Frequency Modulation fm

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CHAPTER 3

• Frequency Modulation (fm)
• Modulation/demodulation

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Noise and Angle Modulation

• With FM, there is a non-uniform noise
  distribution.
• The higher frequencies suffers more noise than
  the lower frequencies.
• Thus for a information signal with uniform signal
  level, the Signal to noise ratio is also
  non-uniform.
• The SNR for higher-modulating frequencies are
  lower than the SNR for the lower frequencies.

fc
Uniform signal level
S/N minimum
Without preemphasis
S/N maximum
Nonuniform noise level
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Preemphasis Deemphasis

• Solution the high-frequency modulating signals
  are emphasized or boosted in amplitude of the
  transmitter prior to performing modulation and
  then deemphasized or attenuated during
  demodulation at receiver.

nonuniform signal level
With preemphasis
Uniform S/N
Nonuniform noise level
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Preemphasis Deemphasis
fc
Uniform signal level
S/N minimum
Without preemphasis
S/N maximum
Nonuniform noise level
nonuniform signal level
With preemphasis
Uniform S/N
Nonuniform noise level
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Preemphasis Deemphasis

• Preemphasis The higher frequencies are
  increased in amplitude before being used to
  modulate the carrier and therefore will be less
  affected to noise.
• Preemphasis Network gt High-Pass Filter
  (Differentiator)
• Deemphasis returns the frequency response to
  its normal flat level.
• Deemphasis Network -gt Low-Pass Filter
  (Integrator)

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Preemphasis Deemphasis

• Preemphasis and Deemphasis circuit diagrams are
  shown below
• PREEMPHASIS CCT DEEMPHASIS CCT
• ? L/R 75 us ?
  RC 75 us

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Preemphasis Deemphasis

• Break frequency is the frequency when preemphasis
  deemphasis begins. It is defined as below
• fb 1 / 2?RC or
• fb 1 / 2?L/R
• where RC or L/R is a time constant (?)
• Preemphasis circuit network in front of FM
  Modulator and Deemphasis network at the output of
  FM Demodulator will improve the signal-to-noise
  ratio for higher modulating-signal frequencies

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Generation of FM signal
2 techniques direct and indirect methods
Require a system that enable the frequency of the
output signal to vary in accordance to an
information signal amplitude.
3.9.1 Direct method

• Varactor diode
• Reactance modulation
• VCO

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Frequency modulation
Direct
Modulating signal source
Frequency modulator
FM wave
Indirect
Integrator
Phase modulator
Modulating signal source
FM wave
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Phase modulation
Direct
Phase modulator
Modulating signal source
PM wave
Modulating signal source
Differentiator
Frequency modulator
PM wave
Indirect
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Direct method

• Varactor diode
• VCO
• Reactance modulation

Direct
Modulating signal source
Frequency modulator
FM wave
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simple direct FM modulator

• The carrier is generated by LC or crystal
  oscillator circuits.

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Frequency Modulators

• In LC oscillators, the carrier frequency can be
  changed by varying either the inductance or
  capacitance.
• The idea is to find a circuit or component that
  converts a modulating voltage to a corresponding
  change in capacitance or inductance.
• In crystal oscillators, the frequency is fixed by
  the crystal.
• This modulatior is impractical

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Varactor diode Modulators
A varactor is a variable capacitance diode used
to change oscillator frequencies.
Figure 6-4 A direct-frequency-modulated carrier
oscillator using a varactor diode.
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Varactor diode Direct FM modulator
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Varactor diode Direct FM modulator

• For voltage controlled oscillator (VCO) Fm
  generator, the center frequency for the
  oscillator is
• Where L inductance of the primary winding
  (henries)
• C varactor diode capacitance (farads)
• When a modulating signal applied, the frequency
  is
• Where f the new frequency of oscillation
• ?C change in varactor diode capacitance
  due to modulating signal.
• The change in frequency is

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Varactor diode Direct FM modulator

• Varactor Modulator
• Most LC oscillators are not stable enough to
  provide a carrier signal.
• The frequency of LC oscillators will vary with
  temperature changes, variations in circuit
  voltage, and other factors.
• As a result, crystal oscillators are normally
  used to set carrier frequency.

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Varactor diode Direct FM modulator

• Frequency-Modulating a Crystal Oscillator
• Crystal oscillators provide highly accurate
  carrier frequencies and their stability is
  superior to LC oscillators.
• The frequency of a crystal oscillator can be
  varied by changing the value of capacitance in
  series or parallel with the crystal.
• By making the series capacitance a varactor
  diode, frequency modulation can be achieved.

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Varactor diode Direct FM modulator
Figure 6-5 Frequency modulation of a crystal
oscillator with a VVC.
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Voltage-Controlled Oscillators Direct FM
Modulators

• Voltage-Controlled Oscillators
• Oscillators whose frequencies are controlled by
  an external input voltage are generally referred
  to as voltage-controlled oscillators (VCOs).
• Voltage-controlled crystal oscillators are
  generally referred to as VXOs.
• VCOs are primarily used in FM.
• VCOs are also used in voltage-to-frequency
  conversion applications.

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Eg (VCO)Linear integrated-circuit direct FM
modulator
High-frequency deviations and high modulation
indices.
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Reactance Modulator Direct FM Modulators

• A reactance modulator is a circuit that uses a
  transistor amplifier that acts like either a
  variable capacitor or an inductor.
• When the circuit is connected across the tuned
  circuit of an oscillator, the oscillator
  frequency can be varied by applying the
  modulating signal to the amplifier.
• Reactance modulators can produce frequency
  deviation over a wide range.
• Reactance modulators are highly linear, so
  distortion is minimal.

