Principles of Electronic Communication Systems

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1
Principles of Electronic
Communication Systems
Third Edition
Louis E. Frenzel, Jr.
© 2008 The McGraw-Hill Companies
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Chapter 9
Communication Receivers
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Topics Covered in Chapter 9
 9-1: Basic Principles of Signal Reproduction
 9-2: Superheterodyne Receivers
 9-3: Frequency Conversion
 9-4: Intermediate Frequency and Images
 9-5: Noise
 9-6: Typical Receiver Circuits
 9-7: Receivers and Transceivers
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9-1: Basic Principles of Signal
Reproduction
4
 In radio communication systems, the transmitted
signal is very weak when it reaches the receiver,
particularly when it has traveled over a long distance.
 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.
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9-1: Basic Principles of Signal
Reproduction
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 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 selectivity will isolate the desired
signal and greatly attenuate other signals.
 A receiver with good sensitivity involves high circuit
gain.
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9-1: Basic Principles of Signal
Reproduction
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Selectivity: Q and Bandwidth
 Selectivity in a receiver is obtained by using tuned
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circuits and/or filters.
LC tuned circuits provide initial selectivity.
Filters 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 and its sidebands but narrow enough to
eliminate signals on adjacent frequencies.
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9-1: Basic Principles of Signal
Reproduction
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Figure 9-1: Selectivity curve of a tuned circuit.
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9-1: Basic Principles of Signal
Reproduction
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Selectivity: Shape Factor
 The sides of a tuned circuit response curve are known
as skirts.
 The steepness of the skirts, or the skirt selectivity, of a
receiver is expressed as the shape factor, the ratio of
the 60-dB down bandwidth to the 6-dB down bandwidth.
 The lower the shape factor, the steeper the skirts and
the better the selectivity.
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9-1: Basic Principles of Signal
Reproduction
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Sensitivity
 A communication receiver’s sensitivity, or ability to pick
up weak signals, is a function of overall gain, the factor
by which an input signal is multiplied to produce the
output signal.
 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.
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9-1: Basic Principles of Signal
Reproduction
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Sensitivity
 Another factor that affects the sensitivity of a receiver is
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the signal-to-noise (S/N) ratio (SNR).
One method of expressing the sensitivity of a receiver is
to establish the minimum discernible signal (MDS).
The MDS is the input signal level that is approximately
equal to the average internally generated noise value.
This noise value is called the noise floor of the
receiver.
MDS is the amount of signal that would produce the
same audio power output as the noise floor signal.
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9-1: Basic Principles of Signal
Reproduction
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Basic Receiver Configuration
 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.
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9-1: Basic Principles of Signal
Reproduction
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Figure 9-4: The simplest receiver—a crystal set.
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9-1: Basic Principles of Signal
Reproduction
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Tuned Radio Frequency (TRF) Receiver
 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.
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9-1: Basic Principles of Signal
Reproduction
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Figure 9-5: Tuned radio-frequency (TRF) receiver.
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9-1: Basic Principles of Signal
Reproduction
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Tuned Radio Frequency (TRF) Receiver
 Many RF amplifiers use multiple tuned circuits.
 Whenever resonant LC circuits tuned to the same
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frequency are cascaded, overall selectivity is improved.
The greater the number of tuned stages cascaded, the
narrower the bandwidth and the steeper the skirts.
The main problem with TRF receivers is tracking the
tuned circuits.
In a receiver, the tuned circuits must be made variable
so that they can be set to the frequency of the desired
signal.
Another problem with TRF receivers is that selectivity
varies with frequency.
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9-2: Superheterodyne Receivers
 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
signal.
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9-2: Superheterodyne Receivers
Figure 9-8: Block diagram of a superheterodyne receiver.
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9-2: Superheterodyne Receivers
RF Amplifier
 The antenna picks up the weak radio signal and feeds it
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to the RF amplifier, also called a low-noise amplifier
(LNA).
RF amplifiers provide some initial gain and selectivity
and are sometimes called 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.
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9-2: Superheterodyne Receivers
Mixers and Local Oscillators
 The output of the RF amplifier is applied to the input of
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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.
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9-2: Superheterodyne Receivers
IF 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 usually lower than
the input frequency, IF amplifiers are easier to design
and good selectivity is easier to obtain.
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9-2: Superheterodyne Receivers
Demodulators
 The highly amplified IF signal is finally applied to the
demodulator, which recovers the original modulating
information.
