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ALP CBT 2 Electronic Mechanic Previous Paper: Held on 21 Jan 2019 Shift 2

Option 1 : QAM

RRB ALP 2018 Full Test (Stage II - Part A)

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**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

__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:

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:

__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: