Base band signals produced by various information sources are not always suitable for direct transmission over a given cannel.
This signals are usually modified to facilitate transmission, this can version process is known modulation.
At the receiver, the modulated signal must pass through a reverse process to reconstruct the base band signal, this process called demodulation.
1- Amplitude (AM)
2- Frequency (FM)
3- Phase modulation (PM)
Need for modulation
1- enable to transmit through air by antenna
Length of antenna = C/F
Efficiency = C/F
2- Enable transmission more than one signal in the same channel.
On this type of modulation the carrier amplitude varies in proportion to the information signal.
On this type of modulation the carrier frequency varies in proportion to the information signal.
Frequency modulation signal
The carrier's frequency is proportional to the base band signal's amplitude.
The carrier increases frequency proportional to the positive magnitude of the base-band and decrease frequency proportional to the negative magnitude of the base band.
There is three types of wirless mobile:
1- fixed device.
2- Movable device and it used in cars.
3- Manual device (mobile).
1-Fixed wirless device:
The device and it has a transmitter circuit and a reciver cicuit.
The battary and it used in case of electrical current cut off.
The describe of the device:
Output power: ideal value is 25w, when the output power is increase the range which the device covered will increase.
Sensitivity: ideal value is 0.2 : 0.3 MV.
The efficiency of the device will increase as the sensitivity decrease.
Reflection: the ideal value is zero and it depends on o/p power.
All wireless device used FM modulation.
2-The movable wireless devices: (cars devices)
it has the same feature of the fixed devices but with out power supply in the component of the device.ÅÐÇ áã ÊÓÊØÚ Ãä ÊäÙÑ ÇãÇãß áÃä ãÓÊÞÈáß ãÙáã æáã ÊÓÊØÚ Ãä ÊäÙÑ ÎáÝß áÃä ãÇÖíß ãÄáã ÝÇäÙÑ Åáì ÇáÃÚáì ÊÌÏ ÑÈß ÊÌÇåßengineer mahmoud
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