AM SCHEME in COMMUNICATION, Slides of Communication

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AMPLITUDE

MODULATION SCHEMES

RABEJE, Robert Jay L.

YOUNG, Rajo Zachiel M.

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 ).

Power Frequency Plsb = 160W Pusb = 160W flsb fc fusb The total power being transmitted is now reduced to 320W No Carrier

 DSBSC helps in reducing power but bandwidth still the

same as DSBFC.

Single Side Band (SSB) System

 (^) Motivation: Both DSBFC and DSBSC occupy a bandwidth of 2 B. 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. Power Frequency Pusb = 160W flsb fc fusb No Carrier No lsf The total power being transmitted is now only 160W

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 ( BW

SSBFC=1/2BWDSBFC).

Amplitude Modulating signal, fm Plsb  0 4 2 m Pusb  P (^) c 4 2 c t m P P  f SSBSC Pc 0

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 Amplitude Modulating signal, fm Plsb  0 4 2 m Pusb  P (^) c^4

2 c t c m P P  P  f SSBRC Pc ( 0. 1 Vc) / R 2 

DSBFC AM Wave

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

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.

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.

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 2 B )

DSBFC is wasteful of Power



2/3 of total transmitted power taken up by carrier.

Power Frequency Pc = 1000W Plsb = 160W Pusb = 160W flsb fc fusb The total power being transmitted is (1000).(1 + 0. 2 ) = 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.

Summary