisition Systems While the DSO can only display voltage over time, a data acquisition (DAQ) system can be used to acquire almost any type of signal, process the information internally, and display “real” values in real time. The DAQ systems can be used to acquire and align many different types of signals at the same time, giving them a distinct advantage over the traditional DSO. The DAQ system can also be used as a supply voltage if necessary. In this laboratory, you will be using the National Instruments NI-cDAQ system combined with LabVIEW software to acquire and analyze data from multiple instruments.
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CGN 3710 LAB 3: DATA ACQUISITION AND ANALYSIS PRE-LAB 4 You want to measure a sine wave that has a period of 0.15 seconds. 1) What is the minimum sampling rate (in Hz) that you must use to obtain the correct frequency? 2) What is the minimum sampling rate (in Hz) that you must use to obtain a reasonable waveform? 3)
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