AM transmitter

Transmitter

In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves. In addition to their use in broadcasting, transmitters are necessary component parts of many electronic devices that communicate by radio, such as cell phones, Wifi and Bluetooth enabled devices, garage door openers, two-way radios in aircraft, ships, and spacecraft, radar sets, and navigational beacons. The term transmitter is usually limited to equipment that generates radio waves for communication purposes; or radiolocation, such as radar and navigational transmitters. Generators of radio waves for heating or industrial purposes, such as microwave ovens or diathermy equipment, are not usually called transmitters even though they often have similar circuits.

Amplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent. For example, changes in the signal strength can be used to specify the sounds to be reproduced by a loudspeaker, or the light intensity of television pixels. (Contrast this with frequency modulation, also commonly used for sound transmissions, in which the frequency is varied; and phase modulation, often used in remote controls, in which the phase is varied)

AMPLITUDE MODULATION TRANSMITTER

                In A.M transmitter, the modulating signal amplitude modulates the carrier. Such transmitters are used for radio broadcast on long, medium and short eaves, radio telephony on short waves, radio telegraphy on short waves, television picture broadcast on every short waves or ultra short waves.

                       In an AM (amplitude modulation) transmitter the amplitude (strength) of the carrier wave is varied in proportion to the audio signal.

                   Amplitude modulation is a form of modulation in which information is carried in the envelope of the carrier. There are different types of amplitude modulation (i)

Amplitude Modulation Suppressed Carrier (AM-SC) (ii) Amplitude Modulation with Carrier (AM-C) (iii) Amplitude Modulation with Single Sideband (AM-SSB). For the suppressed carrier schemes

the receiver must generate a carrier in frequency and phase synchronism with the carrier at

the transmitter. This makes the receiver complex and increases its cost. Regular Amplitude

Modulation transmitter is inefficient but the receiver design is simplified. Hence in terrestrial

broadcast radio Amplitude Modulation is used which can be handled with a very inexpensive

receiver. Mathematically, the regular amplitude modulation can be expressed as:

y(t) (A m(t)) cos(2 f t) C = +μ π

Where A is the maximum amplitude of m(t), fc is the carrier frequency and μ is the

modulation index.

Amplitude Modulation Transmitter

Introduction to amplitude modulation transmitter:

Block diagram of AM transmitter is shown below.

Parts of am Transmitter:

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. The carrier frequency ought to be very stable. Any change in the master oscillator frequency will cause interference with other transmitting stations and receiver will accept programmes from more than one transmitter.
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. Thus, buffer amplifier isolates the MO from the succeeding stages, so that the loading effect may not change the 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. The output of the harmonic generator provides a low power carrier signal. This power is amplified to raise power to desired level to drive the final amplifier stage. This stage is termed as driver amplifier or intermediate power 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. The antenna is normally located at a distance from the transmitter and hence power from the transmitter is fed to the antenna through a properly designed transmission line called feeder.

Modulation index

The modulation depth or modulation index represents the ratio of peak value of modulating

signal and the DC bias which represents the amplitude of the carrier. For amplitude

modulation, modulation index is defined as:

Peak value of the modulating signal

μ =

Amplitude of the carrier

Applications:::

Entertainment - AM Radio (music, talk radio)

Business - CB Radio to relay shipment progress, conference calls with other branches via telephone or speakerphone

Government - police officers keeping contact with the station, or relaying the information and location of a suspect

Personal - cell phones or walkie talkies for chatting with friends

Military - using undetectable radio signals to communicate with other soldiers, or voice commands to control weapons