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Chapter 5 AM, FM, and Digital Modulated Systems Amplitude Modulation (AM) Double Sideband Suppressed carrier (DSSC) Assymetric Sideband Signals Single sideband signals (SSB) Frequency Division Multiplexing (FDM). Huseyin Bilgekul Eeng360 Communication Systems I

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Presentation Transcript
slide1

Chapter 5

  • AM, FM, and Digital Modulated Systems
  • Amplitude Modulation (AM)
  • Double Sideband Suppressed carrier (DSSC)
  • Assymetric Sideband Signals
  • Single sideband signals (SSB)
  • Frequency Division Multiplexing (FDM)

Huseyin Bilgekul

Eeng360 Communication Systems I

Department of Electrical and Electronic Engineering

Eastern Mediterranean University

slide2

Bandpass Signaling Review

Where

Where

  • The modulated bandpass signal can be described by

Modulation Mapping function:Convert m(t) →g(t) Ref :Table4-1

  • The voltage spectrum of the bandpass signal is
  • The PSD of the bandpass signal is
slide3

Amplitude Modulation

  • The Complex Envelope of an AM signal is given by

Ac indicates the power level of AM and m(t) is the Modulating Signal

  • Representation of an AM signal is given by
  • Ac[1+m(t)] In-phase component x(t)
  • If m(t) has a peak positive values of +1 and a peak negative value of -1

AM signal  100% modulated

  • Envelope detection can be used if % modulation is less than 100%.
slide4

Amplitude Modulation

An Example of a message signal m(t)

Waveform for Amplitude modulation of the message signal m(t)

slide5

Amplitude Modulation

B

An Example of message energy spectral density.

Carrier component together with the message

2B

Energy spectrum of the AM modulated message signal.

slide6

AM – Percentage Modulation

  • Definition: The percentage of positive modulation on an AM signal is
  • The percentage of negative modulation on an AM signal is
  • The percentage of overall modulation is

If m(t) has a peak positive values of +1 and a peak negative value of -1

AM signal  100% modulated

slide7

AM Signal Waveform

% Positive modulation= 50%

% Negative modulation =50%

Overall Modulation = 50%

Amax = 1.5Ac

Amin = 0.5 Ac

slide8

AM – Percentage Modulation

Under modulated (<100%)

100% modulated

Over Modulated (>100%)

Envelope Detector

Can be used

Envelope Detector

Gives Distorted signal

slide9

AM – Normalized Average Power

The normalized average power of the AM signal is

If the modulation contains no dc level, then

The normalized power of the AM signal is

Discrete Carrier Power

Sideband power

slide10

AM – Modulation Efficiency

  • Definition : The Modulation Efficiency is the percentage of the total power of the modulated signal that conveys information.

Only “Sideband Components” – Convey information

Modulation Efficiency:

Highest efficiency for a 100% AM signal : 50% - square wave modulation

Normalized Peak Envelope Power (PEP)of the AM signal:

Voltage Spectrum of the AM signal:

Unmodulated Carrier Spectral Component

Translated Message Signal

slide11

The peak envelope power (PEP) is

Example 5-1. Power of an AM signal

Suppose that a 5000-W AM transmitter is connected to a 50 ohm load;

Without

Modulation

Then the constant Acis given by

If the transmitter is then 100% modulated by a 1000-Hz test tone , the total (carrier + sideband) average power will be

The peak voltage (100% modulation) is (2)(707) = 1414 V across the 50 ohm load.

The modulation efficiency would be 33% since < m2(t) >=1/2

double side band suppressed carrier dsbsc

Carrier Power

Sideband power

Spectrum 

Modulation Efficiency 

Double Side Band Suppressed Carrier (DSBSC)
  • Power in a AM signal is given by
  • DSBSC is obtained by eliminating carrier component
  • If m(t) is assumed to have a zero DC level, then

Power 

  • Disadvantages of DSBSC:
    • Less information about the carrier will be delivered to the receiver.
    • Needs a coherent carrier detector at receiver
slide13

DSBSC Modulation

B

An Example of message energy spectral density.

No Extra Carrier component

2B

Energy spectrum of the DSBSC modulated message signal.

slide14

Carrier Recovery for DSBSC Demodulation

  • Coherent reference for product detection of DSBSC can not be obtained by the use of ordinary PLL because there are no spectral line components at fc.
slide15

Carrier Recovery for DSBSC Demodulation

  • A squaring loop can also be used to obtain coherent reference carrier for product detection of DSBSC. A frequency divider is needed to bring the double carrier frequency to fc.
single sideband ssb modulation

LSSB

USSB

Single Sideband (SSB) Modulation
  • An upper single sideband (USSB) signal has a zero-valued spectrum for
  • A lower single sideband (LSSB) signal has a zero-valued spectrum for
  • SSB-AM – popular method ~ BW is same as that of the modulating signal.

Note: Normally SSB refers to SSB-AM type of signal

single sideband signal

–Hilbert transform of m(t) 

Where

and

H(f)

j

f

-j

Single Sideband Signal
  • Theorem :A SSB signal has Complex Envelopeand bandpass form as:

Upper sign (-)  USSB

Lower sign (+)  LSSB

Hilbert Transform corresponds to a -900phase shift

single sideband signal18

Using

Recall from Chapter 4

Single Sideband Signal

Proof: Fourier transform of the complex envelope

Upper sign  USSB

Lower sign  LSSB

Upper sign  USSB

If lower signs were used  LSSB signal would have been obtained

ssb power
SSB - Power

The normalized average power of the SSB signal

Hilbert transform does not change power.

SSB signal power is:

Power of the modulating signal

Power gain factor

The normalized peak envelope (PEP) power is:

slide21

Generation of SSB

SSB signals have bothAM and PM.

The complex envelope of SSB:

For the AM component,

For the PM component,

Advantages of SSB

  • Superior detected signal-to-noise ratio compared to that of AM
  • SSB has one-half the bandwidth of AM or DSB-SC signals
generation of ssb
Generation of SSB
  • SSB Can be generated using two techniques
      • Phasing method
      • Filter Method
  • Phasing method

This method is a special modulation type of IQ canonical form

of Generalized transmitters discussed in Chapter 4 ( Fig 4.28)

generation of ssb23
Generation of SSB
  • Filter Method

The filtering method is a special case in which RF processing (with a

sideband filter) is used to form the equivalent g(t), instead of using

baseband processing to generate g(m) directly. The filter method is the

most popular method because excellent sideband suppression can be

obtained when a crystal oscillator is used for the sideband filter.

Crystal filters are relatively inexpensive when produced in quantity at

standard IF frequencies.