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Which of the following modulation techniques uses two carriers?

Which of the following modulation techniques uses two carriers? Correct Answer QAM

QAM uses two carrier signals which are in quadrature

Quadrature here means out phase by 90°

B-ASK, B-PSK all use a single carrier.

B-FSK use two carriers of very high-frequency carriers which are closely related

The constellation diagram for QAM is shown in the figure

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ASK(Amplitude Shift Keying):

In ASK (Amplitude shift keying) binary ‘1’ is represented with the presence of a carrier and binary ‘0’ is represented with the absence of a carrier:

For binary ‘1’ → S1 (t) = Acos 2π fct

For binary ‘0’ → S2 (t) = 0

The Constellation Diagram Representation is as shown:

[ src="//storage.googleapis.com/tb-img/production/19/10/F2_S.B_Madhu_21.10.19_D%204.png" style="width: 150px; height: 142px;"> 

where ‘I’ is the in-phase Component and ‘Q’ is the Quadrature phase.

PSK(Phase Shift Keying):

In PSK (phase shift keying) binary 1 is represented with a carrier signal and binary O is represented with 180° phase shift of a carrier

For binary ‘1’ → S1 (A) = Acos 2π fct

For binary ‘0’ → S2 (t) = A cos (2πfct + 180°) = - A cos 2π fct

The Constellation Diagram Representation is as shown:

[ alt="F2 S.B Madhu 21.10.19 D 5" src="//storage.googleapis.com/tb-img/production/19/10/F2_S.B_Madhu_21.10.19_D%205.png" style="width: 155px; height: 145px;">

FSK (Frequency Shift Keying):

In FSK (Frequency Shift Keying) binary 1 is represented with a high-frequency carrier signal and binary 0 is represented with a low-frequency carrier, i.e. In FSK, the carrier frequency is switched between 2 extremes.

For binary ‘1’ → S1 (A) = Acos 2π fHt

For binary ‘0’ → S2 (t) = A cos 2π fLt . The constellation diagram is as shown:

[ alt="F2 S.B Madhu 21.10.19 D 6" src="//storage.googleapis.com/tb-img/production/19/10/F2_S.B_Madhu_21.10.19_D%206.png" style="width: 208px; height: 211px;">

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