MICROWAVE COAXIAL CONNECTOR TECHNOLOGY: a CONTINUING EVOLUTION Mario A
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MAURY MICROWAVE 13 Dec 2005 CORPORATION Originally published as a Feature Article in the Microwave Journal 1990 State of the Art Reference, September 1990; Updated December 2005 MICROWAVE COAXIAL CONNECTOR TECHNOLOGY: A CONTINUING EVOLUTION Mario A. Maury, Jr. Maury Microwave Corporation (a) Introduction Coaxial connectors are one of the fundamental tools of microwave technology and yet they appear to be taken for granted in many instances. Unfortunately, many engineers tend to overlook the lowly connec- tor with resulting performance compromises in their applications. A good understanding of connectors, both electrically and mechanically, is required to utilize them properly and derive their full benefit. It should be remembered that performance starts at the (b) connector. Figure 1: Precision coaxial connectors in use today; Coaxial connectors provide a means to connect and (a) 7mm and 14mm sexless connectors and (b) 3.5mm female disconnect transmission lines, components and sys- and male, type N female and male. tems at microwave frequencies. They allow accessing circuits, modularizing, testing, assembling, inter- This paper provides a brief history of coaxial connec- connecting and packaging components into systems. tors, gives an overview of coaxial connector technology today, cites sources of further informa- There is a broad variety of coaxial connectors avail- tion and takes a look into the future as connectors able today due to the various design trade-offs and continue to evolve. applications that exist at microwave frequencies, including impedance (usually 50 ohms), frequency IEEE P287 Committee of operation, power handling, insertion loss, reflec- tion performance, environmental requirements, size, In 1988, the sub-committee P287 for precision co- weight and cost. axial connectors under the IEEE/Instrumentation and Measurement Society was re-activated by the IEEE Precision connectors have played a major role in the Standards Board to carry on the work originally evolution of microwave coaxial connector technol- begun in 1962. ogy, as shown in Figure 1. It is through these connectors that our measurement equipment has The objective of this committee was to revise IEEE improved and thereby connectors, in general, have Standard 287 published in 1968 so that it represents improved. The foresightedness of the C83.2 and IEEE the current state of the art in precision connector P287 connector committees in the early 1960s gave technology; to standardize both laboratory precision us the philosophies and concepts and paved the way connectors (LPC) and general precision connectors to solutions for the fabrication of precision connec- (GPC) in a minimum number of transmission line tors that are in use today. The IEEE P287 committee sizes covering the frequency range from DC to 110 has been re-activated and is carrying on this impor- GHz; to standardize the means for transferring labo- tant work (see IEEE P287 Committee). ratory measurements to devices with field type con- 2900 Inland Empire Blvd. • Ontario, California 91764-4804 application note 5A-021 Tel: 909-987-4715 • Fax: 909-987-1112 • http://www.maurymw.com Copyright 2000 Maury Microwave Inc., all rights reserved. SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 1 of 21 13 Dec 2005 nectors; to produce a standard that is useful and of sexed connectors. This is the predominant cat- meaningful to the manufacturers and users of preci- egory of connectors in use today. sion coaxial connectors; and to incorporate any other items deemed relavant and appropriate into the stan- The type of connector refers to a specific connector dard. configuration that forms the basis of a family of connectors that mate with each other. For example, The committee is in the process of updating the types N, BNC, SMA and 3.5mm are a family. It is also standard that currently covers 7mm and 14mm sex- possible to have mating between families, for ex- less precision connectors. In addition, it has adopted ample, SMA and 3.5mm, 2.4mm and 1.85mm. pin socket type connectors in the line sizes and precision type N connectors for standardization, There are three grades of connectors; production, shown in Table 1. instrument and metrology. The production grade of connectors includes general purpose or field con- TABLE 1 nectors for components and cables. The emphasis Adopted Pin Socket Type Connectors should be one assembly simplicity and low cost. Line Size (mm) Maximum Frequency (GHz) 3.5 33 The instrument grade of connectors includes preci- 2.92 40 sion or test connectors for use with test and 2.4 50 measurement equipment, meeting high performance 1.85 65 1 110 standards, that is, low reflection and good repeat- ability, and moderate cost. The committee is chaired by Harmon Banning of W. L. Gore & Associates, Newark, Delaware and con- The metrology grade of connectors include high sists of 25 members who provie a broad cross section precision connectors