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A guide to cables and connectors.

A GUIDE TO CABLES AND CONNECTORS

TYPES OF TRANSMISSION

LINES

Twin lead transmission cable is generally used where impedance matching alone is important, since it provides only minimal shielding. Impedance values of 300 [omega] and 600 [omega] are common. Lower impedance values require closer spacing of the conductors and are not normally available in this type of cable. A typical application for twin lead cable is in antenna lead wire for television sets.

Twisted pair is a variation of the twin lead type. It consists of two lengths of ordinary hookup wire twisted together. A twisted pair provides relatively constant impedance plus better magnetic shielding than twin lead cables. It is flexible, inexpensive, easy to terminate and is used extensively by the computer industry. However, it should not be used when maximum shielding is required.

Shielded twisted pair cable is used to eliminate inductive and capacitive coupling. Twisting cancels out inductive coupling, while the shield eliminates capacitive coupling. Most applications for this cable are between equipment, racks and buildings.

Flexible (braided) coaxial cable is by far the most common type of closed transmission line because of its flexibility. It is a coaxial cable, meaning that both the signal and the ground conductors are on the same center axis. The outer conductor is made from fine braided wire, hence the name "braided coaxial cable." This type of cable is used in practically all applications requiring complete shielding of the center conductor. The effectiveness of the shielding depends upon the weave of the braid and the number of braid layers. Cable sizes range from less than 1/8-in. diameter, for low-power applications of around 50 W, to over 1/2-in. diameter for power of 850 W at 100 MHz and voltages up to 5,000. In addition to power handling capabilities, cables are available for high-frequency applications, high- and low-temperature applications, severe environmental applications and many other specialized uses.

Triaxial cable is used when higher "shielding" efficiency characteristics are required in applications similar to those using shielded twisted pair cable.

Often you will hear the term "shielded cable." This is very similar to coaxial cable except the spacing between center conductor and shield is not carefully controlled during manufacture, resulting in nonconstant impedance.

Semirigid coaxial cable uses a solid tubular outer conductor rather than the braided type, so that all the RF energy is contained within the cable. One of the drawbacks of braided cable is that the shielding is not 100% effective, especially at higher frequencies. This is because the braided construction can permit small amounts of short wavelength (high-frequency) energy to radiate. Normally this does not present a problem; however, if a higher degree of shielding is required, semirigid coaxial cable is recommended. For applications using frequencies higher than 30 GHz a miniature semirigid cable is recommended. Various connectors are available to terminate these cables.

Ribbon coaxial cable is a relatively recent innovation which combines the advantages of both ribbon cable and coaxial cable. Each individual coaxial cable consists of the signal conductor, dielectric, a foil shield and a drain wire which is in continuous contact with the foil. The entire assembly is then covered with an outer insulating jacket. The unique manufacturing feature of this cable is the precise placement of the drain wires to permit gang stripping of the outer jacket and foil. The major advantage of this cable is the speed and ease with which it can be mass terminated. The conductors can also be separated into individual coaxial lines and terminated with standard coaxial connectors as required.

CONNECTOR TYPES

BNC connectors offer easy engagement and disengagement using bayonet couplings and overlapping dielectrics. They are most useful for frequently coupled and uncoupled RF connections with frequencies below 4 GHz. BNC connectors find applications in flexible networks, instrumentation and computer peripheral interconnections.

TNC connectors have an interface similar to BNC except for a threaded coupling nut. The tighter fit provided by this screw-on connection improves interface control allowing connectors to operate up to 11 GHz. TNC connectors are excellent for mobile units or aircraft where top-notch performance is required under vibration.

SHV high-voltage connectors also feature a bayonet coupling and have an extended overlapping dielectric to handle up to 5,000 V DC. They are recommended for high-pulse EMP instrument applications.

SMA threaded connectors are widely used in avionics, radar and microwave communications and instrumentation. Connectors operate to at least 12.4 GHz on flexible coax cables and up to 26.5 GHz on semi-rigid coax cables. Crimp-on SMA connectors that operate to 26.5 GHz are available.

Blind mate connectors operate to 26.5 GHz for 3.5-mm and 40 GHz for 2.8-mm styles. These connectors offer easy slide-on connection and require less alignment between the cable and the equipment than other connectors with comparable bandwidth. Blind mate connectors are widely used as coaxial interconnects between plug-in modules and motherboards.

