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Mechanical Engineering - 22.302 ME Lab I ME 22.302 Mechanical Lab I Characteristics of Signals 1 Volt Sine Series1 1.5 1 97 93 Normalized Squared Function 101 89 85 81 77 73 69 65 61 57 53 49 45 41 37 33 29 25 21 9 17 5 13 0 1 SIN(X) 0.5 0.07 -0.5 0.06 -1 0.05 -1.5 2*PI 0.04 0.03 0.02 0.01 University of Massachusetts Lowell 96 91 86 81 76 71 66 61 56 51 46 41 36 31 26 21 6 16 1 Dr. Peter Avitabile 11 0 Signal Types - 122601 - 1 Copyright © 2001 Mechanical Engineering - 22.302 ME Lab I There are various types of signals that need to be measured. These can be broken down into Direct Current or Non-Alternating Signals and Alternating Signals Low Frequency Signals High Frequency Signals Dr. Peter Avitabile University of Massachusetts Lowell Signal Types - 122601 - 2 Copyright © 2001 Mechanical Engineering - 22.302 ME Lab I A Direct Current or DC signal is a non-alternating signal. An Alternating Current or AC signal such as a sine wave is described by two characteristics - period or frequency - amplitude and phase Amplitude T = Period Dr. Peter Avitabile University of Massachusetts Lowell Signal Types - 122601 - 3 Copyright © 2001 Mechanical Engineering - 22.302 ME Lab I An AC signal MUST ALWAYS be described in terms of its frequency and amplitude in a correct fashion CORRECT -> 2.828 Volt peak-to-peak sine wave at 25 Hz CORRECT -> 1 Volt RMS sine wave at 25 Hz INCORRECT -> 1.414 Volt sine wave at 5 Hz Peak to Peak Peak T = Period Dr. Peter Avitabile University of Massachusetts Lowell Signal Types - 122601 - 4 Copyright © 2001 A signal may have both a DC component and an alternating component that must be measured AC Mechanical Engineering - 22.302 ME Lab I AC Coupling DC If only the alternating portion of the signal is of interest then the measurement can be made with what is referred to as If we are also interested in the DC portion of the signal as well then the measurement can be made with what is referred to as DC Coupling AC and DC Coupling is used extensively in all labs Dr. Peter Avitabile University of Massachusetts Lowell Signal Types - 122601 - 5 Copyright © 2001 Mechanical Engineering - 22.302 ME Lab I An oscilloscope is used to measure DC signals as well as very high frequencies Oscilloscope A multimeter is used to measure DC signals as well as some relatively high frequencies depending on the model available A function generator can be used to generate a variety of different signals from sinusoids to rectangular pulses. Dr. Peter Avitabile University of Massachusetts Lowell Multimeter Function Generator Signal Types - 122601 - 6 Copyright © 2001 Mechanical Engineering - 22.302 ME Lab I Oscilloscope - used to observe signals Intensity Focus & Power Screen Dr. Peter Avitabile University of Massachusetts Lowell Horizontal Control (Time) Vertical Control (Amplitude) AC/DC Coupling Signal Types - 122601 - 7 Triggering Copyright © 2001 Mechanical Engineering - 22.302 ME Lab I Multimeter - used to measure voltage or current Range Settings AC/DC Coupling Dr. Peter Avitabile University of Massachusetts Lowell Signal Types - 122601 - 8 Copyright © 2001 Mechanical Engineering - 22.302 ME Lab I Function generator - used to create a signal Signal Type Range Settings Output Variable Control Dr. Peter Avitabile University of Massachusetts Lowell Signal Types - 122601 - 9 Copyright © 2001 Mechanical Engineering - 22.302 ME Lab I All measuring instruments have a useful frequency range where the measurement made is not distorted by the measuring instrument. However, when measurements are made beyond this useful frequency range then generally there is an attenuation of the signal obtained from the measuring instrument. This useful frequency range is often defined in terms of the 3 dB down point of the measured signal and called the cut-off frequency. FILTER ROLLOFF G dB Vout = 20 log10 G = 20 log10 Vin fc The cut-off frequency is usually specified at the 3 dB down point (which is where the filter attenuates 3 dB of signal). Dr. Peter Avitabile University of Massachusetts Lowell Signal Types - 122601 - 10 Copyright © 2001