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Wednesday, 19 August 2015

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Know about Electromagnetic Spectrum

When electrons move, they create electromagnetic waves that can propagate through free space. This phenomenon was first predicted to exist by James Maxwell, in 1865, and it was first produced and observed by Heinrich Hertz in 1887. All modern communication depends on manipulating and controlling signals within the electromagnetic spectrum.

The electromagnetic spectrum ranges from extremely low-frequency radio waves of 30Hz, with wavelengths of nearly the earth's diameter, to high-frequency cosmic rays of more than 10 million trillion Hz, with wavelengths smaller than the nucleus of an atom. The electromagnetic spectrum is depicted as a logarithmic progression: The scale increases by multiples of 10, so the higher regions encompass a greater span of frequencies than do the lower regions.
At the high end of the electromagnetic spectrum, signals travel over a band of 10 million trillion Hz (that is, 1022Hz). This end of the spectrum has phenomenal bandwidth, but it has its own set of problems. The wave forms are so miniscule that they're highly distorted by any type of interference, particularly environmental interference such as precipitation. Furthermore, higher-frequency wave forms such as x-rays, gamma rays, and cosmic rays are not very good to human physiology and therefore aren't available for us to use for communication at this point.
An electromagnetic wave
  • Frequency—The number of oscillations per second of an electromagnetic wave is called itsfrequency.
  • Hertz—Frequency is measured in Hertz (Hz), in honor of Heinrich Hertz.
  • Wavelength—The wavelength is the distance between two consecutive maxima or minima of the wave form.
  • Amplitude—Amplitude is a measure of the height of the wave, which indicates the strength of the signal.
  • Phase—Phase refers to the angle of the wave form at any given moment.
  • Bandwidth—The range of frequencies (that is, the difference between the lowest and highest frequencies carried) that make up a signal is called bandwidth.
You can manipulate frequency, amplitude, and phase in order to distinguish between a one and a zero. Hence, you can represent digital information over the electromagnetic spectrum. One way to manipulate frequency is by sending ones at a high frequency and zeros at a low frequency. Devices that do this are called frequency-modulated devices. You can also modulate amplitude by sending ones at a high amplitude or voltage and zeros at a low amplitude. A complementary receiving device could then determine whether a one or a zero is being sent. As yet another example, because the phase of the wave form refers to shifting where the signal begins, you could have ones begin at 90 degrees and zeros begin at 270 degrees. The receiving device could discriminate between these two bit states (zero versus one) based on the phase of the wave as compared to a reference wave.


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