ICT–BVF–4.1 Lecture 6 Analog Modulation

ICT–BVF–4.1

Lecture 6

Analog Modulation

20/04/23

Information & Communication TechnologyModule: ICT–BVF -4.1 Fundamentals of telecommunication

Unit: Analog Modulation

1TTC Riyadh, ICT–BS–1.2

Hassan Mesfer ICT-TE-7

Modulation• Modulation is the process of modifying one of the parameters of a

signal, called carrier, according to the message signal.• Why use modulation:

• Ease of radiation• If we wish to throw a piece of paper (baseband signal), it cannot go too far by

itself. • By wrapping it around a stone (carrier), it can be thrown over a longer distance

• Simultaneous transmission of several signals• FDM (Frequency Division Multiplexing) uses analog modulation• TDM (Time Division Multiplexing) uses pulse modulation

• Reduce noise and interference• Wideband noise reduction

• For frequency assignment• Tune of radio or TV set to a particular station

• Hardware design• To overcome hardware limitations• For certain range of frequencies is easier to design circuits

20/04/23 2TTC Riyadh, ICT–BVF–4.1

Modulation

• There are different types of modulation:• Analog

• Digital

• Pulse modulation

• Digital baseband modulation – line coding

• Analog modulation• Message signal is analog

• Carrier is a sinusoidal signal

• Types of analog modulation:• AM (Amplitude Modulation),• FM (Frequency Modulation)• PM (Phase Modulation)


Modulation• Digital modulation

• Message signal is digital

• Carrier is a sinusoidal signal

• Types of digital modulation: ASK (Amplitude Shift Keying), Multilevel ASK FSK (Frequency Shift Keying), MFSK PSK Phase Shift Keying), QPSK, QAM

• Pulse modulation• Message signal is analog

• Carrier is a train of pulses

• Types of digital modulation: PAM (Pulse Amplitude Modulation) PDF (Pulse Duration Modulation) or PWD (Pulse Width Modulation) PPM (Pulse position Modulation) PCM (Pulse Code Modulation), DPCM (Differential PCM) DM (Delta Modulation), Adaptive DM


• Analog modulation is the process of modifying one of the parameters of a sinusoidal signal, called carrier, according to variation in the amplitude of message signal.

• The carrier signal has a higher frequency than the message signal.

• The spectrum of the message signal is transferred into the higher-frequency band.

• Demodulation is the process of extracting the message signal from the modulated signal.

ModulatorMessage

Carrier

Modulated signal

Demodulator Message

Carrier

Modulated signal

20/04/23 5TTC Riyadh, ICT–BVF-4.1

Analog Modulation

Types of Analog Modulation

Frequency Modulation (FM)

Phase Modulation (PM)

• By changing one of the parameters: amplitude, frequency, or phase of a sinusoidal carrier signal uc(t) = Uc cos(2ft +) ,

according to variation in the amplitude of message signal, the

following analog modulation types are generated.• Amplitude - Amplitude modulation• Frequency - Frequency modulation• Phase - Phase Modulation


Types of Amplitude Modulation AM (Double Side Band with Carrier) DSB-AM (Double Side Band Suppressed Carrier– DSBSC)

BW = 2W = 2 * BW of the message signal SSB-AM(Single Side Band – AM)

BW = W VSB-AM(Vestigial Side Band – AM)

BW = W ~ 2W

Amplitude Modulation


AM is also known as “Linear modulation” Small bandwidth Power inefficient Applications

AM radio, TV video broadcasting(VSB), point-to-point communications (SSB), transmission of many telephone channels over microwave links

• The amplitude of the carrier signal is varied according to the message signal.• The frequency of the modulated signal remains same as the carrier frequency


Carrier signal

Modulating signal

Amplitude modulated signal

cA

cA

mA

mA

c mA A

c mA A


• Modulating signal

– - Maximum amplitude of the modulating signal

– - Frequency of the modulating signal

• Carrier signal

• Amplitude modulated signal

( ) cos( )m mm t A t

mA

m

cos( )c cA t

AM( ) [ cos( )]cos( )c m m cx t A A t t

AM( )Maxx t m cA A

AM( )Minx t m cA A



• By rearranging the equation

• We get

• Take where is the modulation index.

