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Copyright 2006, 2002, 1997, 1993 by The McGraw-Hill Companies, Inc. Click here for terms of use.
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ter is 1 bit. The space between characters in a word is 3 bits. The space between words is 7 bits. The key-down (full-carrier) condition is called mark, and the key-up (no-signal) condition is called space. Morse code is slow. Human operators use speeds ranging from about 5 words per minute (wpm) to 40 or 50 wpm. A few human operators can work at 60 to 70 wpm. These people usually copy the signals in their heads.
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Frequency-Shift Keying Digital data can be sent over wireless links by means of frequency-shift keying (FSK). In some FSK systems, the carrier frequency is shifted between mark and space conditions, usually by a few hundred hertz or less. In other systems, a two-tone audio-frequency (AF) sine wave modulates the carrier. This is known as audio-frequency-shift keying (AFSK). The two most common codes used with FSK and AFSK are Baudot (pronounced baw-DOE ) and ASCII (pronounced ASK-ee ). The acronym ASCII stands for American Standard Code for Information Interchange. In radioteletype (RTTY) FSK and AFSK systems, a terminal unit (TU) converts the digital signals into electrical impulses that operate a teleprinter or display characters on a computer screen. The TU also generates the signals necessary to send RTTY as an operator types on a keyboard. A device that sends and receives AFSK is sometimes called a modem. This acronym stands for modulator/ demodulator. A modem is basically the same as a TU. Figure 25-1 is a block diagram of an AFSK transmitter. The main advantage of FSK or AFSK over on/off keying is the fact that there are fewer errors or misprints, because the space part of the signal is identified as such, rather than existing as a gap or pause in the data. A sudden noise burst in an on/off keyed signal can confuse a receiver into reading the space as a mark signal, but when the space is positively represented by its own signal, this is less likely to happen.
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25-1 Simplified block diagram of an AFSK transmitter.
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Amplitude Modulation An AF voice signal has frequencies mostly in the range between 300 Hz and 3 kHz. Some characteristic of an RF carrier can be varied, or modulated, by these waveforms, thereby transmitting voice information. Figure 25-2 shows a simple circuit for obtaining amplitude modulation (AM). This circuit can be imagined as an RF amplifier for the carrier, with the instantaneous gain dependent on the instantaneous audio input amplitude. Another way to think of this circuit is as a mixer that combines the RF carrier and audio signals to produce sum and difference signals at frequencies just above and below that of the carrier. The circuit shown in Fig. 25-2 works well, provided the AF input amplitude is not too great. If the AF input is excessive, then distortion occurs, intelligibility is degraded, system efficiency is reduced, and the bandwidth of the signal is increased unnecessarily. The extent of AM is expressed as a percentage, from 0 percent (an unmodulated carrier) to 100 percent (full modulation). Increasing the modulation past 100 percent causes the same problems as excessive AF input. In an AM signal modulated 100 percent, 1 3 of the power is used to convey the data, and the other 2 3 is consumed by the carrier wave. Figure 25-3 shows a spectral display of an AM voice radio signal. The horizontal scale is calibrated in increments of 1 kHz per division. Each vertical division represents 3 dB of change in signal strength. The maximum (reference) amplitude is 0 dB relative to 1 mW (abbreviated as 0 dBm). The data exists in sidebands above and below the carrier frequency. These sidebands resemble the sum and difference signals produced by a mixer. In this case the mixing occurs between the AF input signal and the RF carrier. The RF between 3 kHz and the carrier frequency constitutes the lower
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