Download Principles of Electronic Communication Systems

Principles of Electronic
Communication Systems
Second Edition
Louis Frenzel
© 2002 The McGraw-Hill Companies
Principles of Electronic
Communication Systems
Second Edition
Chapter 6
FM Circuits
©2003 The McGraw-Hill Companies
Topics Covered in Chapter 6
There are many circuits used to produce FM and PM
signals. There are two types of frequency modulator
circuits, direct circuits and phase modulation circuits.
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Frequency Modulators
Phase Modulators
Frequency Demodulators
Frequency Modulators
A frequency modulator is a circuit that varies carrier frequency in
accordance with the modulating signal.
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The carrier is generated by LC or crystal oscillator circuits.
In LC oscillators, the carrier frequency can be changed by
varying either the inductance or capacitance.
In crystal oscillators, the frequency is fixed by the crystal.
A varactor is a variable capacitance diode used to change
oscillator frequencies.
Varactor Operation
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A junction diode is created when P- and N-type
semiconductors are formed during the manufacturing
process.
A depletion region, where there are no free carriers,
is formed.
This region acts like a thin insulator that prevents
current from flowing through the device.
A forward bias will cause the diode to conduct.
A reverse bias will prevent current flow.
Varactor Operation (continued)
A reverse-biased diode acts like a small capacitor.
 The P- and N-type materials act as the two plates of
the capacitor.
 The depletion region acts as the dielectric material.
 The width of the depletion layer determines the width
of the dielectric and, therefore the amount of
capacitance.
 All diodes exhibit variable capacitance.
 Varactors are designed to optimize this characteristic.
Varactor Diode Schematic Symbol
Varactor Modulator Circuit
Varactor Modulator
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The capacitance of varactor diode D1 and L1 form the
parallel tuned circuit of the oscillator.
The value of C1 is made very large so its reactance is
very low.
C1 connects the tuned circuit to the oscillator and
blocks the DC bias on the base of Q1.
The values of L1 and D1 fix the center carrier
frequency.
The modulating signal varies the effective voltage
applied to D1 and its capacitance varies.
Frequency-Modulating a Crystal
Oscillator
Crystal oscillators provide highly accurate 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.
 The modulating signal is applied to the varactor diode
which changes the oscillator frequency.
Frequency Modulation of a Crystal
Oscillator
By definition…
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Varactors are made with a wide range of capacitance
values, most units having a nominal capacitance in
the 1- to 200-pF range.
A frequency multiplier circuit is one whose output
frequency is some integer multiple of the input
frequency.
A frequency multiplier that multiplies a frequency by
two is called a doubler.
A frequency multiplier that multiplies a frequency by
three is called a tripler.
Frequency Multipliers
Voltage-Controlled Oscillators
Oscillators whose frequencies are controlled by an
external input voltage are generally referred to as
voltage-controlled oscillators (VCOs).
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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.
Reactance Modulator
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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.
Reactance Modulator Circuit
Phase Modulators
Most modern FM transmitters use some form of phase
modulation (PM) to produce indirect FM.
 In PM the carrier oscillator can be optimized for
frequency accuracy and stability.
 Crystal oscillators or crystal-controlled frequency
synthesizers can be used to set the carrier frequency
accurately and maintain stability.
 The output of the carrier oscillator is fed to a phase
modulator where the phase shift is made to vary in
accordance with the modulating signal.
Varactor Phase Modulators
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A simple phase-shift circuit can be used as a phase
modulator if the resistance or capacitance can be
made to vary with the modulating signal.
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A varactor can be used to vary capacitance and
achieve phase shift modulation.
Varactor Phase Modulator Circuit
Transistor Phase Modulator
A transistor can be used as a variable resistor to create a
phase modulator.
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A standard common emitter class A amplifier biased
into the linear region is used in PM.
The transistor from collector to ground acts like a
resistor.
The transistor’s resistance forms part of the phase
shifting circuit.
Transistor Phase Shifter Circuit
FET Phase Modulator
An improved phase modulator uses a phase shifter made
up of a capacitor and a variable resistance of a fieldeffect transistor (FET).
FET Phase Modulator Circuit
Tuned-Circuit Phase Modulator
Most phase modulators are capable of producing a small
amount of phase shift. The limited phase shift,
therefore, produces a limited frequency shift.
 Phase and frequency shift can be increased by using a
parallel tuned circuit.
 At resonance, a parallel resonant circuit acts like a
large resistor.
 Off resonance, the circuit acts inductively or
capacitively and produces a phase shift.
Frequency Demodulators
Any circuit that will convert a frequency variation in the
carrier back into a proportional voltage variation can
be used to demodulate or detect FM signals. Circuits
used to recover the original modulating signal from
an FM transmission are called:
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Demodulators
Detectors
Discriminators
Frequency Demodulator Types
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Slope detector
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Foster-Seeley discriminator
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Pulse-averaging discriminator
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Quadrature detector
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Phase-locked loop
Slope Detector
The slope detector makes use of a tuned circuit and a
diode detector to convert frequency variations into
voltage variations.
Slope Detector Circuit
Foster-Seeley Discriminator
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Foster-Seeley discriminators are one of the earliest
FM demodulators.
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Foster-Seeley discriminators are typically found in
older equipment.
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Discriminator circuits are sensitive to variations in
amplitude.
Pulse-Averaging Discriminators
A pulse-averaging discriminator uses a zero crossing
detector, a one shot multivibrator and a low-pass filter
in order to recover the original modulating signal.
 The pulse-averaging discriminator is a very highquality frequency demodulator.
 Originally this discriminator was limited to expensive
telemetry and industrial control applications.
 With availability of low-cost ICs, this discriminator is
used in many electronic products.
Pulse-Averaging Discriminator
Quadrature Detector
The quadrature detector is probably the single most
widely used FM demodulator.
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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.
Quadrature FM Detector
Phase-Locked Loop
A phase-locked loop (PLL) is a frequency- or phasesensitive feedback control circuit used in frequency
demodulation, frequency synthesizers, and various
filtering and signal-detection applications. PLLs have
three basic elements. They are:
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Phase detector
Low-pass filter
Voltage-controlled oscillator
Phase-Locked Loop – Block
Diagram