AM TRANSMITTER

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AM TRANSMITTER In an  AM  ( amplitude modulation )  transmitter  the amplitude (strength) of the carrier wave is varied in proportion to the modulation signal. In an FM (frequency modulation)  transmitter  the frequency of the carrier is varied by the modulation signal. There are two basic configuration for transmitters. High level modulation low level modulation  High level modulation In high power modulation signal,the carrier voltage is modulated at highest power level.the require power level is obtained by class c power amplifier.The block diagram of high level modulation as shown in fig(a). low level modulation In the low level modulation system, the carrier is modulated at low power level and the carrier power is subsequently raised to the desire level in the class B amplifier . transmitters using high power level are widely use at present .The block diagram of high level modulation as shown in fig(b). advantage AM sign
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TIME DIVISION MULTIPLEXING

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Time-division Multiplexing (TDM): It is also called synchronous TDM, which iscommonly used for multiplexing digitized voice stream. The users take turns usingthe entire channel for short burst of time.
In frequency division multiplexing, all signals operate at the same time with different frequencies, but in Time-division multiplexing all signals operate with same frequency at different times. This is a base band transmission system, where an electronic commutator sequentially samples all data source and combines them to form a composite base band signal, which travels through the media and is being demultiplexed into appropriate independent message signals by the corresponding commutator at the receiving end. The in coming data from each source are briefly buffered. Each buffer is typically one bit orone character in length. The buffers are scanned sequentially to form a composite data stream. The scan operation is sufficiently rapid so that each buffer is emptied before more data can arrive. Composite data rate must be at least equal to the sum of the individual data rates. The composite signal can be transmitted directly or through a modem. The multiplexing operation is shown in Fig







ADVANTAGE

    1. Time division multiplexing systems are more flexible than frequency division multiplexing.
    2. Time division multiplexing circuitry is not complex.
    3. Problem of cross talk is not severe.Full available channel bandwidth can be utilized for each channel.

DISAVANTAGE

  1. Synchronization is required in time division .
  2. Complex to implement.
  3.  Due to slow narrowband fading, all the TDM channels may get wiped out.


APPLICATION

  1. It used in ISDN (Integrated Services Digital Network) telephone lines.
  2. It is used in PSTN (public switched telephone network).
  3. It is used in wire line telephone lines.
  4. It is used for some telephone system.

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YAGI ANTENNA

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