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TO CONVEY DATA, SOME ASPECT OF A SIGNAL MUST BE VARIED. THERE ARE several different characteristics of a signal that can be made to change in a controlled way, so that data is imprinted on it. Modulation is the process of imprinting data onto an electric current or radio wave. Modulation can be accomplished by varying the amplitude, the frequency, or the phase of a wave. Another method is to transmit a series of pulses, whose duration, amplitude, or spacing is made to change in accordance with the data to be conveyed.
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The carrier wave
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The heart of most communications signals is a sine wave, usually of a frequency well above the range of human hearing. This is called a carrier or carrier wave. The lowest carrier frequency used for radio communications is 9 kHz. The highest frequency is in the hundreds of gigahertz. For modulation to work effectively, the carrier must have a frequency many times the highest frequency of the modulating signal. For example, if you want to modulate a radio wave with hi-fi music, which has a frequency range from a few hertz up to 20 kHz or so, the carrier wave must have a frequency well above 20 kHz. A good rule is that the carrier must have a frequency of at least 10 times the highest modulating frequency. So for good hi-fi music transmission, a radio carrier should be at 200 kHz or higher. This rule holds for all kinds of modulation, whether it be of the amplitude, phase, or frequency. If the rule is violated, the efficiency of transmission will be degraded, resulting in less-than-optimum data transfer.
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The Morse code
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The simplest, and oldest, form of modulation is on-off keying. Early telegraph systems used direct currents that were keyed on and off, and were sent along wires. The first radio transmitters employed spark-generated hash signals that were keyed using the telegraph code. The noise from the sparks, like ignition noise from a car, could be heard in crystal-set receivers several miles away. At the time of their invention, this phenomenon was deemed miraculous: a wireless telegraph! Keying is usually accomplished at the oscillator of a continuous-wave (CW) radio transmitter. A block diagram of a simple CW transmitter is shown in Fig. 26-1. This is the basis for a mode of communications that is, and always has been, popular among radio amateurs and experimenters.
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26-1 A simple CW transmitter.
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While the use of Morse code might seem old-fashioned, even archaic, a CW transmitter is extremely simple to build. A human operator, listening to Morse code and writing down the characters as they are sent, is one of the most efficient data receivers ever devised. Until computers are built to have intuition, there ll always be a place for Morse code radio communications. Besides being efficient, as any CW fanatic radio ham will tell you, it s just plain fun to send and receive signals in Morse code. Morse code is a form of digital communications. It can be broken down into bits, each having a length of one dot. A dash is three bits long. The space between dots and dashes, within a single character, is one bit. The space between characters in a word is three bits. The space between words is seven bits. Punctuation marks are sent as characters attached to their respective words. An amplitude-versus-time rendition of the Morse word eat is shown in Fig. 26-2. Morse code is a rather slow way to send and receive data. Human operators typically use speeds ranging from about 5 words per minute (wpm) to 40 or 50 wpm.
Frequency-shift keying
Morse code keying is the most primitive form of amplitude modulation (AM). The strength, or amplitude, of the signal is varied between two extreme conditions: full-on and full-off. There is another way to achieve two-state keying that works better with
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