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Chapter 8
Communication Receivers
Topics Covered in Chapter 8
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8-1: Basic Principles of Signal Reproduction
8-2: Superheterodyne Receivers
8-3: Frequency Conversion
8-4: Intermediate Frequency and Images
8-5: Noise
8-6: Typical Receiver Circuits
8-7: Receivers and Transceivers
8-1: Basic Principles of Signal Reproduction
• 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.
8-1: 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 selectivity will isolate the
desired signal and greatly attenuate other signals.
• A receiver with good sensitivity involves high
circuit gain.
8-1: Basic Principles of Signal Reproduction
Selectivity: Q and Bandwidth
– Selectivity in a receiver is obtained by using tuned
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.
8-1: Basic Principles of Signal Reproduction
Figure 8-1: Selectivity curve of a tuned circuit.
8-1: Basic Principles of Signal Reproduction
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.
8-1: Basic Principles of Signal Reproduction
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.
8-1: Basic Principles of Signal Reproduction
Sensitivity
– Another factor that affects the sensitivity of a receiver
is 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.
8-1: Basic Principles of Signal Reproduction
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.
8-1: Basic Principles of Signal Reproduction
Figure 9-4: The simplest receiver—a crystal set.
8-1: Basic Principles of Signal Reproduction
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.
8-1: Basic Principles of Signal Reproduction
Figure 8-5: Tuned radio-frequency (TRF) receiver.
8-1: Basic Principles of Signal Reproduction
Tuned Radio Frequency (TRF) Receiver
– Many RF amplifiers use multiple tuned circuits.
– Whenever resonant LC circuits tuned to the same
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.
8-2: 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.
8-2: Superheterodyne Receivers
Figure 8-8 Block diagram of a superheterodyne receiver.
8-2: Superheterodyne Receivers
RF Amplifier
– The antenna picks up the weak radio signal and
feeds it to the RF amplifier, also called a lownoise 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.
8-2: Superheterodyne Receivers
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.
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.
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.
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.
8-2: 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.
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.
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.
8-3: 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.
8-3: Frequency Conversion
Figure 9-9: Concept of a mixer.
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.
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.
9-3: Frequency Conversion
Figure 9-10: A simple diode mixer.
8-3: 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.
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.
9-3: Frequency Conversion
Figure 9-15: NE602 IC
mixer. (a) Block diagram
and pinout. (b)
Simplified schematic.
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.
8-3: Frequency Conversion
Local Oscillator and Frequency Synthesizers: LC
Oscillator
– A local oscillator is sometimes referred to as a
variable-frequency 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.
8-3: Frequency Conversion
Figure 8-17: A VFO for receiver local oscillator service.
8-3: 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.
8-3: Frequency Conversion
Figure 8-18: A frequency synthesizer used as a receiver local oscillator.
8-4: 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
with high gain.
9-4: Intermediate
Frequency and Images
• 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.
9-4: Intermediate
Frequency and Images
Figure 9-19: Relationship of the signal and image frequencies.
9-4: Intermediate
Frequency and Images
Figure 9-20: Signal, local oscillator, and image frequencies in a superheterodyne.
8-4: Intermediate
Frequency and Images
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.
9-4: Intermediate
Frequency and Images
Figure 9-21: A low IF compared to the signal frequency with low-Q tuned circuits causes
images to pass and interfere.
9-4: Intermediate
Frequency and Images
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.
9-4: Intermediate
Frequency and Images
Figure 9-22: A dual-conversion superheterodyne.
8-4: Intermediate
Frequency and Images
Direct Conversion Receivers
– 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).
9-4: Intermediate
Frequency and Images
Figure 9-23: A direct-conversion (zero-IF) receiver.
9-4: Intermediate
Frequency and Images
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 voltage-controlled
oscillator is needed.
• There is no image problem.
9-4: Intermediate
Frequency and Images
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.
8-4: Intermediate
Frequency and Images
Figure 8-24: A direct conversion receiver for FM, FSK, PSK, and digital modulation.
8-4: Intermediate
Frequency and Images
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.
8-4: Intermediate
Frequency and Images
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.
8-5: 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.
8-5: 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.
8-5: 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.
8-5: 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.
8-5: 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
8-5: 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.
8-5: 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.
8-6: Typical Receiver Circuits
• Typical receiver circuits include:
– RF amplifiers
– IF amplifiers
– AGC
– AFC
– Special circuits
8-6: 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.
8-6: Typical Receiver Circuits
Figure 8-30: A typical RF amplifier used in receiver front ends.
8-6: Typical Receiver Circuits
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.
8-6: Typical Receiver Circuits
Figure 8-33: A two-stage IF amplifier using double-tuned transformer coupling for selectivity.
8-6: 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.
8-6: 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.
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.
9-6: Typical Receiver Circuits
Automatic Gain Control Circuits: Controlling 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:
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.
9-6: Typical Receiver Circuits
Figure 9-37: An IF
differential amplifier
with AGC.
8-6: 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.
8-6: 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.
8-6: Typical Receiver Circuits
Figure 8-42: The use of a BFO.
8-6: Typical Receiver Circuits
Integrated Circuits (ICs) in Receivers
– 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.
8-6: 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.
8-7: Receivers and Transceivers
VHF Aircraft Communication Circuit
– A typical VHF receiver is designed to receive twoway 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.
8-7: Receivers and Transceivers
Figure 8-44 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.)
8-7: 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.
8-7: Receivers and Transceivers
Figure 8-45: The Motorola MC3363 dual-conversion receiver IC.
8-7: 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.
8-7: Receivers and Transceivers
Figure 8-47: An SSB transceiver showing circuit sharing.