Digital Modulation Schemes GATE Notes | EduRev

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GATE : Digital Modulation Schemes GATE Notes | EduRev

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Digital Modulation Schemes

This is possible to transmit the analog signal i.e., speech, video etc, in digital format. Some digital modulation schemes are given below

  • Digital Carrier Modulation: Commonly used digital modulation schemes are Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK).
  • Amplitude Shift Keying (ASK): The amplitude of a high-frequency carrier is varied in accordance with digital data (0 or 1).

Digital Modulation Schemes GATE Notes | EduRev

S(t) = Ac cos 2πfct; 0 ≤ t ≤ Tb

= 0; otherwise

Digital Modulation Schemes GATE Notes | EduRev

Digital Modulation Schemes GATE Notes | EduRev

Bandwidth = 2 × 1/Tb

= 2 × bit rate

  • For digital input 1 amplitude level is high and for digital input 0 amplitude level is low.
  • Signalling used is on-off signalling.

Demodulation of ASK:

Digital Modulation Schemes GATE Notes | EduRev

For binary digit 1, Ac cos 2π fct × Ac cos 2πfct = (A2c /2)[1 + cos 4πfct]

Output of LPF = (A2/2)

For binary digit 0 output of LPF = 0

In ASK, probability of error (Pe) is high.

In ASK, SNR is less.

Phase Shift Keying (PSK):

In phase shift, keying phase of high-frequency carrier is varied in accordance with digital data 1 or 0.

Digital Modulation Schemes GATE Notes | EduRev

  • NRZ signalling is used.

S(t) = Ac cos 2πfct for bit 1

= – Ac cos 2pfct for bit 0

Digital Modulation Schemes GATE Notes | EduRev

A frequency of the carrier must be a multiple of a bit rate.

Tb = n/fc

Fc = nrb

  • In case of PSK, a probability of error is less.
  • In case of PSK, SNR is high.
  • Mainly used a technique in wireless transmission.

Frequency Shift Keying (FSK):

In frequency shift keying, a frequency of the carrier is varied in accordance with digital data (1 or 0).

For digital data 1 we use frequency f1 and for digital data 0 we use frequency f2.

Digital Modulation Schemes GATE Notes | EduRev

  • NRZ signalling is used here
  • VCO The schematic diagram of VCO is given below

Digital Modulation Schemes GATE Notes | EduRev

Digital Modulation Schemes GATE Notes | EduRev

Digital Modulation Schemes GATE Notes | EduRev

Bandwidth = 2Δf + 2fm

Digital Modulation Schemes GATE Notes | EduRev

Bandwidth = f1 + (1/Tb) – f2 + (1/Tb)

= f1 – f2 + (2/Tb); f1 – f2 = 2Δf

Key Points

  • In case of FSK, Pe is less but SNR is high.
  • Multiplexing is difficult in FSK.

Differential Phase Shift Keying (DPSK): 

In PSK it needs a complicated synchronising circuit at the receiver, this disadvantage of PSK is removed in DPSK.

Digital Modulation Schemes GATE Notes | EduRev

Digital Modulation Schemes GATE Notes | EduRev

A cos ω0t = ± A cos ω0t

Note: Advantage of DPSK over PSK is, DPSK does not require a coherent carrier for demodulation.

Comparison of Digital Modulation Schemes

Digital Modulation Schemes GATE Notes | EduRev

Quadrature Amplitude Modulation (QAM): 

In QAM, digital information is content in a both amplitude and phase of the signal. It is used in both digital modulation scheme and analog modulation scheme. Digital cable television and in cable modem applications QAM is used.

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