 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.
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9-2: Superheterodyne Receivers
Automatic Gain Control
 The output of a demodulator 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.
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9-2: Superheterodyne Receivers
Automatic Gain Control
 The amplitude of the RF signal at the antenna of a
receiver can range from a fraction of a microvolt to
thousands of microvolts; this wide signal range is known
as the dynamic range.
 Typically, receivers are designed with very high gain so
that weak signals can be reliably received.
 However, applying a very high-amplitude signal to a
receiver causes the circuits to be overdriven, producing
distortion and reducing intelligibility.
 With AGC, the overall gain of the receiver is
automatically adjusted depending on the input signal
level.
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9-3: 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.
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9-3: Frequency Conversion
Mixing Principles
 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.
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9-3: Frequency Conversion
Mixing Principles
 Mixers accept two inputs: The signal to be translated to
another frequency is applied to one input, and the sine
wave from a local oscillator is applied to the other input.
 Like an amplitude modulator, a mixer essentially
performs a mathematical multiplication of its two input
signals.
 The oscillator is the carrier, and the signal to be
translated is the modulating signal.
 The output contains not only the carrier signal but also
sidebands formed when the local oscillator and input
signal are mixed.
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9-3: Frequency Conversion
Figure 9-9: Concept of a mixer.
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9-3: Frequency Conversion
Mixer and Converter Circuits: Diode Mixer
 The primary characteristic of mixer circuits is
nonlinearity.
 Any device or circuit whose output does not vary
linearly with the input can be used as a mixer.
 One of the most widely used types of mixer is the
simple diode modulator.
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9-3: Frequency Conversion
Mixer and Converter Circuits: Diode Mixer
 The input signal 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
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.
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9-3: Frequency Conversion
Figure 9-10: A simple diode mixer.
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9-3: Frequency Conversion
Mixer and Converter Circuits
 Singly balanced mixer: A popular mixer circuit using
two diodes.
 Doubly balanced mixer: This version of the diode
balanced modulator is probably the single best mixer
available, especially for VHF, UHF, and microwave
frequencies.
 FET Mixers: FETs make good mixers because they
provide gain, have low noise, and offer a nearly perfect
square-low response.
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9-3: Frequency Conversion
Mixer and Converter Circuits: IC Mixer
 The NE602, a typical IC mixer, is also known as a
Gilbert transconductance cell or Gilbert cell.
 It consists of a double balanced mixer circuit made up
of two cross-connected differential amplifiers.
Mixer and Converter Circuits: Image Reject Mixer
 An image reject mixer is a special type of mixer used in
designs in which images cannot be tolerated.
 It uses Gilbert cell mixers in a configuration like that
used in a phasing-type SSB generator.
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9-3: Frequency Conversion
Figure 9-15: NE602 IC
mixer. (a) Block
diagram and pinout.
(b) Simplified
schematic.
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9-3: Frequency Conversion
Local Oscillator and Frequency Synthesizers
 The local oscillator signal for the mixer comes from
either a conventional LC tuned oscillator or a frequency
synthesizer.
 The simpler continuously tuned receivers use an LC
oscillator.
 Channelized receivers use frequency synthesizers.
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9-3: Frequency Conversion
Local Oscillator and Frequency Synthesizers: LC
Oscillator
 A local oscillator is sometimes referred to as a variablefrequency oscillator, or VFO.
 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.
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9-3: Frequency Conversion
Figure 9-17: A VFO for receiver local oscillator service.
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9-3: Frequency Conversion
Local Oscillator and Frequency Synthesizers:
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.
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9-3: Frequency Conversion
Figure 9-18: A frequency synthesizer used as a receiver local oscillator.
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9-4: Intermediate
Frequency and Images
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 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 with high
gain.
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9-4: Intermediate
Frequency and Images
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 At low frequencies, image interference is possible. An
image is an RF signal two times the IF above or below
the incoming frequency.
 At higher frequencies, circuit layouts must take into
account stray inductances and capacitances.
 At higher frequencies, there is a need for shielding.
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9-4: Intermediate
Frequency and Images
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Figure 9-19: Relationship of the signal and image frequencies.
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9-4: Intermediate
Frequency and Images
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Figure 9-20: Signal, local oscillator, and image frequencies in a superheterodyne.
