05 - winter 2005 ece ece 766 computer interfacing and protocols 1 modulation conversion of digital...

Winter 2005

ECE

ECE 766Computer Interfacing and Protocols

105 -

ModulationModulation

• Conversion of digital information to analog signals – Example: Telephone lines are band-limited

(300-3300Hz)

• Modem = modulator/demodulator• Characteristics of defining sine waves:

– Amplitude– Frequency– Phase

Change one or more of them to send information

Winter 2005

ECE


205 -

ModulationModulation

• Modulation Techniques– Amplitude Shift Keying (ASK)– Frequency Shift Keying (FSK)– Phase Shift Keying (PSK)

• Aspects of Modulation– Bit rate vs. Baud rate

Bit rate ≥ Baud rate– Carrier Signal

Quadrature Amplitude

Modulation (QAM)

Winter 2005

ECE


305 -

Amplitude Shift Keying (ASK)Amplitude Shift Keying (ASK)

• Basic technique (like AM) change the amplitude of the signal to transfer logical values

• On-Off Keying (OOK)• Highly susceptible to noise• Inefficient

Amplitude

Time

10 10

1 baud 1 baud 1 baud 1 baud

1 second

Winter 2005

ECE


405 -

Amplitude Shift Keying (ASK)Amplitude Shift Keying (ASK)

• Bandwidth requirement

• BW = (1+d) x Nbaud

• d ≥ 0 related to the condition of the line

Minimum Bandwidth = Nbaud

fcfc-(Nbaud/2) fc+(Nbaud/2)

Frequency

Amplitude

Winter 2005

ECE


505 -

Frequency Shift Keying (FSK)Frequency Shift Keying (FSK)

• Represent each logical value with another frequency (like FM)

• Not susceptible to noise as much as ASK• Limiting factor: Physical capabilities of the

carrier

Amplitude

Time

10 10

1 baud 1 baud 1 baud

1 second

1 baud

Winter 2005

ECE


605 -

Frequency Shift Keying (FSK)Frequency Shift Keying (FSK)

• Bandwidth requirement

• How about duplex communication?

Nbaud/2

fc0

Frequency

Amplitude

fc1

fc1--fc0Nbaud/2

BW=fc1--fc0+Nbaud

Winter 2005

ECE


705 -

Phase Shift Keying (PSK)Phase Shift Keying (PSK)

• Use different phased signals to represent binary values– 0 = in phase with reference– 1 = out of phase with reference

• Not practical

Amplitude

Time

10 10


1 second

1 baud

Reference

Winter 2005

ECE


805 -


• Differential PSK– 0 = same phase as last signal element– 1 = 180º shift from last signal element

• Constellation Diagram:

Amplitude

Time

10 10


1 second

1 baud

1 0

Winter 2005

ECE


905 -


• Quadrature Phase Shift Keying (Q-PSK):Do not only use 180º shift

• Min. BW requirement: same as ASK!

• Self clocking (most cases)

10 00

01

11

4-PSK (Q-PSK)

100 000

010

110

011 001

111101

8-PSK

Winter 2005

ECE


1005 -

Quadrature Amplitude Modulation Quadrature Amplitude Modulation (QAM)(QAM)

• Combine ASK and PSK such that each signal corresponds to multiple bits

• More phases than amplitudes

• Minimum bandwidth requirement same as ASK or PSK

3 amplitudes12 phases

05 - winter 2005 ece ece 766 computer interfacing and protocols 1 modulation conversion of digital...

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