Communications Receivers: DPS, Software Radios, and Design, 3rd Edition
656
Communications Receivers: DPS, Software Radios, and Design, 3rd Edition
656eBook
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Overview
From reception basics to cutting-edge software radio design, Communications Receivers, Third Edition brings you a storehouse of task-simplifying and task-clarifying information, examples, and tips. Written by well-known experts Ulrich Rohde, Jerry Whitaker, and Andrew Bateman, this guide covers everything from front end systems to frequency generators and controllers. Topics are thoroughly illuminated for you with hundreds of illustrations, diagrams, and mathematical equations.
You’ll learn the principles and practices involved in receivers and receiver systems, antennas and antenna coupling, amplifiers and gain control, mixers, frequency, oscillators, demodulation and demodulators, digital signal processing, and much more. Discover for yourself why this resource has been prized through two editions by professionals and hobbyists for its ready-to-use insights on the theory and design of all types of communications receivers -- including shortwave, military, broadcast, and direction-finding.
This newly revised edition features:
Advances in DSP, cellular, and software radio design
Details on designing, operating, specifying, installing, and maintaining every kind of receiver in common use
Specific design approaches, circuit examples, and component specs
Help with microprocessors and logic devices
Coverage of important pulse and data operating modes
More than 250 illustrations and diagrams
Handy reference material in tables, charts, and figures
More!
Product Details
| ISBN-13: | 9780071369657 |
|---|---|
| Publisher: | McGraw Hill LLC |
| Publication date: | 11/22/1996 |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 656 |
| File size: | 33 MB |
| Note: | This product may take a few minutes to download. |
About the Author
Read an Excerpt
Chapter 1: Basic Radio Considerations
Because of the earth and its atmosphere, most terrestrial communications links cannot be considered free-space links. Additional losses occur in transmission. Moreover, the received signal field is accompanied by an inevitable noise field generated in the atmosphere or space, or by machinery. In addition, the receiver itself is a source of noise. Electrical noise limits the performance of radio communications by requiring a signal field sufficiently great to overcome its effects.While the characteristics of transmission and noise are of general interest in receiver design, it is far more important to consider how these characteristics affect the design. The following sections summarize the nature of noise and transmission effects in frequency bands through SHF (30 GHz).
ELF and VLF (up to 30 kHz)
Transmission in the extremely-low frequency (ELF) and very-low frequency (VLF) range is primarily via surface wave with some of the higher-order waveguide modes introduced by the ionosphere appearing at the shorter ranges. Because transmission in these frequency bands is intended for long distances, the higher-order modes are normally unimportant. These frequencies also provide the only radio communications that can penetrate the oceans substantially. Because the transmission in saltwater has an attenuation that increases rapidly with increasing frequency, it may be necessary to design depth-sensitive equalizers for receivers intended for this service. At long ranges, the field strength of the signals is very stable, varying only a few decibels diurnally and seasonally, and being minimally affected by changes in solar activity. There is more variation at shorter ranges. Variation of the phase of the signal can be substantial during diurnal changes and especially during solar flares and magnetic storms. For most communications designs, these phase changes are of little importance. The noise at these low frequencies is very high and highly impulsive. This situation has given rise to the design of many noise-limiting or noise-canceling schemes, which find particular use in these receivers. Transmitting antennas must be very large to produce only moderate efficiency; however, the noise limitations permit the use of relatively short receiving antennas because receiver noise is negligible in comparison with atmospheric noise at the earth's surface. In the case of submarine reception, the high attenuation of the surface fields, both signal and noise, requires that more attention be given to receiving antenna efficiency and receiver sensitivity.
LF (30 to 300 kHz) and MF (300 kHz to 3 MHz)
At the lower end of the low frequency (LF) region, transmission characteristics resemble VLF. As the frequency rises, the surface wave attenuation increases, and even though the noise decreases, the useful range of the surface wave is reduced. During the daytime, ionospheric modes are attenuated in the D layer of the ionosphere. The waveguide mode representation of the waves can be replaced by a reflection representation. As the medium-frequency (MF) region is approached, the daytime sky wave reflections are too weak to use.
The surface wave attenuation limits the daytime range to a few hundred kilometers at the low end of the MF band to about 100 km at the high end. Throughout this region, the range is limited by atmospheric noise. As the frequency increases, the noise decreases and is minimum during daylight hours. The receiver noise figure (NF) makes little contribution to overall noise unless the antenna and antenna coupling system are very inefficient. At night, the attenuation of the sky wave decreases, and reception can be achieved up to thousands of kilometers. For ranges of one hundred to several hundred kilometers, where the single-hop sky wave has comparable strength to the surface wave, fading occurs. This phenomenon can become quite deep during those periods when the two waves are nearly equal in strength.
