Analog Transmission

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Analog Transmission

• Chapter 5

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5-1 DIGITAL-TO-ANALOG CONVERSION
Digital-to-analog conversion is the process of
changing one of the characteristics of an analog
signal based on the information in digital data.
Topics discussed in this section
Aspects of Digital-to-Analog ConversionAmplitude
Shift KeyingFrequency Shift Keying Phase Shift
Keying Quadrature Amplitude Modulation
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Figure 5.1 Digital-to-analog conversion
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Figure 5.2 Types of digital-to-analog conversion
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Bit rate is the number of bits per second. Baud
rate is the number of signal elements per second.
In the analog transmission of digital data, the
baud rate is less than or equal to the bit rate.
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Example 5.1
An analog signal carries 4 bits per signal
element. If 1000 signal elements are sent per
second, find the bit rate.
Solution In this case, r 4, S 1000, and N is
unknown. We can find the value of N from
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Example

• Signal rate is 1000 baud, and the bit rate is
  8000bps. How many signal levels are needed?
• S N / r S signal rate, N is in bps.
• r N / S 8 bits/baud (signal)
• r log2(L)
• L 2r 256

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Digital-To-Analog

• ASK
• FSK
• PSK
• QAM (quadrature amplitude modulation)
• Modulation of Amplitude, Frequency, or Phase is
  known as shift keying.

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Binary ASK

• ASK is usually implemented using BASK, or on-off
  keying.
• The frequency and phase are unchanged.
• The amplitude is changed to represent data
  elements

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Figure 5.3 Binary amplitude shift keying
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Figure 5.4 Implementation of binary ASK
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Carrier Frequency

• A frequency (the carrier frequency) acts as the
  base for the information signal.
• The carrier frequency is chosen between the fmin
  and fmax of the bandwidth available.

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BASK Bandwidth

• B (1d) S
• S is the signal rate
• 0 lt d lt 1
• S lt B lt 2S
• d depends on the modulation and filtering process.

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Figure 5.3 Binary amplitude shift keying
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ASK Carrier Frequency

• The carrier frequency can be chosen to match the
  available bandwidth.
• It is possible to choose two (or more) carrier
  frequencies for full duplex communication.

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Figure 5.5 Bandwidth of full-duplex ASK used in
Example 5.4
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Multilevel ASK

• It is possible to have different levels for the
  information signal. However, amplitude is
  sensitive to noise, this makes discerning the
  amplitude differences more difficult.

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FSK Frequency Shift Keying

• Frequency is varied to represent data.
• Each signal element is represented by a different
  frequency. Amplitude and phase are not modified.
• Binary FSK would require two frequencies to
  represent the 0 and 1 data elements.

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Figure 5.6 Binary frequency shift keying
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Multilevel FSK

• A different frequency is used for each data
  element.

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Figure 5.8 Bandwidth of MFSK used in Example 5.6
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PSK Phase Shift Keying

• The phase of the carrier frequency is modified to
  represent different signal elements.
• Noise is less likely to impact phase, giving PSK
  an advantage over ASK.
• Uses only one frequency and requires less
  bandwidth compared to FSK.

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Binary PSK

• The two signal elements (0 and 1) modify the
  carrier signal to be 180 degrees out of phase.

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Figure 5.9 Binary phase shift keying
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QAM Quadrature Amplitude Modulation

• Combines ASK with PSK. 8-QAM is as follows
• A single frequency is divided into multiple
  carriers of different phase 0, 90, 180, 270
  degrees.
• Amplitude is modified on each carrier. .5 or 1
• Each data element is assigned according to a
  phase and amplitude.
• Applications include stereo AM radio.

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Figure 5.11 QPSK and its implementation
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Analog-to-Analog Conversion

• Amplitude Modulation
• Frequency Modulation
• Phase Modulation

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AM Amplitude Modulation

• The information signal is multiplied by the
  carrier signal to produce the modulated signal.
• Stations are assigned a carrier frequency between
  530 and 1700 KHz
• FCC assigns 10kHz bandwidth to each AM radio
  station.
• Stations are separated by 10kHz to avoid
  interference.

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Figure 5.16 Amplitude modulation
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Figure 5.17 AM band allocation
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FM Frequency Modulation

• The information signal is passed through a
  voltage-controlled oscillator to modify the
  carrier frequency according to the intelligence
  signal. The modified carrier frequency is the
  modulated signal.
• FM band in NA is from 88-108Mhz.
• Stations are assigned to odd tenths in the Mhz
  range, resulting in a 200kHz separation.
• FM stations broadcast a 15kHz bandwidth.

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Figure 5.18 Frequency modulation
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Figure 5.19 FM band allocation