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9-4: Intermediate
Frequency and Images
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Solving the Image Problem
 To reduce image interference, high-Q tuned circuits
should be used ahead of the mixer or RF amplifier.
 The IF is made as high as possible for effective
elimination of the image problem, yet low enough to
prevent design problems.
 In most receivers the IF varies in proportion to the
frequencies that must be covered.
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9-4: Intermediate
Frequency and Images
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Figure 9-21: A low IF compared to the signal frequency with low-Q tuned circuits
causes images to pass and interfere.
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9-4: Intermediate
Frequency and Images
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Dual-Conversion Receivers
 Another way to obtain selectivity while eliminating the
image problem is to use a dual-conversion
superheterodyne receiver.
 A typical receiver uses two mixers and local oscillators,
so it has two IFs.
 The first mixer converts the incoming signal to a high
intermediate frequency to eliminate the images.
 The second mixer converts that IF down to a much
lower frequency, where good selectivity is easier to
obtain.
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9-4: Intermediate
Frequency and Images
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Figure 9-22: A dual-conversion superheterodyne.
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Frequency and Images
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Direct Conversion Receivers
 A special version of the superheterodyne is known as
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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).
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9-4: Intermediate
Frequency and Images
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Figure 9-23: A direct-conversion (zero-IF) receiver.
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9-4: Intermediate
Frequency and Images
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Direct Conversion Receivers
 Advantages:
 No separate IF filter is needed.
 No separate detector circuit is needed.
 In transceivers that use half duplex and in which the
transmitter and receiver are on the same frequency,
only one PLL frequency synthesizer voltagecontrolled oscillator is needed.
 There is no image problem.
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Frequency and Images
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Direct Conversion Receivers
 Disadvantages:
 In designs with no RF amplifier (LNA), the LO signal
can leak through the mixer to the antenna and
radiate.
 An undesired dc offset can develop in the output.
 The ZIF receiver can be used only with CW, AM,
SSB, or DSB. It cannot recognize phase or frequency
variations.
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9-4: Intermediate
Frequency and Images
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Figure 9-24: A direct conversion receiver for FM, FSK, PSK, and digital modulation.
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9-4: Intermediate
Frequency and Images
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Direct Conversion Receivers
 To demodulate FM and PM modulations in a zero-IF
receiver, two mixers and filters are needed.
 There must be a 90° phase shift between the LO
signals to produce I and Q signals for the DSP
demodulation.
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9-4: Intermediate
Frequency and Images
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Software-Defined Radio
 A software-defined radio (SDR) is a receiver in which
most of the functions are performed by a digital signal
processor (DSP).
 The benefits of SDRs are improved performance and
flexibility.
 The receiver characteristics (type of modulation,
selectivity, etc.) can be easily changed by running a
different program.
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9-5: Noise
 Noise is an electronic signal that gets added to a radio
or information signal as it is transmitted from one
place to another.
 It is not the same as interference from other
information signals.
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9-5: Noise
 Noise is the static you hear in the speaker when you
tune any AM or FM receiver to any position between
stations. It is also the “snow” or “confetti” that is visible
on a TV screen.
 The noise level in a system is proportional to
temperature and bandwidth, the amount of current
flowing in a component, the gain of the circuit, and the
resistance of the circuit.
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9-5: Noise
Signal-to-Noise Ratio
 The signal-to-noise (S/N) ratio indicates the relative
strengths of the signal and the noise in a
communication system.
 The stronger the signal and the weaker the noise, the
higher the S/N ratio.
 The S/N ratio is a power ratio.
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9-5: Noise
External Noise
 External noise comes from sources over which we have
little or no control, such as:
 Industrial sources
 motors, generators, manufactured equipment
 Atmospheric sources
 The naturally occurring electrical disturbances in the earth’s
atmosphere; atmospheric noise is also called static.
 Space
 The sun radiates a wide range of signals in a broad noise
spectrum.
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9-5: Noise
Internal Noise
 Electronic components in a receiver such as resistors,
diodes, and transistors are major sources of internal
noise. Types of internal noise include:
 Thermal noise
 Semiconductor noise
 Intermodulation distortion
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9-5: Noise
Expressing Noise Levels
 The noise quality of a receiver can be expressed 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.
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9-5: Noise
Noise in Cascaded Stages
 Noise has its greatest effect at the input to a receiver
because that is the point at which the signal level is
lowest.
 The noise performance of a receiver is determined in
the first stage of the receiver, usually an RF amplifier or
mixer.