At MF, the sky wave fades as a result of Faraday rotation and the linear polarization of antennas. At some ranges, additional fading occurs because of interference between the surface wave and sky wave or between sky waves with different numbers of reflections. When fading is caused by two (or more) waves that interfere as a result of having traveled over paths of different lengths, various frequencies within the transmitted spectrum of a signal can be attenuated differently. This phenomenon is known as selective fading and results in severe distortion of the signal. Because much of the MF band is used for AM broadcast, there has not been much concern about receiver designs that will offset the effects of selective fading. However, as the frequency nears the high frequency (HF) band, the applications become primarily long-distance communications, and this receiver design requirement is encountered. Some broadcasting occurs in the LF band, and in the LF and lower MF bands medium-range narrow-band communications and radio navigation applications are prevalent.
HF (3 to 30 MHz)
Until the advent of satellite-borne radio relays, the HF band provided the only radio signals capable of carrying voiceband or wider signals over very long ranges (up to 10,000 km). VLF transmissions, because of their low frequencies, have been confined to narrow-band data transmission. The high attenuation of the surface wave, the distortion from sky-wave-reflected near-vertical incidence (NVI), and the prevalence of long-range interfering signals make HF transmissions generally unsuitable for short-range communications. From the 1930s into the early 1970s, HF radio was a major medium for long-range voice, data, and photo communications, as well as for overseas broadcast services, aeronautical, maritime and some ground mobile communications, and radio navigation. Even today, the band remains active, and long-distance interference is one of the major problems. Because of the dependence on sky waves, HF signals are subject to both broad-band and selective fading. The frequencies capable of carrying the desired transmission are subject to all of the diurnal, seasonal, and sunspot cycles, and the random variations of ionization in the upper ionosphere. Sunspot cycles change every 11 years, and so propagation tends to change as well. Significant differences are typically experienced between day and night coverage patterns, and between summer to winter coverage. Out to about 4000 km, E-layer transmission is not unusual, but most of the very long transmission-and some down to a few thousand kilometers-is carried by F-layer reflections. It is not uncommon to receive several signals of comparable strength carried over different paths. Thus, fading is the rule, and selective fading is common. Atmospheric noise is still high at times at the low end of the band, although it becomes negligible above about 20 MHz.
Receivers must be designed for high sensitivity, and impulse noise reducing techniques must often be included. Because the operating frequency must be changed on a regular basis to obtain even moderate transmission availability, most HF receivers require coverage of the entire band and usually of the upper part of the MF band. For many applications, designs must be made to combat fading. The simplest of these is automatic gain control (AGC), which also is generally used in lower-frequency designs. Diversity reception is often required, where signals are received over several routes that fade independently-using separated antennas, frequencies, and times, or antennas with different polarizations-and must be combined to provide the best composite output. If data transmissions are separated into many parallel low-rate channels, fading of the individual narrow-band channels is essentially flat, and good reliability can be achieved by using diversity techniques. Most of the data sent over HF use such multitone signals.
In modern receiver designs, adaptive equalizer techniques are used to combat multipath that causes selective fading on broadband transmissions. The bandwidth available on HF makes possible the use of spread-spectrum techniques intended to combat interference and, especially, jamming. This is primarily a military requirement.
VHF (30 to 300 MHz)
Most very-high frequency (VHF) transmissions are intended to be relatively short-range, using line-of-sight paths with elevated antennas, at least at one end of the path. In addition to FM and television broadcast services, this band handles much of the land mobile and some fixed services, and some aeronautical and aeronavigation services. So long as a good clear line of sight with adequate ground (and other obstruction) clearance exists between the antennas, the signal will tend to be strong and steady. The wavelength is, however, becoming sufficiently small at these frequencies so that reflection is possible from ground features, buildings, and some vehicles. Usually reflection losses result in transmission over such paths that is much weaker than transmission over line-of-sight paths. In land mobile service, one or both of the terminals may be relatively low, so that the earth's curvature or rolling hills and gullies can interfere with a line-of-sight path. While the range can be extended slightly by diffraction, in many cases the signal reaches the mobile station via multipath reflections that are of comparable strength or stronger than the direct path. The resulting interference patterns cause the signal strength to vary from place to place in a relatively random matter.
There have been a number of experimental determinations of the variability, and models have been proposed that attempt to predict it. Most of these models apply also in the ultrahighfrequency (UHF) region. For clear line-of-sight paths, or those with a few well-defined intervening terrain features, accurate methods exist for predicting field strength. In this band, noise is often simply thermal, although man-made noise can produce impulsive interference. For vehicular mobile use, the vehicle itself is a potential source of noise. In the U.S., mobile communications have used FM, originally of a wider band than necessary for the information, so as to reduce impulsive noise effects. However, recent trends have reduced the bandwidth of commercial radios of this type so that this advantage has essentially disappeared. The other advantage of FM is that hard limiting can be used in the receiver to compensate for level changes with the movement of the vehicle...