Amplitude Modulation 96l24

AMPLITUDE MODULATION TRANSMISSION

AM Transmission • AM Transmitters are used in medium wave (MW) and short wave (SW) frequency bands for AM broadcast. • Medium wave – between 550 kHz and 1650 kHz • Short wave –between 3 MHz and 30 MHz

Block Diagram of AM Transmitters

Block Diagram of AM Transmitters Oscillator • The oscillator generates the RF carrier at a preset frequency and maintains it within close tolerances. • The oscillator outputs must be amplified many times to be radiated any distance.

Block Diagram of AM Transmitters

Buffer Amplifier • The buffer stage is a voltage amplifier that increases the amplitude of the oscillator signal to a level that drives the power amplifier. • Without a buffer, changes in the amplifier caused by keying or variations in source voltage would vary the load of the oscillator and cause it to change frequency. It may also be used as a frequency multiplier

Block Diagram of AM Transmitters Power Amplifier • Amplifies the power of the carrier frequency

Block Diagram of AM Transmitters Microphone • Converts the audio frequency (af) input (a person's voice) into corresponding electrical energy. Driver • Amplifies the audio Modulator • the modulator further amplifies the audio signal to the amplitude necessary to fully modulate the carrier. The output of the modulator is applied to the power amplifier. The power amplifier combines the RF carrier and the modulating signal in the power amplifier to produce the amplitude-modulated signal output for transmission.

High Level and Low Level Modulation • High Level Modulation • The carrier voltage is modulated at the highest power level. The required power level is obtained by using class C amplifiers. • High level transmitters are widely used at present.

High Level and Low Level Modulation • Low Level • The carrier is modulated at low power level and the carrier power is subsequently raised to the desired level in class B Tuned power amplifiers.

Block Diagram of High Level Modulation

Block Diagram of High Level Modulation Carrier oscillator Generate carrier signal Buffer amplifier Match the output impedance of the oscillator circuit with the input impedance of the frequency multiplier. Isolates the carrier oscillator and frequency multiplier.

Frequency multiplier Generates higher harmonics of the carrier oscillator frequency

Power amplifier The power of the carrier signal is then amplified

Block Diagram of High Level Modulation MIC Produces the intelligence signal Audio Driver amplifier Amplifies the voltage of the intelligence signal Audio power amplifier Amplifies the power of the intelligence signal Modulated Class C amplifier The modulating audio signal and the and the carrier signal, after power amplification are applied to this modulating stage

Antenna Radiates the signal into space of transmission through electromagnetic waves

Block Diagram of Low Level Modulation Carrier Oscillator Generates the carrier signal Buffer amplifier Isolates the carrier oscillator and frequency multiplier.

Modulated Class C amplifier The modulating audio signal and the and the carrier signal, after power amplification are applied to this modulating stage

Block Diagram of Low Level Transmitters MIC Produces the intelligence signal

Audio amplifier Amplifies the intelligence signal

AM Transmission Circuit

RF Amplifier Circuit