Download Characteristics of Signals

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