Analog data to analog signal (AM, FM, PM) Digital data to analog signal (ASK, FSK, BPSK, QAM) ...

Analog data to analog signal(AM, FM, PM)

Digital data to analog signal(ASK, FSK, BPSK, QAM)

Analog data to digital signal(PCM, DM)

Digital data to digital signal(line codes)

Signal Encoding Techniques (modulation and encoding)

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Analog Signals

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Digital Signals

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Analog and Digital Transmission

AMPLITUDE MODULATION

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Modulation The process by which some characteristics of a

carrier wave is varied in accordance with an information-bearing signal.

Continuous-wave modulation Amplitude modulation Frequency modulation

AM modulation family Amplitude modulation (AM) Double sideband-suppressed carrier (DSB-SC) Single sideband (SSB) Vestigial sideband (VSB)

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AMPLITUDE MODULATION

1. DEFINING AM A carrier wave whose amplitude is varied

in proportion to the instantaneous amplitude of a modulating voltage

2. GENERATING THE AM nonlinear device: diode or transistor biased in

its nonlinear region

Carrier wave: is a waveform (usually sinusoidal) that is modulated (modified) with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal. 

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3. ANALYSIS OF THE AM WAVE tfVv ccc 2sin

tffVm

tffVm

tfVv sccscccc )(2cos2

)(2cos2

2sin

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4. Different Carriers and AM

Carriers are spaced at 20 kHz, beginning at 100kHz. Each carrier is modulated by a signal with 5kHz bandwidth. Is there interference from sideband overlap?

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5. MODULATION INDEX AND SIGNAL POWER

minmax

minmax

VV

VV

V

Vm

c

m

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Moduiation Index and Power

R

V

R

V

R

VP cccarrc 2

)2/( 222

42

2m

R

VPP cUSBLSB

)2

1()2

1(2

222 mP

m

R

VP c

CT

21

P

P 2

c

T m )1(2

C

T

P

Pm

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Current Calculations

21

2m

I

I

c

T

ExampleA carrier of 1000 W is modulated with a resulting modulation index of 0. 8. What is the total power?

What is the carrier power if the total power is 1000 W and the modulation index is 0.95?

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6.2 Double Sideband Suppressed Carrier (DSBSC)

When the carrier is reduced, this is called double-sideband suppressed-carrier AM, or DSB-SC. If the carrier could somehow be removed or reduced, the transmitted signal would consist of two information-bearing sidebands, and the total transmitted power would be information

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6.3 Single-Sideband (SSB) suppressing the carrier and one of the sidebands

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6.4 Filtering the SSB LSB or USB Dual Conversion: up-converting the mod ulating

frequency twice and selecting the upper or lower sideband for transmission.

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AM: Features and Drawbacks:

the AM signal is greatly affected by noise

impossible to determine absolutely the original signal level

conventional AM is not efficient in the use of transmitter power

AM is useful where a simple, low-cost receiver and detector is desired

Angle Modulation

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ANGLE MODULATION:

The intelligence of the modulating signal can be conveyed by varying the frequency or phase of the carrier signal. When this is the case, we have angle modulation, which can be subdivided into two categories: frequency modulation (FM), and phase modulation (PM).

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Frequency Modulation. The carrier's instantaneous frequency deviation from its unmodulated value varies in proportion to the instantaneous amplitude of the modulating signal.

Phase Modulation. The carrier's instantaneous phase deviation from its unmodulated value varies as a function of the instantaneous amplitude of the modulating signal;

)sinsin( tmtAe mfccFM

)sinsin( ttAe mmccPM

23

FIGURE 4-1 The FM and PM waveforms for sine-wave modulation: (a) carrier wave; (b) modulation wave; (c) FM wave; (d) PM wave. (Note: The derivative of the modulating sine wave is the cosine wave shown by the dotted lines. The PM wave appears to be frequency modulated by the cosine wave.)

