Transcript Chapter Six: Receivers

Chapter Six:
Receivers
Introduction
• Two important specifications are fundamental to
all receivers:
– Sensitivity: signal strength required to achieve a given
signal-to-noise ratio
– Selectivity: the ability to reject unwanted signals
Receiver Topologies
• Nearly all modern receivers use the
superheterodyne principle
• The simplest receiver would consist of a
demodulator connected directly to an antenna
• Adding a tuned circuit would improve the
performance
Simple Receiver
Tuned-radio-frequency Receiver
• In a receiver
with multiple
RF stages, all
tuned circuits
must track
together,
typically by
ganged-tuning
methods as
shown:
The Superheterodyne Receiver
• The superheterodyne receiver was invented in
1918 by Edwin H. Armstrong and is still almost
universally used
• A superheterodyne receiver is characterized by
one or more stages of RF amplification and the RF
stage may be tuned or broadband
Functional Elements of
Superheterodyne Receivers
• The input filter and RF stage are referred to as the
Front End of a receiver
• The mixer combines the signal frequency with a
sine-wave signal generated by a local oscillator
creating an intermediate frequency
Receiver Characteristics
• Sensitivity - the ability to receive weak signals with an
acceptable signal-to-noise ratio
• One common specification for AM receivers is the signal
strength required for a 10-dB signal-plus-noise-to-noise
ratio at a specified power level
• Adjacent channel sensitivity is another way of specifying
selectivity
• Techniques like alternate channel rejection are also used to
specify selectivity
Receiver Characteristics: Distortion
• Distortion comes in several forms:
– Harmonic distortion is when the frequencies generated
are multiples of those in the original signal
– Intermodulation distortion occurs when frequency
components in the original signal mix and produce sum
and difference signals
– Phase distortion consists of irregular shifts in phase and
is common when signals pass through filters
Dynamic Range
• The ratio between between the receiver’s response to weak
signals and signals that are overload one or more stages is
referred to as Dynamic Range
• Blocking may occur when two adjacent signals, one of
which is much stronger than the other, cause a reduction in
sensitivity to the desired channel. This is also referred to
as desensitization or desense
Spurious Responses
• Superheterodyne receivers have a tendency to receive
signals they are not tuned to
• Image Frequencies are signals that are produced as a result
of the generation of intermediate frequencies
Demodulators
• The demodulator, also known as the detector, is
the part of the receiver that recovers the baseband
signal. It performs the inverse operation to the
transmitter modulator
• Several types of modulators are used, depending
upon the type of modulation in use
Full-Carrier AM
• The simplest, most popular demodulator for fullcarrier AM is the envelope detector
• To recover the baseband signal, the incoming
signal is simply rectified to remove half the
envelope, then filtered to remove the highfrequency components
SSBSC AM
• A diode detector alone will not work for SSB or
DSBSC because the envelope is different from
that of AM
• Typically, a product detector using a balanced
modulator is used
FM
• FM demodulators must convert frequency
variations of the input signal into amplitude
variations at the output
• The amplitude of the output must be proportional
to the frequency deviation of the input
FM Detectors
• There are four major types of FM detectors:
–
–
–
–
Foster-Seely discriminator
Ratio detector
Quadrature detector
PLL detector
Foster-Seely Detector
Ratio Detector
Quadrature Detector
Communications Receivers
• The term communications receiver is used mainly
for general-purpose receivers that cover a wide
range of frequencies from 100 kHz to 30 MHz
• Generally, communications receivers divide their
coverage over several bands
Components of
Communications Receivers
• Squelch - disables the receiver audio in the absence of a signal
• Noise limiters typically use a diode limiter or clipper in the
audio section of the receiver
Transceivers
• A transceiver is simply a transmitter and receiver in
one box
• Transceivers are convenient and allow certain
economies to be made
• Most transceivers operate in the half-duplex mode
Receiver Measurements
• Sensitivity - measured with a calibrated RF
signal generator and audio voltmeter
• Selectivity - measured with an RF generator