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 9
Communication Receivers
©2003 The McGraw-Hill Companies
Communication Receivers
In radio communication systems, the transmitted signal
is very weak when it reaches the receiver, particularly
when it has traveled over a long distance.
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The signal has also picked up noise of various kinds.
Receivers must provide the sensitivity and selectivity
that permit full recovery of the original signal.
The radio receiver best suited to this task is known as
the superheterodyne receiver.
Topics Covered in Chapter 9
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Basic Principles of Signal Reproduction
Superheterodyne Receivers
Frequency Conversion
Intermediate Frequency and Images
Noise
Typical Receiver Circuits
Receivers and Transceivers
Basic Principles of Signal
Reproduction
A communication receiver must be able to identify and
select a desired signal from the thousands of others
present in the frequency spectrum (selectivity) and to
provide sufficient amplification to recover the
modulating signal (sensitivity).
 A receiver with good sensitivity will isolate the
desired signal and greatly attenuate other signals.
 A receiver with good sensitivity involves high circuit
gain.
Selectivity
Selectivity in a receiver is obtained by using tuned
circuits and/or filters.
 LC tuned circuits provide initial selectivity.
 Filters, which are used later in the process, provide
additional selectivity.
 By controlling the Q of a resonant circuit, you can set
the desired selectivity.
 The optimum bandwidth is one that is wide enough to
pass the signal but narrow enough to eliminate signals
on adjacent frequencies.
Sensitivity
A communication receiver’s sensitivity, or ability to pick up weak
signals, is mainly a function of overall gain, the factor by
which an input signal is multiplied to produce the output
signal.
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The higher the gain of a receiver, the better its sensitivity.
The more gain that a receiver has, the smaller the input signal
necessary to produce a desired level of output.
High gain in receivers is obtained by using multiple
amplification stages.
Basic Receiver Configuration
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The simplest radio receiver is a crystal set consisting
of a tuned circuit, a diode (crystal) detector, and
earphones.
The tuned circuit provides the selectivity.
The diode and a capacitor serve as an AM
demodulator.
The earphones reproduce the recovered audio signal.
A Crystal Receiver
Tuned Radio Frequency (TRF)
Receiver
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In the tuned radio frequency (TRF) receiver
sensitivity is improved by adding a number of stages
of RF amplification between the antenna and detector,
followed by stages of audio amplification.
The RF amplifier stages increase the gain before it is
applied to the detector.
The recovered signal is amplified further by audio
amplifiers, which provide sufficient gain to operate a
loudspeaker.
TRF Receiver
Superheterodyne Receiver
Superheterodyne receivers convert all incoming signals
to a lower frequency, known as the intermediate
frequency (IF), at which a single set of amplifiers is
used to provide a fixed level of sensitivity and
selectivity.
 Gain and selectivity are obtained in the IF amplifiers.
 The key circuit is the mixer, which acts like a simple
amplitude modulator to produce sum and difference
frequencies.
 The incoming signal is mixed with a local oscillator.
Superheterodyne Receiver
RF Amplifier
The antenna picks up the weak radio signal and feeds it
to the RF amplifier, also called a low-noise amplifier
(LNA).
 RF amplifiers provide some initial gain and
selectivity and they are sometimes referred to as
preselectors.
 Tuned circuits help select the frequency range in
which the signal resides.
 RF amplifiers minimize oscillator radiation.
 Bipolar and FETs can be used as RF amplifiers.
Mixers and Local Oscillators
The output of the RF amplifier is applied to the input of the
mixer.
 The mixer also receives an input from a local oscillator or
frequency synthesizer.
 The mixer output is the input signal, the local oscillator signal,
and the sum and difference frequencies of these signals.
 A tuned circuit at the output of the mixer selects the difference
frequency, or intermediate frequency (IF).
 The local oscillator is made tunable so that its frequency can
be adjusted over a relatively wide range.
Amplifiers
The output of the mixer is an IF signal containing the
same modulation that appeared on the input RF
signal.
 The signal is amplified by one or more IF amplifier
stages, and most of the gain is obtained in these
stages.
 Selective tuned circuits provide fixed selectivity.
 Since the intermediate frequency is lower than the
input frequency, IF amplifiers are easier to design and
good selectivity is easier to obtain.
Demodulators
The highly amplified IF signal is finally applied to the
demodulator, or detector, which recovers the original
modulating information.
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The demodulator may be a diode detector (for AM), a
quadrature detector (for FM), or a product detector
(for SSB).
The output of the demodulator is then usually fed to
an audio amplifier.
Automatic Gain Control
The output of a detector is usually the original
modulating signal, the amplitude of which is directly
proportional to the amplitude of the received signal.
 The recovered signal, which is usually AC, is
rectified and filtered into a DC voltage by a circuit
known as the automatic gain control (AGC) circuit.
 This DC voltage is fed back to the IF amplifiers, and
sometimes the RF amplifier, to control receiver gain.