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MODULATION INDEX

modulation index for an FM signal

mf f

m

δ = maximum frequency deviation of the carrier caused by the amplitude of the modulating signal

fm = frequency of the modulating signal

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FREQUENCY ANALYSIS OF THE FM WAVE

where: eFm = the instantaneous amplitude of the modulated FM waveAc = the peak amplitude of the carrierJn = solution to the nth order Bessel function for a modulation index mf.

mf = FM modulation index, Δf/fm

etc ...,

]})3sin()3)[sin(({

]})2sin()2)[sin(({

]})sin())[sin(({

sin

3

2

1

0

ttmJA

ttmJA

ttmJA

tJAe

mcmcfc

mcmcfc

mcmcfc

ccFM

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Spectral components of a carrier of frequency, fc, frequency modulated by a sine wave with frequency fm

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FM signal characters• The FM wave is comprised of an infinite number of

sideband components

• bandwidth of an FM signal must be wider than that of an AM signal

• As the modulation index increases from mf = 0, the spectral energy shifts from the carrier frequency to an increasing number of significant sidebands.

• Jn(mf) coefficients, decrease in value with increasing order, n.

• negative Jn(mf) coefficients imply a 1800 phase inversion.

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Carrier Frequency Eigenvalues in some cases the carrier frequency component, JO,

and the various sidebands, Jn go to zero amplitudes at specific values of m. These values are called eigenvalues.

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Bandwidth Requirements for FM The higher the modulation index, the greater

the required system bandwidth

)(2 mfnBW where n is the highest number of significant (least 1%, or -40 dB; (20 log 1/100 ), of the voltage of the unmodulated carrier) sideband components and fm is the highest modulation frequency.

Carson's Rule

)1(2)(2 fmm mffBW

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Amplitude versus frequency spectrum for various modulation indices (fm fixed, & varying): (a) mf = 0.25; (b) mf = 1; (c) mf = 2; (d) mf = 5; (e) mf = 10.

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Copyright ©2001 by Prentice-Hall, Inc.Upper Saddle River, New Jersey 07458

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FIGURE 4-6 Commercial FM broadcast band.

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Commercial FM broadcast band

• The maximum permissible carrier deviation, δ, is ±75 kHz

• Modulating frequencies (voice or music) is ranging from 50 Hz to 15 kHz

• The modulation index can range from as low as 5 for fm = 15 kHz (75 kHz/15 kHz) to as high as 1500 for fm = 50 Hz (75 kHz/50 Hz).

• The ±75-kHz carrier deviation results in an FM bandwidth requirement of 150 kHz for the receiver.

• A 25-kHz guard band above and below the upper and lower FM sidebands.

• Total bandwidth of one channel is 200Hz.

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Narrowband FM (NBFM)

NBFM uses low modulation index values, with a much smaller range of modulation index across all values of the modulating signal.

An NBFM system restricts the modulating signal to the minimum acceptable value, which is 300 Hz to 3 KHz for intelligible voice.

10 to 15 kHz of spectrum.

Used in police, fire, and Taxi radios, GSM, amateur radio, etc.

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POWER IN THE FM WAVE

power of the unmodulated carrier

For a modulated carrierR

VP crmsT

2

R

V

R

V

R

V

R

V

R

V

PPPPPP

n

n

JJJJJ

JJJJJT

22222 2...

222

...

3210

3210

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FM NOISE

Increased bandwidth of an FM – to enhance the signal-to-noise ratio (SNR). Advantages of FM over AM.

To take this advantage, large mf is necessary– high order sidebands are important – wider bandwidth is required.

Phase Analysis of FM Noise

c

N

V

V1sin

where α = the maximum phase deviation of the carrier frequency caused by the noise

VN = noise voltage

Vc= carrier voltage

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Phasor addition of noise on an FM signal’s carrier frequency causes a phase shift, whose maximum value is .

Warren HiokiTelecommunications, Fourth Edition

Copyright ©2001 by Prentice-Hall, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.DCTC, By Ya Bao

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The ratio of carrier voltage to noise voltage, is the SNR (voltage)n

c

V

V

SNR

V

VSNR

N

c

1sin 1

α represents the equivalent modulation index produced by the noise.

mN f

NFMSNR

40

mN fSNR

• The effect of noise on an FM carrier signal is directly proportional to the modulation frequency fm.

• Increasing fm, degrades the

Voice, data, and music contain many frequencies, which are distributed throughout the given modulation passband. Therefore, the SNR is not uniform throughout. To maintain a flat SNR, some techniques are employed.

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