• Amplitude modulated signal is:

Modulation Index (Degree of Modulation)

AM( ) [ cos( )]cos( )c m m cx t A A t t

1AM( ) [ cos( )]cos( )mc m c

c

Ax t A t tA

m cA A /

1AM( ) [ .cos( )]cos( )c m cx t A t t


• The maximum occurs when (100% modulation)

• Once the modulation index is higher than 1 (i. e. ), over modulation.

m cA A .

m cA A

Figure : (a) 100% modulation. (b) Over modulation.

(a) (b)

Phase reversal

1

Modulation Index


AM Frequency Spectrum & Bandwidth• Assume again that the modulating (message) signal is a

cosinusoidal signal of frequency fm

• Then, the amplitude modulated signal:

Consists of three components:• unmodulated carrier and two sidebands, located around the

carrier frequency fc

• upper sideband fc + fm and lower sideband fc – fm.

• the bandwidth of the amplitude modulated signal is 2fm



Efficiency of AM Signal

• The efficiency of the amplitude modulated signal:

is given by:



Power/sidebandEfficiency ( ) =

Total power transmitted

The output voltage of a transmitter is given by 500(1 + 0.4 cos 3140t)cos 2π x105t. Calculate:

a. Carrier frequency.b. Modulating frequency.c. Bandwidth of the modulated signal.

Solution: On the white bord

Example of AM


• Normally, the carrier does not convey any information.

• To increase the efficiency of amplitude modulation, the carrier can be suppressed. Therefore, the efficiency of AM can be increased up to 50%.

• However, the bandwidth of the DSB-SC remains same as amplitude modulated signal bandwidth (i.e. 2W).

Double-Sideband Suppressed Carrier (DSB-SC)

DSB-SC( ) . ( ).cos( )c cx t A m t t

DSB-SC Signal

cc

DSB-SC( )x t DSB-SC( )X


• Using an analog multiplier chip (for low power applications in the labs)

Generation of DSB-SC

( )m t

.cos( )c cA t

DSB-SC( )x t

DSB-SC( ) . ( ).cos( ) c cx t A m t t


Comparison – Different AM Schemes

Bandwidth Efficiency

AM(DSB + Carrier)

2W <25%

DSB-SC(No carrier)

2W 50%

SSB(No carrier)

W 100%


• Demodulation is the process of extracting the message signal from the modulated signal.

• Two types– Multiplication detector (coherent detector)

• More complicated• More expensive

– Envelope detection (Diode detection)• Simple• Cheaper

AM Demodulation


• To demodulate, the double sideband modulated signal is multiplied with the reference signal in time domain.

• As a reference signal a sinusoid with the same frequency and phase of the carrier signal is used.

Coherent Demodulation

AM( )x t

.cos( )cK t

LPF

(coherent carrier )

( )m t

LPF


• No reference signal is needed.

• Once the amplitude modulated signal is passed through the diode (D), it is rectified by a half.

• The capacitor is used to remove the high frequency carrier in the positive detected half.

Envelope Demodulation

AM( )x t

D

RC ( )m t


• Non-linear modulation scheme.

• Either the phase or the frequency of the carrier signal is varied according to the message signal.

• The general form of angle modulated signal is

Angle Modulation



Angle Modulation

c cx t A t t FM/PM( ) .cos ( ) .


• The phase of the modulated signal is directly proportional to the modulating signal.

• Therefore, the phase modulated signal can be expressed as

where is the peak phase deviation

• In PM modulation index μ is equal to: μ = K Am = ΔΦ


c cx t A t m t PM( ) .cos . ( ) .

t m t ( ) ( )

0180 | | .


• For the phase modulated signal, maximum or minimum frequency deviations takes place near zero crossings of the modulating signal.

