Chapter 2 Radio Receiver Characteristics

Source: Communications Receivers: DSP, Software Radios, and Design Chapter 1 Basic Radio Considerations 1.1 Radio Communications Systems The capability of radio waves to provide almost instantaneous distant communications without interconnecting wires was a major factor in the explosive growth of communications during the 20th century. With the dawn of the 21st century, the future for communications systems seems limitless. The invention of the vacuum tube made radio a practical and affordable communications medium. The replacement of vacuum tubes by transistors and integrated circuits allowed the development of a wealth of complex communications systems, which have become an integral part of our society. The development of digital signal processing (DSP) has added a new dimension to communications, enabling sophis- ticated, secure radio systems at affordable prices. In this book, we review the principles and design of modern single-channel radio receivers for frequencies below approximately 3 GHz. While it is possible to design a receiver to meet specified requirements without knowing the system in which it is to be used, such ig- norance can prove time-consuming and costly when the inevitable need for design compro- mises arises. We strongly urge that the receiver designer take the time to understand thor- oughly the system and the operational environment in which the receiver is to be used. Here we can outline only a few of the wide variety of systems and environments in which radio receivers may be used. Figure 1.1 is a simplified block diagram of a communications system that allows the transfer of information between a source where information is generated and a destination that requires it. In the systems with which we are concerned, the transmission medium is radio, which is used when alternative media, such as light or electrical cable, are not technically feasible or are uneconomical. Figure 1.1 represents the simplest kind of communications system, where a single source transmits to a single destination. Such a system is often referred to as a simplex system. When two such links are used, the second sending information from the destination location to the source location, the system is referred to as duplex. Such a system may be used for two-way communication or, in some cases, simply to provide information on the quality of received information to the source. If only one transmitter may transmit at a time, the system is said to be half-duplex. Figure 1.2 is a diagram representing the simplex and duplex circuits, where a single block T represents all of the information functions at the source end of the link and a single block R represents those at the destination end of the link. In this simple diagram, we encounter one of the problems which arise in communications systems—a definition of the boundaries between parts of the system. The blocks T and R, which might be thought of as transmitter and receiver, incorporate several functions that were portrayed separately in Figure 1.1. 1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Basic Radio Considerations 2 Communications Receivers Figure 1.1 Simplified block diagram of a communications link. Figure 1.2 Simplified portrayal of communications links: (a) simplex link, (b) duplex link. Many radio communications systems are much more complex than the simplex and duplex links shown in Figures 1.1 and 1.2. For example, a broadcast system has a star configuration in which one transmitter sends to many receivers. A data-collection network may be organized into a star where there are one receiver and many transmitters. These configurations are indicated in Figure 1.3. A consequence of a star system is that the peripheral ele- ments, insofar as technically feasible, are made as simple as possible, and any necessary complexity is concentrated in the central element. Examples of the transmitter-centered star are the familiar amplitude-modulated (AM), frequency-modulated (FM), and television broadcast systems. In these systems, high-power transmitters with large antenna configurations are employed at the transmitter, whereas most receivers use simple antennas and are themselves relatively simple. An example of the receiver-centered star is a weather-data-collection network, with many unattended measur- ing stations that send data at regular intervals to a central receiving site. Star networks can be configured using duplex rather than simplex links, if this proves desirable. Mobile radio networks have been configured largely in this manner, with the shorter-range mobile sets transmitting to a central radio relay located for wide coverage. Cellular radio systems incorporate a number of low-power relay stations that provide contiguous coverage over a large area, communicating with low-power mobile units. The relays are interconnected by various means to a central switch. This system uses far less spectrum than conventional mobile systems because of the capability for reuse of frequencies in noncontiguous cells. Packet radio transmission is another example of a duplex star network. Stations transmit at random times to a central computer terminal and receive responses sent from the com- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Basic Radio Considerations Basic Radio Considerations 3 Figure 1.3 Star-type communications networks: (a) broadcast system, (b) data-collection network. puter. The communications consist of brief bursts of data, sent asynchronously and contain- ing the necessary address information to be properly directed. The term packet network is applied to this scheme and related schemes using similar protocols. A packet system typically incorporates many radios, which can serve either as terminals or as relays, and uses a flooding-type transmission scheme. The most complex system configuration occurs when there are many stations, each hav- ing both a transmitter and receiver, and where any station can transmit to one or more other stations simultaneously. In some networks, only one station transmits at a time. One may be designated as a network controller to maintain a calling discipline. In other cases, it is necessary to design a system where more than one station can transmit simultaneously to one or more other stations. In many radio communications systems, the range of transmissions, because of terrain or technology restrictions, is not adequate to bridge the gap between potential stations. In such a case, radio repeaters can be used to extend the range. The repeater comprises a receiving system connected to a transmitting system, so that a series of radio links may be established to achieve the required range. Prime examples are the multichannel radio relay system used by long-distance telephone companies and the satellite multichannel relay systems that are used extensively to distribute voice, video, and data signals over a wide geographic area. Satellite relay systems are essential where physical features of the earth (oceans, high moun- tains, and other physical restrictions) preclude direct surface relay. On a link-for-link basis, radio relay systems tend to require a much higher investment than direct (wired) links, depending on the terrain being covered and the distances involved. To make them economically sound, it is common practice in the telecommunications indus- try to multiplex many single communications onto one radio relay link. Typically, hundreds of channels are sent over one link. The radio links connect between central offices in large population centers and gather the various users together through switching systems. The hundreds of trunks destined for a particular remote central office are multiplexed together into one wider-bandwidth channel and provided as input to the radio transmitter. At the other central office, the wide-band channel is demultiplexed into the individual channels and distributed appropriately by the switching system. Telephoneand data common carriers are probably the largest users of such duplex radio transmission. The block diagram of Fig- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Basic Radio Considerations 4 Communications Receivers Figure 1.4 Block diagram of simplified radio relay functions: (a) terminal transmitter, (b)repeater (without drop or insert capabilities), (c) terminal receiver. ure 1.4 shows the functions that must be performed in a radio relay system. At the receiving terminal, the radio signal is intercepted by an antenna, amplified and changed in frequency, demodulated, and demultiplexed so that it can be distributed to the individual users. In addition to the simple communications use of radio receivers outlined here, there are many special-purpose systems that also require radio receivers. While the principles of design are essentially the same, such receivers have peculiarities that have led to their own design specialties. For example, in receivers used for direction finding, the antenna systems have specified directional patterns. The receivers must accept one or more inputs and pro- cess them so that the output signal can indicate the direction from which the signal arrived. Older techniques include the use of loop antennas, crossed loops, Adcock antennas, and other specialized designs, and determine the direction from a pattern null. More modern Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Basic Radio Considerations Basic Radio Considerations 5 systems use complex antennas, such as the Wullenweber.

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