Need for modulation

Need for modulation:-

Modulation is the fundamental need for communications, the following are the basic needs for modulation

Distance:-

As low frequency signals can not travel longer distances, low frequencies can be translated into higher frequencies by using modulation schemes. 

Improved Signal to Noise Ratio:-

Signal to Noise Ratio has been improved because of modulation at the Receiver.

Practicability of antennas:-

If the communication medium is free space , then messages are transmitted and received with the help of antennas.

∴ The height of the antenna is of the order of the wavelength ( λ ) of the signal being transmitted, when the signals from transmitter are transmitted without modulation then the height required for the antenna is very high, for example to transmit a message signal of frequency  f = 5 KHz

height of the antenna required would be

 h \simeq \frac{\lambda }{4} \simeq \frac{c}{4f}

h \simeq \frac{3 X 10^{8}}{4 X 5X10^{3}}

h\simeq 15000 meters

Designing an antenna with 15 Km length is almost impractical. To reduce the height of the antenna , instead of sending low frequency (5 KHz) signal as it is modulation is preferred , modulation technique reduces the height of the antenna and makes the antenna to be more practical both at Txr and Rxr.

Narrow Banding:-

There is a problem caused by direct transmission of base band signal which can be explained as follows , suppose a base band signal spectrum ranges from (50 Hz-10 KHz) then the height of the antenna must be 1.5 X 106 meters to receive 50 Hz signal at the receiving end , where as for 10 KHz , the height would be 7500 meters that means height of the antenna is not same for all frequencies . 

∴ A wide band antenna which can operate for band edge ratio of 200 is required which is impractical, so modulation is required to use same antenna at the receiver to receive certain range of frequencies.

∴ A wide band message signal from 50 Hz to 10 KHz gets converted to a narrow band signal by a carrier frequency of 1 MHz.

This Narrow banding of Base band signal is possible with modulation which in turn eliminates the complexity of antenna height at the receiver .

Multiplexing:-

Simultaneous transmission of multiple messages over a channel is known as multiplexing. suppose number of messages from different transmitters are transmitted without modulation then there is a possibility of interference (one with other)  since the base band spectrum is identical for all the messages. Hence the transmitted messages will not be received properly at the receiver.

one technique to eliminate interference is by the use of  modulation  and the other technique is multiplexing.

  • Frequency division multiplexing  (FDM)  which uses analog modulation techniques.
  • Time division multiplexing (TDM) uses pulse modulation techniques.

Multiplexing reduces number of channels needed and reduce the cost of installation and maintenance.

Radio Frequency spectrum from ELF to EHF:-

frequency Range Description of frequency band
upto 300 Hz Extreme Low frequency(ELF)
300 Hz-3 KHz Voice frequency (VF)
3 KHz-30 KHz Very Low frequency (VLF)
30 KHz-300 KHz Low Frequency (LF)
300 KHz-3 MHz Medium frequency (MF)
3 MHz-30 MHz High frequency (HF)
30 MHz-300 MHz Very High frequency (VHF)
300 MHz-3 GHz Ultra High frequency (UHF)
3 GHz-30 GHz Super High frequency (SHF)
30 GHz-300 GHz Extreme High frequency (EHF)

*Audio Frequency range: 20 Hz-20 KHz.

*UHF,SHF and EHF are Micro wave frequencies.

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Author: Lakshmi Prasanna Ponnala

Completed M.Tech in Digital Electronics and Communication Systems and currently working as a faculty.