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Reactance Modulator Direct FM Modulators
Figure 6-10 A reactance modulator.
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Direct FM Modulator

• Disadvantages
• Relatively unstable LC oscillators must be used
  to produce the carrier frequency which prohibits
  using crystal oscillators
• Requires the addition of some form of automatic
  frequency control circuitry to maintain the
  carrier frequency
• Advantages
• Relatively high-frequency deviations and
  modulation indices are easily obtained because
  the oscillator are inherently unstable

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Demodulation of FM signal

• Demodulation process is done in order to
  recover/get back the information signal
  transmitted.
• Basic concepts of demodulation circuit is to
  detect the frequency variation.
• Two techniques can be used

FM Demodulation
Indirect
Direct

• Discriminator

Phase Lock Loop(PLL)/ Quadrature detector
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Demodulator

• Five most commonly used demodulator are

Slope detector Foster-Seeley discriminator Ratio
detector PLL demodulator Quadrature detector
Tuned-circuit frequency discriminator
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Conversion circuit - FM to AM (Discriminator)
Direct

• This technique is required to convert FM signal
  to AM signal and then by using AM demodulation
  circuit is to get back the information signal.
• This technique is called (slope detection) or
  discriminator.
• Block diagram of the detection circuit is as
  shown below

vFM(t)
t
t
Pemodulatan Sudut
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Demodulator Slope detector
Slope Detector
Envelope Detector

• Convert FM to AM and then demodulate the AM
  signal with conventional peak detector
• Single-ended slope detector.
• Tuned circuit produces an output voltage
  proportional to the input frequency
• When ?f is above fc ( ?f ), Vout increase
• When ?f is below fc (- ?f ), Vout decrease

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Foster Seeley
The Foster-Seeley discriminator is a widely used
FM detector. The detector consists of a special
center-tapped transformer feeding two diodes in a
full wave DC rectifier circuit. When the input
transformer is tuned to the signal frequency, the
output of the discriminator is zero when there is
no deviation of the carrier both halves of the
center tapped transformer are balanced. As the FM
signal swings in frequency above and below the
carrier frequency, the balance between the two
halves of the center-tapped secondary are
destroyed and there is an output voltage
proportional to the frequency deviation.
http//en.wikipedia.org/wiki/Detector_(radio)
Pemodulatan Sudut
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Ratio detector
The ratio detector is a variant of the
Foster-Seely discriminator, but, the diodes
conduct in opposite directions. The output in
this case is taken between the sum of the diode
voltages and the center tap. The output across
the diodes is connected to a large value
capacitor, which eliminates AM noise in the ratio
detector output. While unlike the Foster-Seely
discriminator, the ratio detector will not
respond to AM signals, however the output is only
50 of the output of a discriminator for the same
input signal.
Pemodulatan Sudut
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Phase-Locked Loop (PLL) Indirect Method
adjusts the VCO frequency in an attempt to
correct for the original frequency or phase
difference.
compare the two input signals and generate an
output signal that, when filtered, will control
the VCO.
Figure 6-21 Block diagram of a PLL.
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Frequency Demodulators

• Quadrature Detector
• The quadrature detector is probably the single
  most widely used FM demodulator.
• The quadrature detector is primarily used in TV
  demodulation.
• This detector is used in some FM radio stations.
• The quadrature detector uses a phase-shift
  circuit to produce a phase shift of 90 degrees at
  the unmodulated carrier frequency.

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Frequency Demodulators
Figure 6-19 A quadrature FM detector.
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FM Transmitter
Crosby
Direct
Phase Locked Loop
Modulating signal source
Frequency modulator
FM wave
Indirect
Armstrong
Integrator
Phase modulator
Modulating signal source
FM wave
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Group Task

• Each member of the group must search and prepare
  notes on one of the following FM transmitters
• Crosby
• Phase-locked loop
• Amstrong
• Explain how each system works
• Sumbit 11/3/09

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FM Stereo
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FM Stereo broadcasting

• The first is a normal audio signal made up of the
  Sum of the left and right channels. This is the
  signal you hear on a Mono radio and is the same
  as switching the Stereo/Mono Switch on an
  amplifier to "Mono'.
• In addition a difference signal (Left - Right) is
  generated and then used to modulate a 38 Khz
  subcarrier using Double sideband suppressed
  carrier (DSBSC) modulation. This is an AM
  modulation of the subcarrier.
• To keep the receiver decoder locked into the
  38Khz subcarrier a 19 Khz pilot tone (EXACTLY 1/2
  of 38 Khz) is transmitted at well. The relative
  percentage of modulation put into the pilot is
  10.

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FM Stereo broadcasting
Compatible with mono receivers
SCA channel is only used in the US
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FM Stereo Transmitter
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FM Stereo Receiver
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FM Stereo matrix decoder
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• FM Stereo Transmitter uses FDM (Frequency
  Division Multiplexing).
• It consists of 3 channels
• 1. Left (L) and right (R) Audio channels
• 2. Combination of LR and L-R Audio channels
• 3. Combination of LR and L-R Stereo channels

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Summary

• Angle modulation especially FM has many
  advantages compared to AM.
• One of the major advantage is that it is more
  immune to noise compared to AM. The use of
  pre-emphasis and de-emphasis is another way of
  improving FM immunity against noise.
• However, FM has a major drawback in terms of
  bandwidth. Since FM produces infinite numbers of
  sidebands, the bandwidth can be very large.
• Bessel function is used in investigating the
  sidebands and bandwidth of FM signal.
• Modulation/demodulation of FM can be done
  directly or indirectly.
• FM stereo is an example of FM modulation used
  today.