primarily used on measure- of the microwave industry. ment standards where the highest accuracy is required and would allow traceability to national standards. Terminology and Definitions of The cost of these connectors is high. Coaxial Connectors There are basically two distinct categories of con- Obviously it would be difficult for all connectors to nectors, sexless and sexed. meet this criteria. However, most can at least be made to meet production and instrument grade. New A sexless connector is a connector where both halves connectors, as they are designed, should follow these of the connector mated pair are identical. They are general guidelines. coplanar. The outer conductor coupling mechanism and the center conductor contacts are captive to the Interface is the mechanical configuration (dimen- individual halves. There are sexed outer conductor sions and tolerances) of a connector mating properties coupling versions of sexless connectors. For ex- that must be clearly defined in order to insure me- ample, 7mm GPC connectors typically are limited to chanical mating compatibility and electrical precision connectors. These also are referred to as repeatability. Different dielectrics present at the hermaphroditic connectors. interface include air dielectric and dielectric. Sexed connectors have a female and a male configu- Air dielectric simplifies connector construction and ration to form a mated pair. They can be coplanar or generally is used on precision connectors so accu- non-coplanar, and generally are sexed in both con- rate standards can be created. These connectors ductors. Type N and SMA connectors are examples include type N, 7mm and 3.5mm. 5A-021 application note 2900 Inland Empire Blvd. • Ontario, California 91764-4804 Tel: 909-987-4715 • Fax: 909-987-1112 • http://www.maurymw.com SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 2 of 21 MAURY MICROWAVE 13 Dec 2005 CORPORATION A solid dielectric like teflon usually is used and there the only way they can be measured; this is particu- are two basic configurations, flush and overlapping. larly true of non coplanar connectors. Flush connectors are SMA and SSMA; and overlap- ping connectors are BNC, TNC, C and SC. There are many ways to describe connectors and Overlapping constructions generally are used for usually a combination of these terms is involved. higher power application and to prevent voltage The following two important equations are helpful in breakdown. defining coaxial connector parameters1, impedance, Reference plane is the outer conductor mating plane 59.9586 D Z = ln (Ω) of a coaxial connector. It is desirable to have both √ε d the outer and center conductors coplanar at this cutoff frequency (TE ), plane, that is, in the identical plane, for electrical 11 and mechanical reasons. 7512 f = (GHz) c √ε Coplanar connectors are connectors where the cen- (d + D) ter and outer conductor mate in the same plane. where These connectors include 7mm, SMA and 3.5mm. D=outer conductor inner diameter Non coplanar connectors are connectors where the d=center conductor outer diameter center conductors don't mate in the same plane. ε = the dielectric constant in transmission line These connectors include N, BNC, C and TNC. air = 1.00059 Coupling type describes how the outer conductors teflon = 2.02 are connected. There are basically three types, threaded, twist or bayonet locks and snap-on. Type The predominant impedance of coaxial connectors N, TNC, SMA and 7mm are threaded; type BNC and used is 50 Ω (see Why 50 Ω Connectors?). C are twist locks; and SMB is a snap-on. Connector electrical specifications are sometimes All sexed connectors are of the pin-and-socket type, difficult to specify or verify because their perfor- where there is a male contact (pin) and a female mance depends on how they are assembled to the contact (socket). There are currently two types of transmission line media, that is, flexible cable, female contacts slotted and slotless, used for preci- stripline or microstrip. The only connectors that can sion applications. N, BNC, SMA and 3.5mm meet be accurately specified are connectors for mounting this definition. or rigid coaxial lines. Millimeter-wave coaxial connectors are coaxial con- Connectors covered in this paper are generally lim- nectors for use above 18 GHz. The term is appropriate ited to those developed or in primary use in the because they operate in the mm-wave region. Gen- United States. Also all dimensions are in inches erally these are pin-and-socket type connectors, such unless otherwise specified. as SMA, 3.5mm and 2.4mm. Sexless connectors also have been made above 18 GHz. Some helpful hints are: use the largest coaxial transmission line and connector possible for your As a general rule, a coaxial connector's electrical application, as this will provide the best perfor- performance (SWR and insertion loss) must be de- mance; check the manufacturer's specifications (both fined based on a mated connector pair since this is mechanical and electrical), sometimes they are bet- 2900 Inland Empire Blvd.