Lecture 5.PPt.pdf

Chapter 3 Data and Signals 3.1 Note To be transmitted, data must be transformed to electromagnetic signals. 3.2  3-1 ANALOG AND DIGITAL Data can be analog or digital. The term analog data refers to information that is continuous; digital data refers to information that...

Chapter 3 Data and Signals 3.1 Note To be transmitted, data must be transformed to electromagnetic signals. 3.2  3-1 ANALOG AND DIGITAL Data can be analog or digital. The term analog data refers to information that is continuous; digital data refers to information that has discrete states. Analog data take on continuous values. Digital data take on discrete values. Topics discussed in this section: Analog and Digital Data Analog and Digital Signals Periodic and Nonperiodic Signals 3.3 Note Data can be analog or digital. Analog data are continuous and take continuous values. Digital data have discrete states and take discrete values. 3.4 Note Discrete data is the type of data that has clear spaces between values. Continuous data is data that falls in a constant sequence 3.5 Note discrete data is a finite value that can be counted whereas continuous data has an infinite number of possible values that can be measured; continuous data that will change over time; 3.6 Discrete vs. Continuous Data 3.7 Note Signals can be analog or digital. Analog signals can have an infinite number of values in a range; digital signals can have only a limited number of values. 3.8 Figure 3.1 Comparison of analog and digital signals 3.9 Note In data communications, we commonly use periodic analog signals and nonperiodic digital signals. 3.10  3-2 PERIODIC ANALOG SIGNALS Periodic analog signals can be classified as simple or composite. A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals. A composite periodic analog signal is composed of multiple sine waves. Topics discussed in this section: Sine Wave Wavelength Time and Frequency Domain Composite Signals Bandwidth 3.11 Figure 3.2 A sine wave 3.12  Example 3.1 The power in your house can be represented by a sine wave with a peak amplitude of 155 to 170 V. However, it is common knowledge that the voltage of the power in U.S. homes is 110 to 120 V. This discrepancy is due to the fact that these are root mean square (rms) values. The signal is squared and then the average amplitude is calculated. The peak value is equal to 2½ × rms value. 3.14  Figure 3.3 Two signals with the same phase and frequency, but different amplitudes Egypt U.S.A 3.15  Note 3.2 The voltage of a battery is a constant; this constant value can be considered a sine wave, as we will see later. For example, the peak value of an AA battery is normally a battery that is typically used in small electronic devices and that is standardized at 51 mm in length and roughly 14 mm in diameter. called also AA, AA cell, double-A, double-A battery 1.5 V. 3.16 Note: Time and Frequency Domain Frequency and period are the inverse of each other. 3.17 Figure 3.4 Two signals with the same amplitude and phase, but different frequencies 3.18  Table 3.1 Units of period and frequency ÷ × 3.19  Example 3.3 The power we use at home has a frequency of 60 Hz. The period of this sine wave can be determined as follows: 3.20  Example 3.4 Express a period of 100 ms in microseconds. Solution From Table 3.1 we find the equivalents of 1 ms (1 ms is 10−3 s) and 1 s (1 s is 10-6 μs). We make the following substitutions:. 3.21  Example 3.5 The period of a signal is 100 ms. What is its frequency in kilohertz? Solution First we change 100 ms to seconds, and then we calculate the frequency from the period (1 Hz = 10−3 kHz). 3.22 Note Frequency is the rate of change with respect to time. Change in a short span of time means high frequency. Change over a long span of time means low frequency. 3.23 Note If a signal does not change at all, its frequency is zero. If a signal changes instantaneously, its frequency is infinite. 3.24 Note Phase describes the position of the waveform relative to time 0. 3.25 Figure 3.5 Three sine waves with the same amplitude and frequency, but different phases 3.26