 AGC circuits help maintain a constant output level
over a wide range of RF input signal levels.
Frequency Conversion
Frequency conversion is the process of translating a
modulated signal to a higher or lower frequency
while retaining all the originally transmitted
information.
 In radio receivers, high-frequency signals are
converted to a lower, intermediate frequency. This is
called down conversion.
 In satellite communications, the original signal is
generated at a lower frequency and then converted to
a higher frequency. This is called up conversion.
Mixing Principles
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Frequency conversion is a form of amplitude
modulation carried out by a mixer circuit or
converter.
The function performed by the mixer is called
heterodyning.
Diode Mixer
The primary characteristic of mixer circuits is
nonlinearity.
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Any device or circuit whose output does not vary
with the input can be used as a mixer.
One of the most widely used types of mixer is the
simple diode modulator.
Diode Mixer
Diode Mixer Operation
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The input signal, directly from the antenna, is applied
to the primary winding of the transformer.
The signal is coupled to the secondary winding and
applied to the diode mixer, and the local oscillator
signal is coupled to the diode by way of a capacitor.
The input and local oscillator signals are linearly
added this way and applied to the diode, which
produces the sum and difference frequencies.
The output signals are developed across the tuned
circuit which selects the difference frequency.
Bipolar Transistor Mixer
The primary benefit of transistor mixers over diode
mixers is that gain is obtained.
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Transistor bias is set to provide class AB.
Bipolar mixers with their inherent high noise are
often replaced with field-effect transistors (FETs).
IC Mixer
The NE602, IC mixer, also known as a Gilbert
transconductance cell or Gilbert cell, consists of a
double balanced mixer circuit made up of two crossconnected differential amplifiers.
IC Mixer NE602
Local Oscillator and Frequency
Synthesizer
The local oscillator signal for the mixer comes from
either a conventional LC tuned oscillator or a
frequency synthesizer.
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The simpler continuously tuned receivers use an LC
oscillator.
Channelized receivers use frequency synthesizers.
LC Oscillator
A local oscillator, which is sometimes referred to as a
variable-frequency oscillator, or VFO is used for
frequencies up to 100 MHz.
 An amplifier (e.g. FET) is connected as a Colpitts
oscillator.
 Feedback is developed by a voltage divider made up
of capacitors.
 The frequency is set by a parallel tuned circuit.
 The output is taken across an RFC and it is buffered
by a direct-coupled emitter follower.
Variable Frequency Oscillator
Frequency Synthesizer
Most new receiver designs incorporate frequency
synthesizers for the local oscillator, which provides
some important benefits over simple VFO designs.
 The synthesizer is usually of the phase-locked loop
(PLL) design and the output is locked to a crystal
oscillator reference which provides high stability.
 Tuning is accomplished by changing the frequency
division factor in the PLL, resulting in incremental
rather than continuous frequency changes.
Frequency Synthesizer
Intermediate Frequency and
Images
The primary objective in the design of an IF stage is to
obtain good selectivity.
 Narrow-band selectivity is best obtained at lower
frequencies.
 At low frequencies, circuits are more stable.
 At low frequencies, image interference is possible.
 At higher frequencies, circuit layouts must take into
account stray inductances and capacitances.
 At higher frequencies, there is a need for shielding.
By Definition…
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To reduce image interference, which is an RF signal
two times the IF above or below the incoming
frequency, high-Q tuned circuits should be used
ahead of the mixer or RF amplifier.
Noise is the static you hear in the speaker when you
tune an AM or FM receiver to any position between
stations.
Direct Conversion Receiver
A special version of the superheterodyne is known as
the direct conversion (DC) or zero IF (ZIF) receiver.
 DC receivers convert the incoming signal directly to
baseband without converting to an IF.
 They perform demodulation as part of the translation.
 The low-noise amplifier (LNA) boosts the signal
before the mixer.
 The local oscillator (LO) frequency is set to the
frequency of the incoming signal.
 Baseband output is passed via a low-pass filter (LPF).
Direct-Conversion Receiver
By Definition…
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A software-defined radio (SDR) is a receiver in which
most of the functions are performed by a digital
signal processor (DSP).
Noise consists of an electronic signal that is a mixture
of many frequencies at many amplitudes that gets
added to a radio or information signal as it is
transmitted from one place to another.
The signal-to-noise (S/N) ratio indicates the relative
strengths of the signal and the noise in a
communication system.
External Noise
External noise comes from sources over which we have
little or no control. Examples include:
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Industrial noise
Atmospheric (static) noise
Extraterrestrial noise
Internal Noise
Electronic components is a receiver such as resistors,
diodes, and transistors are major sources of internal
noise. Types of internal noise include:
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Thermal noise
Semiconductor noise
Intermodulation (distortion) noise
Expressing Noise Levels
The noise quality of a receiver can be expressed as in the
following terms.