SMB connectors feature a snap coupling for fast connection. A self-centering outer spring and overlapping dielectric allows easy snap-on and excellent performance even in moderate vibration. The SMB is smaller in size than the SMA and excellent where engineers are concerned about circuit miniaturization. Typical application is inter- or intraboard connection of RF or digital signals. Commercial 50-[Omega] versions operate to 4 GHz, and 75-[Omega] versions reach 2 GHz.

SMC connectors are similar to SMB versions, but use a screw-on connection for increased bandwidth (up to 10 GHz) and greater mechanical stability. These products are used primarily in military or high-vibration environments.

UHF connectors are relatively inexpensive screw-on products. They have large impedance discontinuities that limit their range to about 500 MHz. Miniature versions, however, offer 2-GHz bandwidth. These products are used extensively in commercial communications and instrument applications.

N threaded connectors have an air dielectric interface, are low cost and are available in 50- and 75-[Omega] impedance types. These connectors operate to 11 GHz and are commonly used in cable-based local-area networks (LANs) medium-power transmitters and test equipment.

C connectors are bayonet-coupled versions of N connectors with overlapping dielectrics in the interface. They feature a substantial 11-GHz bandwidth; however, they are sensitive to mechanical vibration.

Triaxial, twinaxial (twin coaxial) connectors find widespread use in computers, computer peripherals and control systems. The connectors accommodate balanced-mode transmission lines that are driven differentially for immunity to common-mode noise. Triaxial connectors feature bayonet coupling and accommodate several cable groups. Twinaxial connectors are available in a variety of soldered and crimped jacks and plugs. These include twin-threaded, twin-BNC and twin-video versions.

Miscellaneous connector types include multiple-circuit connectors that use coaxial contacts in a pin-and-socket configuration, crimp-on ferrules that offer fast, reliable connections for attaching one or more ground taps to shielded wire and braided shield terminations for connecting cable shields to printed circuit (pc) boards. Also available are network/premises interconnect products for Ethernet/IEEE 802.3 systems and coaxial taps for simple, dependable connections from transceiver to LAN without cutting the cable.

CONNECTOR SELECTION

Choosing the proper coaxial connector is a balance between performance and economics. The performance of the connector must meet the overall electrical requirements of the system in which it is to be used. The economics of connector selection should include the cost of the connector as well as the cost of terminating the connector. There are three major considerations in the selection of an RF coaxial connector:

* connector interface (BNC, TNC, etc.) * method of termination (pc board, cable, etc.) * physical construction (Mil-type, commercial, plating, etc.).

CONNECTOR INTERFACE

Most often the type of interface is dictated by the application. However, the electrical performance requirements must be considered.

Impedance - The connector should be matched to the system and cable impedance. Mismatches can cause reflections resulting in poor system performance. It should be noted that not all connector interfaces are available in 50, 75 and 95+ [Omega] impedances.

Voltage - Make sure the application will not exceed the maximum voltage rating of the connector. Figure 1 illustrates the maximum ratings of each interface type. If the application requires a high-voltage pulse, the SHV connector is recommended.

Maximum Operating Frequency - Each connector interface has an upper frequency limitation. Figure 2 illustrates the comparative limitations of each design. Some designs have a lower frequency limit for the commercial or 75-[Omega] offering.

In addition to the electrical performance characteristics, each interface type has advantages to the user. The BNC interface is the most popular because of its easy connect/disconnect bayonet coupling. The TNC interface offers a threaded coupling that assures excellent performance even in applications where vibration is common, such as in aircraft and automobiles. The SMB has a snap detent feature which makes it easy to install and prevents the connector from becoming unmated during high levels of vibration.

PHOTO : FIGURE 1 MAXIMUM VOLTAGE RATINGS OF DIFFERENT INTERFACE TYPES

PHOTO : FIGURE 2 COMPARATIVE LIMITATIONS OF CONNECTOR INTERFACE DESIGNS
COPYRIGHT 1992 Horizon House Publications, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Title Annotation:EW Design Engineers' Handbook & Manufacturers Directory; transmission cables
Publication:Journal of Electronic Defense
Date:Jan 1, 1992
Words:1472
Previous Article:Historical review of the high-speed IC issue.
Next Article:Vibration and acoustics.
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