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9-6: Typical Receiver Circuits
 Typical receiver circuits include:
 RF amplifiers
 IF amplifiers
 AGC
 AFC
 Special circuits
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9-6: Typical Receiver 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.
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9-6: Typical Receiver Circuits
Figure 9-30: A typical RF amplifier used in receiver front ends.
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9-6: Typical Receiver Circuits
IF Amplifier
 Most of the gain and selectivity in a superheterodyne
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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.
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9-6: Typical Receiver Circuits
Figure 9-33: A two-stage IF amplifier using double-tuned transformer coupling for
selectivity.
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9-6: Typical Receiver Circuits
Traditional IF Amplifier Circuits: Coupled Circuit
Selectivity
 Changing the amount of coupling between the primary
and secondary windings allows the desired amount of
bandwidth to be obtained. At some particular degree of
coupling, known as critical coupling, the output
reaches a peak value.
 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.
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9-6: Typical Receiver Circuits
Traditional IF Amplifier Circuits: Coupled Circuit
Selectivity
 Most modern receivers do not use LC tuned filters but
instead use crystal, ceramic, mechanical, SAW or
DSP filters.
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9-6: Typical Receiver Circuits
Automatic Gain Control Circuits
 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 can be achieved by using a
potentiometer in RF and IF stages.
 Receivers include volume controls in audio circuits.
 AGC circuits are more effective in handling large signals
and give the receiver a very wide dynamic range.
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9-6: Typical Receiver Circuits
Automatic Gain Control Circuits: Controlling Circuit Gain
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The gain of a bipolar transistor amplifier is proportional
to the amount of collector current flowing.
 Two methods of applying AGC are as follows:
1. The gain can be decreased by decreasing the
collector current. This is called reverse AGC.
2. 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.
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9-6: Typical Receiver Circuits
Figure 9-37: An IF
differential amplifier
with AGC.
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9-6: Typical Receiver Circuits
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.
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9-6: Typical Receiver Circuits
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.
 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.
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9-6: Typical Receiver Circuits
Figure 9-42: The use of a BFO.
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9-6: Typical Receiver Circuits
Integrated Circuits (ICs) in Receivers
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In new designs, virtually all receiver circuits are ICs.
A complete receiver usually consists of three or four
ICs, plus coils, transformers, capacitors, and filters.
Most modern receivers are contained on a single IC.
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9-6: Typical Receiver Circuits
Integrated Circuits (ICs) in Receivers
 IC receivers are typically broken down into three major
sections:
1. The tuner, with RF amplifier, mixer, and local
oscillator
2. The IF section, with amplifiers, demodulator, and
AGC and muting circuits
3. The audio power amplifier.
 The second and third sections are entirely implemented
with ICs. The tuner may or may not be, for often the
LNA is separate.
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9-7: Receivers and Transceivers
VHF Aircraft Communication Circuit
 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.
© 2008 The McGraw-Hill Companies
77
9-7: Receivers and Transceivers
Figure 9-44 The aviation receiver—a superheterodyne unit built around four ICs—is
designed to receive AM signals in the 118- to 135-MHz frequency range. (Popular
Electronics, January 1991, Gernsback Publications, Inc.)
© 2008 The McGraw-Hill Companies
78
9-7: Receivers and Transceivers
Single-IC FM Receiver
 The Motorola MC3363 FM receiver IC chip contains all




receiver circuits except for the audio power amplifier (a
separate chip).
It is designed to operate at frequencies up to about 200
MHz
It is widely used in cordless telephones, paging
receivers, and other portable applications.
This dual-conversion receiver contains two mixers, two
local oscillators, a limiter, a quadrature detector, and
squelch circuits.
The first local oscillator has a built-in varactor that
allows it to be controlled by an external frequency
synthesizer.
© 2008 The McGraw-Hill Companies
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9-7: Receivers and Transceivers
Figure 9-45: The Motorola MC3363 dual-conversion receiver IC.
© 2008 The McGraw-Hill Companies
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9-7: Receivers and Transceivers
Transceiver
 Most two-way radio communication equipment is
packaged so that both transmitter and receiver are in a
unit known as a transceiver.
 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.
© 2008 The McGraw-Hill Companies
81
9-7: Receivers and Transceivers
Figure 9-47: An SSB transceiver showing circuit sharing.
© 2008 The McGraw-Hill Companies
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