CAREER POINT UNIVERSITY Major assignment on

Modulation And De-modulation

Submitted to:Somesh chaturvedi Submitted by Brijesh KUMAR ME-3rd year UID-K10668

Modulation And De-modulation Radio Frequency Transmission

Information signal are transported between a transmitter and a receiver over some form of transmission medium.

The information signal typically a voice and a digital signal is very low frequency and not suitable for transmission.

Therefore they must be transformed from their original form into a form that is more suitable for transmission.

The process of impressing low frequency information signal onto a high-frequency carrier signal is called modulation.

Demodulation is the reverse process where the received signals are transformed back to their original form.

Principles of Amplitude Modulation Amplitude modulation (AM) is the process of changing the

amplitude of a relatively high frequency carrier signal in accordance with the amplitude of modulating information signal.

AM modulators are nonlinear devices with two input signals: a single-frequency constant amplitude carrier signal and the information signal.

The output from the modulator is the frequencies that are high enough to be efficiently radiated by an antenna and propagated through free space and are commonly called radio frequency or simply RF.

Principles of Amplitude Modulation Using trigonometric functions, the sine-wave carrier can be

expressed as:

A modulating sine-wave signal can be expressed as:v V f tc c c sin2

v V f tm m m sin 2

wherevm instantaneous value of intelligence signalV = peak amplitude of intelligence signalf = frequency of modulating

m

m

3-1: AM Concepts

Figure 3-3: Amplitude modulator showing input and output signals.

Vmsin2fmt sin2fct is the characteristic of AM. The AM wave is the product of the carrier and modulating signals. The circuit used for producing AM is called a modulator.

The AM Envelope Although there are several types of amplitude modulation, AM

double-sideband full carrier (DSBFC) is the most commonly used. This type of modulation is sometimes called conventional AM or simply AM.

Figure 28 shows how an AM waveform is produced when a single-frequency modulating signal acts on a high-frequency carrier signal.

The output waveform contains all the frequencies that make up the AM signal and it is used to transport the information through the system. The shape of the AM signal is called the AM envelope.

Note that with no modulating signal, the output waveform is simply the carrier signal, however when the modulating signal is applied, the amplitude of the output wave varies in accordance with the modulating signal.

The repetition rate of the envelope is equal to the frequency of the modulating signal and the shape of the envelope is identical to the shape of the modulating signal.

Modulation Index and Percentage of Modulation:Modulation index is a term that is used to describe the amount of amplitude change (modulation) present in an AM waveform. Percentage of modulation is a coefficient of modulation stated as percentage.

m VVm

c

(volts) ltagecarrier vo dunmodulate theof amplitudepeak V voltageeformoutput wav theof amplitude in the changepeak V

(unitless)t coefficien modulation = m:Where

c

m

The amount, or depth, of AM is then expressed as the percentage of modulation (100 × m) rather than as a fraction.

Modulation index can also be read directly from the oscilloscope:

m V VV V

max min

max min

Amplitude Modulation Reception AM reception is the reverse process of AM modulation. Its

simply to converts an amplitude-modulated wave back to the original source information.

A typical AM receiver block diagram is shown below:

Peak Detector (Diode Detector)The AM signal will on and off the diode. RC low pass filter will filter out the high frequency signal (carrier +harmonic) and only the original information are recovered.

Angle Modulation Transmission Three properties of analog signal can be varied: amplitude,

frequency and phase. Frequency and phase modulation are both form of angle modulation.

The basic equation for an angle modulated sine wave is:

Where: m(t) = angle-modulated wave Vc = peak carrier amplitude c= carrier radian frequency (t) = instantaneous phase deviation

( ) cos( ( ))m t V t tc c (*#)

Frequency Modulation Frequency modulation (FM) is defined as varying the

frequency of a constant-amplitude carrier directly proportional to the amplitude of the modulating signal at a rate equal to the frequency of the modulating signal.

In FM, the carrier amplitude remains constant while the carrier frequency is changed by the modulating signal. As the modulating signal amplitude increases, the carrier frequency increases. If the amplitude of the modulating signal decreases, the carrier frequency decreases.

Figure 36 shows the waveform for a sinusoidal carrier for which frequency modulation is occurred.

Figure 36: Frequency modulation of the sinusoidal signal.

Phase Modulation Phase modulation (PM) is defined as varying the phase of a

constant-amplitude carrier directly proportional to the amplitude of the modulating signal at a rate equal to the frequency of the modulating signal.

In PM, the carrier amplitude remains constant while the carrier phase is changed by the modulating signal. As the modulating signal amplitude increases, the carrier phase increases. If the amplitude of the modulating signal decreases, the carrier phase decreases.

Figure 37 shows the waveform for a sinusoidal carrier for which angle modulation is occurred.

Frequency DeviationFrequency deviation is the change in frequency in the carrier when it is acted on by a modulating-signal frequency denoted as f:

Therefore the FM modulation index can be rewritten as:

f KVm 1 (Hz)

m ffm

unitless

Frequency Modulation TransmissionDirect FM Modulators Direct FM is angle modulation in which the frequency of the

carrier is varied directly by the modulating signal. Figure 38 shows a basic schematic diagram of the direct FM

modulator. L and Cm is the frequency-determining section for the LC oscillator.

Figure 38: Simple direct FM modulator

The capacitor microphone is a transducer that converts acoustical energy to mechanical energy which is used to vary the distance between the plates of the microphone and consequently change its capacitor, Cm.

Because of the Cm varied, the resonant frequency is varied, thus, the oscillator output frequency varies directly with the external sound source.

This is direct FM because the oscillator frequency is changed directly by the modulating signal.

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