Carrier signal

Modulating signal

Phase modulated signal

Zero crossings


Demodulation of PM• In phase modulation the message is contained in

the amount and rate of phase shift in a carrier wave.

• Because of demodulation reasons, the frequency of m(t) is always kept much smaller than that of the carrier signal.

• Main advantages of PM are improved signal to noise ratio and less radiated power

20/04/2324TTC Riyadh, ICT–BVF-4.1

• In frequency modulation, the amplitude of the modulated carrier signal is kept constant while its frequency is varied by the modulating message signal.

• The carrier frequency is controlled at each instant by the voltage of the modulating signal.

• The frequency of the modulated signal is increased if the input signal is positive, whereas the frequency is reduced if the input signal is negative.



• For the frequency modulation, the phase of the modulated signal can be modified as

• Then, the frequency modulated signal is

where is the peak frequency deviation.

• Modulation index in FM

2t

c cx t A t f m t dt

FM( ) .cos ( ). .

2t

t f m t dt

( ) ( ).

f

( )

2

Peak frequency deviation

Modulating frequency m

f



Example 1

For a given FM signal eFM(t) = 12cos(16 108 t + 5sin1250t) find the:

a.Carrier frequencyb.Modulating frequencyc.Modulation indexd.Maximum frequency deviation

Example 2

The output voltage of a transmitter is given by 5cos (2π x108t + 25 sin 2π x103t). Calculate:

a. Carrier frequency.b. Modulating frequency.c. Frequency deviation Δf

Example 3Carrier with an amplitude of 5 V and a frequency of 100 MHz is frequency

modulated (FM) by a cosinusoidal signal of 2 kHz. If frequency deviation is Δf = 75 kHz write the expression of the FM signal.

Examples FM


• For FM, maximum or minimum frequency deviation takes place at negative or positive peaks of the modulating signal.

Carrier signal

Modulating signal

Frequency modulated signal

Positive peak

Negative peak



Narrow Band Frequency Modulation (NBFM)• For small values of the frequency modulation index ( <<1), we have Narrow

Band Frequency Modulation (NBFM).

Wide Band Frequency Modulation (NBFM)• As the modulation index increases, the signal occupies more bandwidth. • In this case the modulation scheme is called Wide Band Frequency

Modulation (WBFM).

Frequency modulated signals are classified into two categories based on the value of modulation index

mm

mff f

f

f

Ak

KfAm - frequency deviation

fm – Modulated Signal Frequency

Frequency Modulation Index β


Frequency Modulation Index


FM Demodulation

• The frequency demodulator should produce an output voltage with instantaneous amplitude that is directly proportional to the instantaneous frequency of the input frequency modulated signal.

• Thus, a frequency demodulator is a frequency-to-amplitude converter circuit

• Various techniques are used to demodulate FM signal: – Slope detection

– Zero-crossing detection

– Phase locked discrimination

– Quadrature detection


Comparison of AM and FM

Amplitude Modulation Frequency Modulation

Narrow bandwidth Larger bandwidth (more fidelity)

Less efficient (Most of the power in the carrier)

All the transmitted power is used

More noise interference Minimum noise interference

Adjacent channel interference Adjacent channel interference can be avoided by using wide frequency spectrum

Operates in MF and HF range Operates in VHF and UHF range

One carrier and two sidebands Infinite number of sidebands

Simple transmitter Complex transmitter


Comparison of FM and PM

Frequency Modulation Phase Modulation

Maximum frequency deviation depends on amplitude of modulating voltage and modulating frequency.

Maximum phase deviation only depends on amplitude of modulating.

Frequency of the carrier is modulated according to the message.

Phase of the carrier is modulated according to the message.

Modulation index is increased when the frequency is reduced and vice versa.

No change in the modulation index with the changes in modulating frequency.

20/04/23 33TTC Riyadh, ICT–BVF– 4.1

ICT–BVF–4.1 Lecture 6 Analog Modulation

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