EE 551451, Fall, 2006 Communication Systems

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EE 551/451, Fall, 2006Communication Systems

• Zhu Han
• Department of Electrical and Computer Engineering
• Class 9
• Sep. 19th, 2006

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FM Modulator and Demodulator

• FM modulator
• Direct FM
• Indirect FM
• FM demodulator
• Direct use frequency discriminator
  (frequency-voltage converter)
• Ratio detector
• Zero crossing detector
• Indirect using PLL
• Superheterodyne receiver
• FM broadcasting and Satellite radio
• Next class FM in noisy channel, real
  implementation demo, projects

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FM Direct Modulator

• Direct FM
• Carrier frequency is directly varied by the
  message through voltage-controlled oscillator
  (VCO)
• VCO output frequency changes linearly with input
  voltage
• A simple VCO implemented by variable capacitor
• Capacitor Microphone FM generator

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FM Direct Modulator cont.

• Direct method is simple, low cost, but lack of
  high stability accuracy, low power application,
  unstable at the carrier frequency
• Modern VCOs are usually implemented as PLL IC
• Why VCO generates FM signal?

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Indirect FM

• Generate NBFM first, then NBFM is frequency
  multiplied for targeted ?f.
• Good for the requirement of stable carrier
  frequency
• Commercial-level FM broadcasting equipment all
  use indirect FM
• A typical indirect FM implementation Armstrong
  FM
• Block diagram of indirect FM

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Indirect FM cont.

• First, generate NBFM signal with a very small ß1

m(t)
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Indirect FM cont.

• Then, apply frequency multiplier to magnify ß
• Instantaneous frequency is multiplied by n
• So do carrier frequency, ?f, and ß
• What about bandwidth?

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Analysis of Indirect FM
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Armstrong FM Modulator

• Invented by E. Armstrong, an indirect FM
• A popular implementation of commercial level FM
• Parameter message W15 kHz, FM s(t) ?f74.65
  kHz.
• Can you find the ?f at (a)-(d)?
• Example 5.6

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FM Demodulator

• Four primary methods
• Differentiator with envelope detector/Slope
  detector
• FM to AM conversion
• Phase-shift discriminator/Ratio detector
• Approximates the differentiator
• Zero-crossing detector
• Frequency feedback
• Phase lock loops (PLL)

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FM Slope Demodulator

• Principle use slope detector (slope circuit) as
  frequency discriminator, which implements
  frequency to voltage conversion (FVC)
• Slope circuit output voltage is proportional to
  the input frequency. Example filters,
  differentiator

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FM Slope Demodulator cont.

• Block diagram of direct method (slope detector
  slope circuit envelope detector)

so(t) linear with m(t)
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Slope Detector
Magnitude frequency response of transformer BPF.
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Bandpass Limiter

• A device that imposes hard limiting on a signal
  and contains a filter that suppresses the
  unwanted products (harmonics) of the limiting
  process.
• Figure 5.12
• Input Signal
• Output of bandpass limiter
• Bandpass filter
• Remove the amplitude variations

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Ratio Detector

• Foster-Seeley/phase shift discriminator
• uses a double-tuned transformer to convert the
  instantaneous frequency variations of the FM
  input signal to instantaneous amplitude
  variations. These amplitude variations are
  rectified to provide a DC output voltage which
  varies in amplitude and polarity with the input
  signal frequency.
• Example
• Ratio detector
• Modified Foster-Seeley discriminator, not
  response to AM, but 50

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Zero Crossing Detector
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FM Demodulator PLL

• Phase-locked loop (PLL)
• A closed-loop feedback control circuit, make a
  signal in fixed phase (and frequency) relation to
  a reference signal
• Track frequency (or phase) variation of inputs
• Or, change frequency (or phase) according to
  inputs
• PLL can be used for both FM modulator and
  demodulator
• Just as Balanced Modulator IC can be used for
  most amplitude modulations and demodulations

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PLL FM

• Remember the following relations
• SiAcos(wct?1(t)), SvAvcos(wct?c(t))
• Sp0.5AAvsin(2wct?1?c)sin(?1-?c)
• So0.5AAvsin(?1-?c)AAv(?1-?c)
• Equations 5.24-5.28

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Small Error Analysis

• Phase error
• Block Diagram 5.13.b, and 5.14
• Noise model s/(sAKH(s))
• First order filter 5.31, phase error
• Second order filter 5.32, unstable
• FM signal
• Equations
• AK large
• PLL better when SNR low
• First Order Loop Analysis
• Acquisition Figure 5.35

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Superheterodyne Receiver

• Radio receivers main function
• Demodulation ? get message signal
• Carrier frequency tuning ? select station
• Filtering ? remove noise/interference
• Amplification ? combat transmission power loss
• Superheterodyne receiver
• Heterodyne mixing two signals for new frequency
• Superheterodyne receiver heterodyne RF signals
  with local tuner, convert to common IF
• Invented by E. Armstrong in 1918.

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Advantage of superheterodyne receiver

• A signal block (of circuit) can hardly achieve
  all selectivity, signal quality, and power
  amplification
• Superheterodyne receiver deals them with
  different blocks
• RF blocks selectivity only
• IF blocks filter for high signal quality, and
  amplification, use circuits that work in only a
  constant IF, not a large band

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FM Broadcasting

• The frequency of an FM broadcast station is
  usually an exact multiple of 100 kHz from 87.5 to
  108.5 MHz . In most of the Americas and Caribbean
  only odd multiples are used.
• fm15KHz, ?f75KHz, ?5, B2(fm?f)180kHz
• Pre-emphasis and de-emphasis
• Random noise has a 'triangular' spectral
  distribution in an FM system, with the effect
  that noise occurs predominantly at the highest
  frequencies within the baseband. This can be
  offset, to a limited extent, by boosting the high
  frequencies before transmission and reducing them
  by a corresponding amount in the receiver.
• Block diagram and spectrum figure 5.19
• Equation 5.40
• Relation of stereo transmission and monophonic
  transmission

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TV FM broadcasting

• fm15KHz, ?f25KHz, ?5/3, B2(fm?f)80kHz
• Center fc4.5MHz

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Satellite Radio

• WorldSpace outside US, XM Radio and Sirius in
  North Americ.

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Questions?