COMMUNICATION SYSTEM EEEB453Chapter 2

AMPLITUDE MODULATION

Dept of Electrical EngineeringUniversiti Tenaga Nasional

2

Comment on AM-DSBFC

Both modulator and demodulator have simple structure (low cost and reliable)

DSB is wasteful of Power (carrier does not carry any information)

DSB is wasteful of Bandwidth (B vs 2B)

3

DSBFC is wasteful of Power 2/3 of total transmitted power taken up by carrier.

P

ower

Frequency

Pc = 1000W

Plsb = 160W Pusb = 160W

flsb fusb fc

The total power being transmitted is (1000).(1 + 0.82) = 1320W 2

In transmitting 1320W of the total power, the carrier contains 1000W and does not contain any information being transmitted. The side freq each have 160W and each carries a copy of the same info signal.

So, 1320W is being used in order to transmit only 160W.

4

DSB is wasteful of Bandwidth

DSB has a Wide Bandwidth wasteful BW usage i.e info in USB = info in LSB

If so much of the transmitted wave is not required, then why transmit it?

ANY ALTERNATIVE?

DOUBLE SIDEBAND SUPPRESSED CARRIER?

5

DSB Suppressed Carrier (DSBSC)

Generated by circuit called balanced modulator where it produces sum (fusb) and difference (flsb) freq but cancel or balance out the carrier (fc).

Pow

er

Frequency

Plsb = 160W Pusb = 160W

flsb fusb fc

The total power being transmitted is now reduced to 320W

No Carrier

DSBSC helps in reducing power but bandwidth still the same as DSBFC.

6

Single Side Band (SSB) System

Motivation: Both DSBFC and DSBSC occupy a bandwidth of 2B. How can we reduce the bandwidth requirements?

Due the symmetric condition (info in USB = info in LSB), one of the sidebands is sufficient to provide the complete information in the original signal.

Pow

er

Frequency

Pusb = 160W

flsb fusb fc

No Carrier No lsf

The total power being transmitted is now only 160W

7

Single Side Band (SSB) System AM Single Sideband Full Carrier (SSBFC)

Carrier is transmitted at full power with only one of the sidebands.

Half as much bandwidth will be required (BWSSBFC=1/2BWDSBFC).

R

VPc

2

Am

plit

ude Modulating

signal, fm 4

2mPP clsb

4

2mPP cusb

2

2c

ctPm

PP

f

DSBFC

Am

plit

ude Modulating

signal, fm 0lsbP

4

2mPP cusb

4

2c

ctPm

PP

f

SSBFC R

VPc

2

8

Single Side Band (SSB) System AM Single Sideband Suppressed Carrier (SSBSC)

The carrier is totally removed together with one of the sidebands.

Half as much bandwidth is required ( BWSSBFC=1/2BWDSBFC).

Am

plit

ude Modulating

signal, fm 0lsbP

4

2mPP cusb 4

2c

tPm

P

f

SSBSC

0cP

9

Single Side Band (SSB) System AM Single Sideband Reduced Carrier (SSBRC)

Conserve BW and considerably power One sideband is totally removed and carrier voltage is reduced to approx. 10 % of its

unmodulated amplitude or carrier power is reduced to approx. 1% of its unmodulated power The carrier is totally suppressed during modulation and to be reinserted at reduced

amplitude for the purpose of demodulation

Am

plit

ude Modulating

signal, fm 0lsbP

4

2mPP cusb 4

01.02

cct

PmPP

f

SSBRC

RVP cc /)1.0( 2

10

Single Side Band (SSB) System AM Independent Sideband (ISB)

It is a form of DSB transmission in which the transmitter consists of two independent SSBSC modulators.

Output consists of two totally independent sidebands each of different information, with suppressed carrier.

It conserves both power and BW as two info sources are transmitted within the same freq spectrum.

Modulating signal, fm

4

2mPP cusb 2

01.02

cct

PmPP

f

ISB

RVP cc /)1.0( 2

4

2mPP clsb

Ch A Ch B

11

Single Side Band (SSB) System AM Vestigal Sideband (VSB)

The carrier and one complete SB are transmitted, but only part (a vestige) of the second SB is transmitted. The carrier is transmitted at full power (Carrier and full 1st SB & part of 2nd SB).

The BW is typically 25% greater than that of SSBSC.

R

VPc

2

Am

plit

ude Modulating

signal, fm usblsb PP 4

2mPP cusb

lsbc

ct PPm

PP 4

2

f

VSB

12

DSBFC AM Wave

13

SSBFC AM Wave PEAK CHANGE IN THE ENVELOPE IS HALF THAT OF THE DSB WAVE (ONLY ONE SIDEBAND)

100% modulated SSBFC wave with a single frequency modulating wave

14

SSBSC AM Wave THE WAVEFORM IS NOT AN ENVELOPE; IT IS A SINE WAVE AT A SINGLE FREQUENCY EQUAL TO THE CARRIER FREQUENCY PLUS/MINUS THE MODULATING SIGNAL FREQUENCY

15

ISB AM Wave WAVE IS SIMILAR TO A DSBSC WAVE BUT WITH A REPETITION RATE TWICE THAT OF THE MODULATING SIGNAL FREQUENCY

16

Advantages of SSB Transmission

Bandwidth conservation - Only half the bandwidth is required.

Power conservation - Only one sideband with carrier removed or suppressed. Hence total transmitted power will be less. This allows smaller transmitters to be used.

Selective fading - In double sideband, the two sidebands may experience different impairments as the propagate along different paths in the medium. This could result in carrier phase shift. This cannot happen if only one sideband is transmitted.

Noise Reduction - Thermal noise is reduced to half, because the bandwidth is also half.

17

Disadvantages of SSB Transmission

Complex receivers – Require more expensive receivers because envelope detection cannot be used

Tuning Difficulties - More difficult to tune than conventional AM receivers. Receivers need a precise tuning.

18

AM Modulation

Carrier Signal Amplitude = 15V

Frequency = 500kHz

Modulating Signal Amplitude = ±6.5V Frequency = 5kHz

Modulated Output

Revision

Refer to Figure above, determinea. Upper and lower side frequencies [1 marks]b. Modulation coefficient and percent modulation [1 marks]c. Peak amplitude of the modulated carrier and the upper and lower side

frequency voltages [2 marks]d. Maximum and minimum amplitude of the envelope [2 marks]e. Expression of the modulated wave [1 marks]f. The total average power required for a load resistance of 10Ω [2 marks]g. Efficiency

If the waveform goes further modification,a. Find the total average power required for transmitting a DSBSC wave [1

marks]b. Find the total average power required for transmitting a SSBSC wave [1

marks]

Briefly explain two disadvantages of SSBSC compared to DSBSC [2 marks]