E1C02 09/14/2010
13:35:15 Page 41
Chapter 2
Static and Dynamic Characteristics
of Signals
2.1 INTRODUCTION
A measurement system takes an input quantity and transforms it into an output quantity that can be
observed or recorded, such as the movement of a pointer on a dial or the magnitude of a digital
display. This chapter discusses the characteristics of both the input signals to a measurement system
and the resulting output signals. The shape and form of a signal are often referred to as its waveform.
The waveform contains information about the magnitude and amplitude, which indicate the size of
the input quantity, and the frequency, which indicates the way the signal changes in time. An
understanding of waveforms is required for the selection of measurement systems and the
interpretation of measured signals.
2.2 INPUT/OUTPUT SIGNAL CONCEPTS
Two important tasks that engineers face in the measurement of physical variables are (1) selecting a
measurement system and (2) interpreting the output from a measurement system. A simple example of
selecting a measurement system might be the selection of a tire gauge for measuring the air pressure in
a bicycle tire or in a car tire, as shown in Figure 2.1. The gauge for the car tire would be required to
indicate pressures up to 275 kPa (40 lb/in.
2
), but the bicycle tire gauge would be required to indicate
higher pressures, maybe up to 700 kPa (100 lb/in.
2
). This idea of the range of an instrument, its lower to
upper measurement limits, is fundamental to all measurement systems and demonstrates that some
basic understanding of the nature of the input signal, in this case the magnitude, is necessary in
evaluating or selecting a measurement system for a particular application.
A much more difficult task is the evaluation of the output of a measurement system when the
time or spatial behavior of the input is not known. The pressure in a tire does not change while we are
trying to measure it, but what if we wanted to measure pressure in a cylinder in an automobile
engine? Would the tire gauge or another gauge based on its operating principle work? We know that
the pressure in the cylinder varies with time. If our task was to select a measurement system to
determine this time-varying pressure, information about the pressure variations in the cylinder
would be necessary. From thermodynamics and the speed range of the engine, it may be possible to
estimate the magnitude of pressures to be expected and the rate with which they change. From that
41
13:35:15 Page 41
Chapter 2
Static and Dynamic Characteristics
of Signals
2.1 INTRODUCTION
A measurement system takes an input quantity and transforms it into an output quantity that can be
observed or recorded, such as the movement of a pointer on a dial or the magnitude of a digital
display. This chapter discusses the characteristics of both the input signals to a measurement system
and the resulting output signals. The shape and form of a signal are often referred to as its waveform.
The waveform contains information about the magnitude and amplitude, which indicate the size of
the input quantity, and the frequency, which indicates the way the signal changes in time. An
understanding of waveforms is required for the selection of measurement systems and the
interpretation of measured signals.
2.2 INPUT/OUTPUT SIGNAL CONCEPTS
Two important tasks that engineers face in the measurement of physical variables are (1) selecting a
measurement system and (2) interpreting the output from a measurement system. A simple example of
selecting a measurement system might be the selection of a tire gauge for measuring the air pressure in
a bicycle tire or in a car tire, as shown in Figure 2.1. The gauge for the car tire would be required to
indicate pressures up to 275 kPa (40 lb/in.
2
), but the bicycle tire gauge would be required to indicate
higher pressures, maybe up to 700 kPa (100 lb/in.
2
). This idea of the range of an instrument, its lower to
upper measurement limits, is fundamental to all measurement systems and demonstrates that some
basic understanding of the nature of the input signal, in this case the magnitude, is necessary in
evaluating or selecting a measurement system for a particular application.
A much more difficult task is the evaluation of the output of a measurement system when the
time or spatial behavior of the input is not known. The pressure in a tire does not change while we are
trying to measure it, but what if we wanted to measure pressure in a cylinder in an automobile
engine? Would the tire gauge or another gauge based on its operating principle work? We know that
the pressure in the cylinder varies with time. If our task was to select a measurement system to
determine this time-varying pressure, information about the pressure variations in the cylinder
would be necessary. From thermodynamics and the speed range of the engine, it may be possible to
estimate the magnitude of pressures to be expected and the rate with which they change. From that
41