Communication Systems
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COMMUNICATION SYSTEMS
Barriers to Human Communication
Language
Distance
Electronic Communications
The transmission, reception, and processing of information using electronic circuits.
History
mid-19th century ÐŽV James Clark Maxwell studied electromagnetic wave and predicted that it can be propagated through free space.
1837 ÐŽV Samuel Morse invented the telegraph.
1876 ÐŽV Alexander Graham Bell and Thomas A. Watson transmitted human conversation over a functional telephone system.
1888 ÐŽV Heinrich Hertz radiated electromagnetic energy from a machine he called oscillator.
1894 ÐŽV Guglielmo Marconi was the first to accomplish wireless transmission.
1908 ÐŽV Lee DeForest invented the triode vacuum tube.
1933 ÐŽV Major Howard Armstrong invented frequency modulation.
1948 ÐŽV William Shockley, Walter Brattain and John Bardeen invented the transistor.
Electromagnetic Spectrum
Electromagnetic Spectrum
The entire range of frequency.
Frequency
The number of times an alternating current goes through its complete cycle per second is known as its frequency. The international unit of measurement of frequency is hertz, abbreviated Hz. The English unit is cycles per second, abbreviated cps (1 Hz = 1 cps). It is rarely used.
To simplify terminology, 1000 Hz is called kilohertz, abbreviated kHz, and 1000000 Hz is called a megahertz, abbreviated MHz.
The vibration rate of sound waves in air may also use the term frequency. When middle C is played on a musical instrument, for example, an air disturbance with a frequency of 262 Hz is set up. The lowest tone that can be heard by human beings is about 15 Hz. The highest audible, audio, or sonic tones are usually 12 to 22 kHz (22 kHz for youngsters, 12 kHz for seniors). A microphone is a device or transducer that can change sound waves in air to an equivalent-frequency ac in wires.
Frequencies that produce sound waves audible to humans are said to be audio frequencies (AF). Frequencies that can be fed to antennas and will radiate electromagnetic and electrostatic waves in space are considered to be radio frequencies (RF).
FCC Band Designations
Frequency Range
Designations
30 ÐŽV 300 Hz
ELF (Extremely Low Frequencies)
0.3 ÐŽV 3 kHz
VF (Voice Frequencies)
3 ÐŽV 30 kHz
VLF (Very Low Frequencies)
30 ÐŽV 300 kHz
LF (Low Frequencies)
0.3 ÐŽV 3 MHz
MF (Medium Frequencies)
3 ÐŽV 30 MHz
HF (High Frequencies)
30 ÐŽV 300 MHz
VHF (Very High Frequencies)
0.3 ÐŽV 3 GHz
UHF (Ultra High Frequencies)
3 ÐŽV 30 GHz
SHF (Super High Frequencies)
30 ÐŽV 300 GHz
EHF (Extremely High Frequencies)
0.3 ÐŽV 3 THz
Infrared
3 ÐŽV 30 THz
Infrared
30 ÐŽV 300 THz
Infrared
0.3 ÐŽV 3 PHz
Visible Light
3 ÐŽV 30 PHz
Ultraviolet
30 ÐŽV 300 PHz
X-rays
0.3 ÐŽV 3 EHz
Gamma rays
3 ÐŽV 30 EHz
Cosmic rays
„` FCC stands for Federal Communications Commission
Wavelength, єЬ
The length that one cycle of an electromagnetic wave occupies in space, also, the distance between similar points in a repetitive wave.
where
= velocity factor (equal to 1 in free space)
= velocity of light in free space
= 299793000 m/s „l 3 „e 108 m/s
= frequency
@ high frequencies, wavelength is too short and is usually expressed as Angstrom.
Bandwidth and Information Capacity
Limitations of Communication Systems
noise
bandwidth
Bandwidth, BW
unit:
Hz (Hertz)
A portion of electromagnetic spectrum occupied by a system.
Minimum range of frequencies required propagating the source information through the system.
must be sufficiently large (wide) to pass all significant information frequencies.
difference between the upper and lower limit frequencies.
Information Capacity
A measure of how much source information can be carried through the system in a given period of time.
HartleyÐŽ¦s Law
Information Capacity єС Bandwidth „e time
Signal Analysis
Factors Affecting the Signal
Distortion ÐŽV signal alteration due to imperfect response of the system to the desired signal.
Interference ÐŽV contamination of extraneous signal usually man-made to a form similar to the desired signal.