Chapter 20: Communication System

Radio Wave
Electromagnetic waves covering a vast range of wavelengths; used in variety of communication depending on the frequency. Its frequency is in between 104 and 1011 or 1012 Hz. To allow several radio stations to broadcast, each signal has a different carrier wave frequency which is altered and modulated.

Modulation
Modulation is the variation of either amplitude or frequency of the carrier wave. The modulated carrier wave is actual wave transmitted due to high frequency wave or the amplitude or the frequency is varied in synchrony with the displacement of the information signal.

Amplitude Modification (AM)
In this, the amplitude of the carrier wave varies in synchrony with the displacement of the information signal. The frequency of the modulated carrier wave is constant.

Advantages:

  • The actual receiver and transmitter used for AM are less complicated and cheaper.
  • The bandwidth needed for each AM transmission is less then FM transmission, hence more stations are available in any given frequency range.

Frequency Modulation
In this, frequency of the carrier wave varies in synchrony with the displacement of the information signal. The amplitude of the modulated carrier wave is constant.

Advantages:

  • Electrical interference affects AM more than FM.
  • Higher bandwidth can be used, hence better quality of sound.

Sidebands and Bandwidth
Side bands are each of two frequency band either side of the carrier wave, which contain the modulated signal.  The carrier wave is modulated by range of frequencies which change with time resulting to band of frequencies which are upper and lower sidebands.

Bandwidth of a signal is the range of frequencies occupied by the amplitude modulated waveform, the difference between the highest-frequency and lowest frequency signal component.

Analogue and Digital Signal
Those signals that consists of a series of zeros (0) and ones (1) is digital signal. 1 in digital signal is presence of voltage pulse and 0 is absence of voltage pulse.

Those signal that is continuously variable, having a continuum of possible values.

Analogue to digital conversion

The digital transmission of speech or music involves analogue-to-digital conversion (ADC) before transmission and digital-to analogue conversion (DAC) after reception.

There are difference in the digital signal in ADC and DAC i.e. the level of detail in the transmitted signal can be increased by:

Increasing the number of bits in each number to reduce the step height, resulting to smaller changes in input signal can be seen.

Increasing in the sampling rate, which is the number of sample made per second.

Channels of Communication
There are different types of signal channel and they are:

  1. Wire-pairs
    These are applicable for linking a landline telephone to local exchange. The potential difference between the two wires is signal. It is the cheapest transmission medium and has a small bandwidth. Each wire is an aerial, picks up wanted electromagnetic waves and distorts the signal.
    Wire pairs are easily tapped so there is low security. It suffers from cross-linking where signal in one pair is picked up by other wires.
  2. Coaxial Cables
    These are applicable in connecting aerial to a television. It has copper core and the finely woven copper braid which acts are two conductors that transmit the signal. There is greater bandwidth than wire pair. Thy can transmit data faster over long distance. It is more expensive.
  3. Radio Waves
    Electromagnetic waves covering a vast range of wavelengths which is used in a variety of communication depending on frequency. It is of three types;
    1. Surface waves ( Up to 3 MHz)
    2. Sky waves ( above 3 MHz)
    3. Space Waves ( Above 30 MHz)
  1. Microwave Links
    These are above 1 GHz. These are applicable for linking ground station to satellite. They are able to pass through the ionosphere to reach satellites in space.
  1. Optic Fibers
    They consists of thin glass core surrounded by a material of slightly lower refractive index called the cladding, to cause total internal reflection. They have large bandwidth and carries more information. It is highly secure and costs less than the same length of copper wire.