6.3 ANALOG SIGNALS

6.3.1 Analog Signals

Figure 6-1 Analog wave of a typical voice signal.


6.3.2 Signal Frequency

   Frequency (for sound waves) is the number of vibrations per second that cause the particular sound. This pure, simple signal is commonly diagrammed as a sine wave ,as shown in figure 6-2. Each complete wave is called a cycle ,and the frequency of the signal is the number of cycles that occur in one second.The unit of measure for frequency is the Hertz(HZ) .

Figure 6-2 Sine waves of differing frequencies.

Figure 6-3 The frequency ranges of some common sounds.

    When referring to very high frequencies,we commonly use the designations kilohertz(KHZ) for thousands of Hertz, megahertz(MHZ) for millions of Hertz,and gigahertz (GHZ) for billions of Hertz,to more easily describe the frequencies.

Figure 6-4 The frequency spectrum showing the common names applied to certain frequency ranges.

Figure 6-5 A more detailed view of the frequency spectrum relevant to telecommunications.

Figure 6-6 Common abbreviations for very large and very small quantities.

6.3.3 Bandwidth
    The difference between the upper and lower frequency is called the bandwidth. For example: The FM radio band in the United States extends from 88 MHz to 108 MHz. Its bandwidth is 20MHZ . The guard channel or guard band provides a buffer area so that telephone conversations or data signals on adjacent circuits don't interfere with each other.

Figure 6-7 Bandwidth of a voice channel.

6.3.4 Signal Amplitude
    Another characteristic of analog signals is their loudness , or amplitude . The amplitude of the signal is also called its level .

Figure 6-8 Analog wave with constant frequency and varying amplitude.

6.3.4 Signal Amplitude
    Analog signal level is measured in decibels (dB), which is a logarithmic ratio of signal input and output power . Because the dB is a logarithmic measure, doubling the strength of the signal increases its level by 3 dB. The mathematical formula for the relationship between power and signal strength is power out dB = 10 log 10 ( ) power in

Figure 6-9 The relative power of a signal measured in decibels.

6.3.4 Signal Amplitude
   Crosstalk is interference that occurs when the signals from one communications channel interfere with those on another channel. The loss of signal strength between two points on a communication circuit is called attenuation .

Figure 6-10 A signal loses strength as the distance it travels increases. This loss of strength is called attenuation.


6.3.5 Signal Phase
    A signal's phase is the relative position of the sine wave measured in degrees.

Figure 6-11 Example of a phase shift.

The three attributes of an analog signal discussed in the chapter are frequency, amplitude, and phase .

 

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