# 14 - 14: ALIASING I. PRELAB FOR ALIASING LAB You might...

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P108 Lab 14: Page 1 14: A LIASING I. PRELAB FOR ALIASING LAB You might expect that to record a frequency of 4000 Hz you would have to sample at a rate of at least 4000 Hz. It turns out, however, that you actually have to sample at TWICE that rate (the Nyquist theorem) This is because you need to record a peak and a valley from each cycle. If you sampled at the same rate that the signal was oscillating, you would get a peak every time you sampled, and the output would be a constant voltage. When you sample at a rate that is less than twice the signal frequency, the output frequency is lower than it should be. This phenomenon, called “aliasing” or “foldover”, is best studied by trying it. Fill in the following worksheet-style pages. A colored pen or pencil works best. The procedure is: 1. Start at the left edge of the paper; at this point the signal is at its maximum value. 2. Draw a straight line to the right for the number of squares indicated in the left hand margin. 3. Drop straight down until your pen is BELOW OR EQUAL TO the signal. 4. Go right again for the appropriate number of squares. Each time you get to sample, move your pen to the highest corner which is below or equal to the signal. 5. Continue. ALWAYS STAY ON THE GRID. Never cut across squares. Trace the example to get a feel for it. Notice how the quality of reproduction decreases as the sampling rate decreases. In the last examples on the first workpage (page 2) the sampling frequency is more than half the signal frequency. Observe the consequences. This is aliasing. On the pages 3 and 4, the frequency of the signal stays the same , but we keep changing the sampling period. What is the apparent period (in divisions) of each signal after it is sampled?

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P108 Lab 14: Page 2
P108 Lab 14: Page 3

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P108 Lab 14: Page 4
P108 Lab 14: Page 5 II. ALIASING ( FOLDOVER ) LAB A. Set Up Your ADC and DAC should be intact from last lab. Attach the scope and function generator. Make sure that everything still works. Set up the scope so that you can look at the original and final signals simultaneously.

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## This note was uploaded on 01/20/2012 for the course PHYSICS 108 taught by Professor Kesmodel during the Fall '08 term at Indiana.

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14 - 14: ALIASING I. PRELAB FOR ALIASING LAB You might...

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