Chapter Six: Receivers
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Transcript of Chapter Six: Receivers
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Chapter Six:Receivers
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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
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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
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Simple Receiver
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Tuned-radio-frequency Receiver• In a receiver
with multiple RF stages, all tuned circuits must track together, typically by ganged-tuning methods as shown:
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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
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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
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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
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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
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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
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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
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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
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Full-Carrier AM• The simplest, most popular demodulator for full-
carrier 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 high-frequency components
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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
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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
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FM Detectors
• There are four major types of FM detectors:– Foster-Seely discriminator
– Ratio detector
– Quadrature detector
– PLL detector
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Foster-Seely Detector
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Ratio Detector
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Quadrature Detector
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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
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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
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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
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Receiver Measurements• Sensitivity - measured with a calibrated RF signal
generator and audio voltmeter• Selectivity - measured with an RF generator