 The noise factor is the ratio of the S/N power at the input to
the S/N power at the output.
 When the noise factor is expressed in decibels, it is called the
noise figure.
 Most of the noise produced in a device is thermal, which is
directly proportional to temperature. Therefore, the term noise
temperature (TN) is used.
 SINAD is the composite signal plus noise and distortion
divided by noise and distortion contributed by the receiver.
Typical Receiver Circuits
Typical receiver circuits include:
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RF amplifiers
IF amplifiers
AGC
AFC
Special circuits
RF Input Amplifier
The RF amplifier, also called a low-noise amplifier
(LNA), processes the very weak input signals,
increasing their amplitude prior to mixing.
 Low-noise components are used to ensure a
sufficiently high S/N ratio.
 Selectivity should be such that it effectively
eliminates images.
 The RF amplifier is typically a class A circuit that can
be configured with bipolar or field-effect transistors.
RF Amplifier Circuit
IF Amplifier
Most of the gain and selectivity in a superheterodyne
receiver are obtained in the IF amplifier.
 If amplifiers are tuned class A circuits capable of
providing gain in the 10- to 30-dB range.
 Usually two or more IF amplifiers are used to provide
adequate receiver gain.
 Ferrite-core transformers are used for coupling
between stages.
 Selectivity is provided by tuned circuits.
Two-Stage IF Amplifier
By Definition…
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Crystal, ceramic, or SAW filters are used for better IF
selectivity. They are smaller than LC circuits and
provide higher selectivity and require no tuning.
In FM receivers, one or more of the IF amplifier
stages is used as a limiter, to remove any amplitude
variations on the FM signal before the signal is
applied to the demodulator.
Automatic Gain Control
Receiver gain is typically far greater than required for
adequate reception. Excessive gain usually causes the
received signal to be distorted and the transmitted
information to be less intelligible.
 Manual gain control by using a potentiometer in RF
and IF stages can be achieved.
 Receivers include volume controls in audio circuits.
 AGC circuits are more effective and give the receiver
a very wide dynamic range, which is the ratio of the
largest signal handled to the lowest signal.
AGC and Circuit Gain
The gain of a bipolar transistor amplifier is proportional
to the amount of collector current flowing. Two
methods of applying AGC are as follows:
 The gain can be decreased by decreasing the collector
current. This is referred to as reverse AGC.
 The gain can be reduced by increasing the collector
current. A stronger signal increases AGC voltage and
base current and, in turn, increases collector current,
reducing the gain. This method of gain control is
known as forward AGC.
AGC Applied to IF Amplifier
AGC Operation
A common emitter IF amplifier with bias derived from
voltage divider resistors can be configured.
 A DC voltage is fed by way of a resistor to the base
of the common emitter amplifier.
 As the level of the signal amplitude increases, the
negative DC voltage increases, decreasing the base
current.
 The reduced base current, in turn, decreases the
collector current and lowers the circuit gain.
Squelch Circuit
A squelch circuit, or muting circuit, is found in most
communications receivers.
 The squelch is used to keep the receiver audio turned
off until an RF signal appears at the receiver input.
 In AM systems such as CB radios, the noise level is
high and can be very annoying.
 Squelch circuits provide a means of keeping the audio
amplifier turned off during the time that noise is
received in the background and enabling it when an
RF signal appears at the input.
SSB and Continuous-Wave
Reception
Communication receivers designed for receiving SSB or
continuous-wave signals have a built-in oscillator that
permits recovery of the transmitted information.
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A circuit called the beat frequency oscillator (BFO),
is usually designed to operate near the IF.
The BFO signal is applied to the demodulator along
with the IF signal containing the modulation.
Beat Frequency Oscillator (BFO)
Integrated Circuits (ICs) in
Receivers
In new designs, virtually all receiver circuits are ICs.
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A complete receiver usually consists of three or four
ICs.
IC receivers include components such as coils,
transformers, capacitors, and filters.
Most modern receivers are contained on a single IC.
Integrated Circuits (ICs) in
Receivers (Continued)
IC receivers are typically broken down into three major
sections:
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The tuner, with RF amplifier, mixer, and local
oscillator
The IF section, with amplifiers, demodulator, and
AGC and muting circuits
The audio power amplifier
VHF Receiver
A typical VHF receiver is designed to receive two-way
aircraft communication between planes and airport
controllers.
 They have a typical frequency range of 118 to 135
MHz.
 Amplitude modulation is typical with these receivers.
 VHF receivers are designed to use a combination of
discrete components and ICs.
Transceiver
Most two-way radio communication equipment is
packaged so that both transmitter and receiver are in a
unit known as a transceiver.
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Transceivers range from large, high-power desktop
units to small, pocket-sized, handheld units.
Transceivers have a common housing and power
supply.
Transceivers can share circuits, thereby achieve cost
savings, and in some cases are smaller in size.