Amplitude modulation process is completed in two stages, one is modulation and other is demodulation. Modulation is performed at transmitter while demodulation is done at receiver.A transmitter not only performs the modulation process, but also raises the power level of a modulated signal to the desired extent for effective radiation.
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Amplitude Modulation:categories of am TransmittersThe AM transmitters are divided into two categories:-(i) Low level modulation: The block diagram for this type of transmitter is shown in figure.Low level modulation(ii) High level modulation: The block diagram for this type of transmitter is shown in figure.High level modulationVarious component of transmitter are described below:-Master Oscillator:The master oscillator generates a stable sub harmonic carrier frequency (i.e. the fraction of a desired carrier frequency).
This stable sub-harmonic oscillation is generated by using a crystal oscillator and then frequency is raised to the desired value by harmonic generator.Buffer Amplifier:This is a tuned amplifier providing high input impedance at the master oscillator frequency. Any variation in load current does not affect the master oscillator due to this high input impedance of buffer amplifier at the operating frequency of the master oscillator.Harmonic Generator:It is an electronic circuit that generates harmonics of its input frequency.
The principle of harmonic generation is the same as that of a non-linear modulator. When a signal is applied to a non-linear circuit, it generates harmonics of input frequency. The desired harmonic is selected by a properly tuned circuit. The circuit uses a class C tuned amplifier.Driver Amplifier or Intermediate Power Amplifier:One or more stages of a class C tuned amplifier are used to increase the power level of a carrier signal to provide a large drive to the modulated class C amplifier.
Modulation system:The collector modulation circuit is used for modulation in high power transmitters. The modulating amplifier is a class A, or class B amplifier amplifying the base-band signal.Feeder and Antenna:The transmitter power is fed to a transmitting antenna for effective radiation. The length of the antenna (a conductor) should be of the order of the wavelength for effective radiation.Block Diagram of Amplitude Modulaion (receiver)AM ReceiversA radio receiver is an electronic circuit that picks up a desired modulated radio frequency signal, and recovers the base-band signal from it. A receiver performs the following functions which are explained with the help of the block diagram of a simple receiver known as TRF .
TRF receiverInterception:This function is performed by a receiving antenna (small conductor). The radio waves coming from various transmitting stations arrive at this antenna. These radio waves contain electrical energy in the form of an electromagnetic wave.Selection:This is the process by which the receiver selects a particular desired carrier frequency, and rejects others, so that at any time a signal from only one transmitter is received. This is done by Radio frequency amplifier.R.F.
Amplification:The selected carrier is amplified by a class C tuned amplifier. This is necessary to raise the carrier voltage level so that the linear diode detector following this stage may operate in linear region. This is also done by Radio frequency amplifier.Detection:Detection is the process of recovering a base-band signal from a modulated carrier. This is done by the detector here.Audio Amplification:The detected output audio signal is further amplified so that it can drive the speaker. This stage consists of an RC coupled amplifier followed by class B push-pull amplifier. Here this is shown by Audio voltage amplifier and Audio power amplifier.Reproduction:Reproduction is the process by which an electrical signal is converted into a desired physical message. Here it is converted in audio signal with the help of speaker.