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INTRODUCTION TO MODULATION
Modulationis the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with respect to a modulating signal
Here the three key parameters of a periodic waveform are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low frequency signal to obtain the modulated signal. Typically a high-frequency sinusoid waveform is used as carrier signal
FREQUENCY MODULATION
o Frequency modulationis a type of modulation where the frequency of the carrier is varied in accordance with the modulating signal. The amplitude of the carrier remains constant.
o The information-bearing signal (the modulating signal) changes the instantaneous frequency of the carrier. Since the amplitude is kept constant, FM modulation is a low-noise process and provides a high quality modulation technique which is used for music and speech in hi-fidelity broadcasts
o For FM: the carrier’s frequency is proportional to the baseband’s amplitude, thecarrier increases frequency proportional to the positive magnitude of the baseband and decreases frequency proportional to the negative magnitude of the baseband.
ADVANTAGES OF FREQUENCY MODULATION
• Frequency modulation have some advantages over amplitude modulation, FM is not sensitive to the amplitude noise, and have high efficient use of transmitted power
• Lesser distortion: Frequency modulated wave is less susceptible to intereferences from buildings, traffic etc which provides improved signal to noise ratio (about 25dB) w.r.t. to man made interference.
• Waves at higher frequencies can carry more data than the waves at low frequency.
• Smaller geographical interference between neighboring stations.
• Less radiated power.
• Well defined service areas for given transmitter power
WORKING OF A FM TRANSMITTER
FM transmission is done by the process of audio pre amplification, modulation and then transmission.
Here we have adapted the same formula by first amplifying the audio signal, generating a carrier signal using an oscillating and then modulating the carrier signal with the amplified audio signal
The amplification is done by an amplifier, whereas the modulation and
carrier signal generation is done by an variable frequency oscillator circuit.
The frequency is set at anywhere between the FM frequency range from 88MHz to 108MHz. The power of the FM signal from the oscillator is then amplified using a power amplifier to produce a low impedance output, matching that with the antenna.