AM AM TRANSMITTERTRANSMITTER

Amplitude modulationFrequncy modulation

WHAT IS MODULATION?

It is modulation of amplitude of the wave.

This type of transmission is simple and easily disturbed.

AM radio ranges from 535 to 1705 kilohertz (OR) Up to 1200 Bits per second.

Can send only one channel at a time.

It is modulation frequency of the wave.

This type of transmission is complex.

FM radio ranges in a higher spectrum from 88 to 108 megahertz. (OR) 1200 to 2400 bitsper second.

Can send two channels at a time.

• Modulation is process of varying one or more properties like frequency , amplitude, phase of periodic waves.

• AM and FM are two very popular and very different methods of sending information over the airwaves.

Amplitude modulationAmplitude modulation 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.

In AM, a radio wave known as the "carrier" or "carrier wave" is modulated in amplitude by the signal that is to be transmitted.

It is used in both analog and digital communication and telemetry.

Range of modulator is small as compared to FM.

How does the signal transmitted?• This circuit designed to transmit the input signal with the

help of a carrier wave.• The input signal is too weak to travel long distance , so

we assist a carrier wave to the wave.• Now here we create carrier wave by the help of RC oscillator in 555 timer chip.• The electrical signals are change to electromagnetic wave with the help of antenna.

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Principles of AM

Amplitude Modulation – is a process of changing the amplitude of a relatively high frequency carrier signal with the instantaneous value of the modulating signal (information signal)

2 inputs to the modulation devise (modulator) A single, high frequency RF carrier signal of constant amplitude Low frequency information signals that maybe a single frequency or a

complex waveform made up of many frequencies In the modulator, the information signal modulates the RF carrier

signal to produce a modulated waveform made up of many frequencies

This modulated waveform also called as AM envelope

Transmitters• Transmitter concept is in the block diagram below:-

• Foundation Licence only permits use of commercial equipment to minimise the risk of interference and/or out-of-band operation.

• Avoid over-deviating, and operating PAs into poor matches !!

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3

2 4Mic

1 - Audio Stage2 - Modulator eg AM, FM, SSB3 - RF Frequency Generator4 - RF Power Amplifier

Transmitter

Modulator

Power AmplifierDriver StagesPower Stage

Antenna

Oscillator

Signal

Receiver

Low Noise Amplifier

Low Noise AmplifierGain-Stage Amplifier

Filter

Antenna Oscillator

Mixer

Filter

Transceiver = Transmitter + Receiver

Modulator

Power Amplifier

Antenna

Oscillator

Signal

Low Noise Amplifier

Filter

Mixer

Filter

The AM Radio “System”

Transmitter Receiver

AM is a nonlinear processSum and difference frequencies are created that carry the information

Basic analog communications system

Modulator

Demodulator

Transmission Channel

Input transducer

Transmitter

Receiver

Output transducer

Carrier

EM waves (modulated signal)

EM waves (modulated signal)

Baseband signal (electrical signal)

Baseband signal (electrical signal)

Amplitude Modulation

Carrier wave

Baseband signal

Modulated wave

Amplitude varying-frequency constant

SIMPLE AM TRANSMITTER CIRCUIT

ABOUT THE CIRCUIT

Potentiometers are commonly used to control electrical devices such as volume controls on audio equipment.

The 555 timer chip makes easy to connect the circuit.This chip creates the carrier wave of higher frequency. The audio signal is weak signal which cannot travel long distances . So we assist carrier wave to carry the our to the destination.

AM TransmittersAM transmitters are a “mature” technology, but are still in widespread use

Examples include:

Standard AM broadcast stationsCB radioVHF aircraft radio

AM Transmitter Stages

All of the stages of a transmitter (except the power amplifier and possibly the driver) operate at low power levels

This part of the transmitter, exclusive of the power-handling stages, is called the exciter

Other transmitter components include: The oscillator stage The buffer and multiplier stages The driver stage The power amplifier/modulator

AM Transmitter

Each AM station is allocated a frequency band of 10kHz in which to transmit its signal.

This frequency band is centered around the carrier frequency of the station A station at 610 on your dial transmits at a carrier frequency of

610kHz The signal that is broadcast occupies the frequency range from

605kHz to 615kHz

AM Transmitter

Transmitter input (signal source) is an audio signal. Speech, music, advertisements

The input is modulated to the proper carrier frequency.

Modulated signal is amplified and broadcast

Transmitter Block Diagram

Signal

Source

Modulator

Power

Amplifier

Antenna

In the block diagram of the a-m transmitter , the r-f section consists of an oscillator feeding a buffer, which in turn feeds a system of frequency multipliers and/or intermediate power amplifiers. If frequency multiplication is unneccessary, the buffer feeds directly into the intermediate power amplifiers which, in turn, drive the final power amplifier. The input to the antenna is taken from the final power amplifier.

RF SECTION

Frequency domain representation of output

Modulator-Time DomainInput Signal

Output Signal

Antenna

The antenna converts a current or a voltage signal to an electromagnetic signal which is radiated throughout space.

In the frequency domain, the antenna output is

0 frequency

Undesired Signals

Desired Signal

Carrier Frequencyof desired station

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AM Transmitters1 : Low-level Transmitters Block diagram for a low-level AM DSBFC transmitter :

Preamplifier Linear voltage amplifier with high input impedance. To raise source signal amplitude to a usable level with minimum nonlinear

distortion and as little thermal noise as possible. Modulating signal driver

Amplifies the information signal to an adequate level to sufficiently drive the modulator.

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Low-level Transmitters Block diagram for a low-level AM DSBFC transmitter :

RF Carrier oscillator To generate the carrier signal. Usually a crystal-controlled oscillator is used.

Buffer amplifier Low gain, high input impedance linear amplifier. To isolate the oscillator from the high power amplifiers.

Modulator : can use either emitter collector modulation Intermediate and final power amplifiers (pull-push modulators)

Required with low-level transmitters to maintain symmetry in the AM envelope

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Low-level Transmitters

Coupling network Matches output impedance of the final amplifier to the transmission line/antenn

Applications are in low-power, low-capacity systems : wireless intercoms, remote control units, pagers and short-range walkie-talkie

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High-level Transmitters Block diagram for a high-level AM DSBFC transmitter

Modulating signal is processed similarly as in low-level transmitter except for the addition of power amplifier

Power amplifier To provide higher power modulating signal necessary to achieve 100% modulation (carrier power is

maximum at the high-level modulation point). Same circuit as low-level transmitter for carrier oscillator, buffer and driver but with addition of

power amplifier

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High-level Transmitters

Primary functions of modulator circuit Provide the necessary circuitry for the modulation to occur The final power amplifier Frequency-up converter : translates low-frequency information signals to radio-frequency

signals that can be efficiently radiated from the antenna and propagates through the free space