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 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

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