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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.
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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
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
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 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
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
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
