Electronic Communication
As information is transferred over increasingly large distances, the capabilities of a person's primary senses are insufficient for reliable, efficient transmission. The use of electrical signals becomes necessary, and electrical energy is used to transmit the information to be communicated. Electrical energy is relatively easy to generate, control, and has the desirable property of traveling at the speed of light. Thus, electronic communication offers a near instantaneous link between transmitter and receiver of the information, even when the two are separated by vast distances.
The transmission link, or channel, via which the electrical energy travels from the transmitter to the receiver will be dictated by the requirements of the communication system. Where two stations can be physically linked, a simple wire provides a low cost, efficient channel. Telephone and cable TV signals are examples of electronic communications transmitted over wires.
When a physical link between the transmitter and receiver is not practical, electromagnetic waves radiated from the transmitter site can be used to transmit information to a remotely located receiver. The atmosphere is the channel via which radiated electromagnetic waves propagate. Communication systems that utilize electrical energy radiated through the atmosphere include radio, radar, and cellular telephone systems. The nature of electromagnetic waves traveling through the atmosphere, and the communication distance required, will dictate the physical characteristics of the electrical energy used to produce the radiated waves.
Modulation of Electrical Signals
If an electronic signal is to be used to convey information, some characteristic of the signal must be varied, or modulated, in such a way that the information is contained, or carried, by the signal. The receiver of the signal must have the capability of recovering, or demodulating the information from the received signal. In this sense, the transmitter and receiver pair, in combination with the transmission channel, form a communication system. The modulator is used to prepare the information signal for efficient transmission through the channel, and the demodulator recovers the information signal from the received carrier signal.
In a simple electronic communication system, the presence or absence of a signal can be used to convey information, such as the click of a telegraph key which closes in the presence of a current on the telegraph line. The telegraph line carries the current pulses that are recognized by the human receiver as Morse code. In this case, human operators acted to modulate and demodulate the information signal onto and off of the telegraph line. Pulse code modulation (PCM) uses a similar technique for transmitting a digital information signal, usually in binary code, as a pulse pattern. An example is a computer's modulator/demodulator (MODEM) used to encode and decode digital data on and off of phone lines as a series of pulsed tones.
For more complex information, such as speech or audio, the information signal is time-varying and continuous, and more complex modulation techniques are used. Sinusoidal voltage or current signals optimized for a specific channel are used to 'carry' the information signal. These carrier waves have some physical property modulated such that the information signal is contained in the carrier wave. Amplitude modulation (AM) and frequency modulation (FM) are two common techniques of modulating a sinusoidal carrier wave, and will be studied in detail.
WAVE CHARACTERISTICS
Many forms of energy propagate as traveling waves. Mechanical waves, such as sound waves in air or pressure waves in water, require a physical medium to propagate. The wave speed of mechanical waves is highly dependent upon the material characteristics of the medium. In contrast, electromagnetic waves (radio, radar, light, etc.) require no medium and will propagate through free space. The wave speed of electromagnetic waves in free space is the speed of light (c), and very nearly the speed of light in mediums such as the atmosphere and water. Whether the traveling wave is mechanical or electromagnetic in nature, one set of characteristics is sufficient to describe the traveling wave.
Although much of the development of wave characteristics is independent of the shape of the wave, a sinusoidal waveform is used since it is easy to generate, easy to model, and more complex waveforms can be described by sums of sinusoidal waves. The easiest sinusoidal wave to describe is the standing wave, as given by the standing wave equation:
y(r)~ = ~ {A } ~{ sin } ~({2 pi r } OVER lambda )
With no time dependence, and thus no motion, the standing wave conveys no energy or information, and varys only with distance (r). We look at the standing wave, then, in order to easily recognize some characteristics that will be common to more complex waves.
Amplitude (A)
Wavelength ()
One of the fundamental characteristics of any wave is wavelength (). Defined as the shortest distance at which the wave pattern fully repeats itself, wavelength has the physical units of length (meters). Wavelength of a sinusoidal wave is most easily found by measuring the distance from one peak to the successive peak.
Frequency and Period
The second fundamental characteristic of a wave is its frequency (f).
Electromagnetic Waves
Electronic communications over large distances usually requires the use of electromagnetic energy waves as the transmission link between transmitter and receiver.
it is necessary to develop the set of properties that describe waves. The electromagnetic spectrum spans an immense range of frequencies ranging from below the 60 Hz of power in an ac outlet to above the six hundred thousand billion Hz of green light in the range of visible light. For convenience, the electromagnetic spectrum is partitioned into frequency ranges which share some similar characteristic: the radio
The process of transmitting speech or audio over transmission lines (wires) presents few problems, as evidenced by the number of telephones and telephone lines. Transmitting the same signals to remote locations via radiated electromagnetic waves, however, creates a host of physical limitations. A tone at 300 Hz captured by a microphone would produce a voltage oscillating at the same frequency. As an electromagnetic wave, the 300 Hz signal would have a wavelength of one million meters. Given that antennas shorter than -wavelength are inefficient radiators, transmitting electromagnetic waves at this frequency would require an antenna at least 155 miles in length. There needs to be some method of using higher frequency - shorter wavelength - waves that can be modulated to carry the lower frequency information. These high frequency carrier waves can then be efficiently radiated from reasonably-sized antennas.
One method of encoding a low frequency information signal, such as speech or audio, onto a higher frequency carrier is to allow the amplitude of the carrier wave to vary directly with the amplitude of the information signal. In the transmitter, an oscillator at the desired frequency will produce the required carrier wave. The amplitude of the carrier wave is varied directly with the amplitude of the information signal, and the information is contained in the outline, or envelope of the amplitude modulated (AM) signal.
An oscillator is used to generate a continuous, sinusoidal carrier wave at the desired carrier frequency (fc), here expressed in the form of a voltage, vc (t):
v SUB {c}(t)~=~V SUB {c} cos (2 pi f SUB {c}t)
The information signal is modeled using a single sinusoidal wave at the modulating frequency (fm). This general modulating signal forms the basis for more complicated signals, and will be sufficient for this development. The information signal, expressed in the form of a voltage, vm (t):
v SUB {m}(t)~=~V SUB {m} cos (2 pi f SUB {m}t)
Amplitude modulation varies the amplitude of the carrier wave directly with the amplitude of the information signal, and can be represented by summing the two amplitude components in the following form:
v SUB {AM}(t)~=~