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5.2

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5.4

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Modulation

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What is Modulation

Modulation

In the modulation process, some characteristic of ahigh-frequency carrier signal (bandpass), is changedaccording to the instantaneous amplitude of the

information (baseband) signal.

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AM Modulation/Demodulation

Modulator Demodulator

Baseband Signal

with frequencyfm

(Modulating Signal)

Bandpass Signal

with frequencyfc

(Modulated Signal)

Channel

Original Signal

with frequencyfm

Source Sink

fc >> fm

Voice: 300-3400Hz GSM Cell phone: 900/1800MHz

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Need Of Modulation

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1) Size of the antenna : - For efficienttransmission the transmitting antennas should have

length at least equal to a quarter of the wavelength

of the signal to be transmitted. For an

electromagnetic wave of frequency 15 kHz, thewavelength is 20 km and one-quarter of this will

be equal to 5 km. Obviously, a vertical antenna of

this size is impractible. On the other hand, for a

frequency of 1 MHz, this height is reduced to 75m.

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Multiplexing :-Audio frequencies are within the range of 20Hz to 20 kHz. Without modulation all signals

at same frequencies from differenttransmitters would be mixed up. There bygiving impossible situation to tune to any

one of them. In order to separate thevarious signals, radio stations mustbroadcast at different frequencies.

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Figure 5.15 Types of analog-to-analog modulation

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Amplitude Modulation

A carrier signal is modulated only inamplitude value

The modulating signal is the envelope of thecarrier

The required bandwidth is 2B, where B is thebandwidth of the modulating signal

Since on both sides of the carrier freq. fc, thespectrum is identical, we can discard onehalf, thus requiring a smaller bandwidth fortransmission.

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Figure 5.16 Amplitude modulation

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The total bandwidth required for AM

can be determined

from the bandwidth of the audio

signal: BAM = 2B.

Note

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Figure 5.17 AM band allocation

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Frequency Modulation

The modulating signal changes the freq.fc of the carrier signal

The bandwidth for FM is high

It is approx. 10x the signal frequency

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The total bandwidth required for FM canbe determined from the bandwidth

of the audio signal: BFM= 2(1 + )B.

Where is usually 4.

Note

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Figure 5.18 Frequency modulation

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Figure 5.19 FM band allocation