RF and Microwave Connectors Part - 1

Contents

1. Introduction
   

2. What is a connector
   

3. Theory behind connector
   

4. Modes of propagation and connector
   

5. Co-axial transmission line
   

6. Types of connectors

1. Male/Plug and Female/Jack connectors
2. Gender neutral connectors
3. Reverse polarity connectors
4. N-connector
5. BNC Connector
6. SMA Connector
7. SMB Connector
8. SMC Connector
9. TNC Connector
10. 7/16 DIN connector
11. GR874 Connector
12. GR900BT Connector
    
    
     

1. Introduction

In the field of electronics, we come across various types of connectors. They play an important part in the functioning of the electronic system, as they are the point of contact through which a signal enters or leaves the system. These connectors are either a part of the chassis consisting of the electronic circuit or constructed at the ends of a cable connecting two such electronic circuit system.

What are these connectors, why there are different types of connectors, what are their characteristics? This article aims to explore the topic of interconnect system or connectors, and to understand the different aspect of this interesting subject. To begin with the discussion on connectors, it becomes necessary to provide a brief introduction of connectors.

2. What is a connector?

A connector is essentially a point of transition for a signal, in the sense that it travels through this junction to another point in the system. This could be a point where a connector is located, through which the signal leaves an electronic circuit and arrives at an antenna to be transmitted into the space or vice versa. The basic purpose of a connector is to transport a signal but at the same time it is required that it should not make the signal noisy or cause reflections due to this discontinuity.

A poor connector in a system can distort a signal or lower the signal strength or cause it to radiate etc. Even the material used for connector construction (metal strength, metal type, dielectric material used etc) of a connector plays a role in determining the intended application (space, satellite, military, commercial etc) and its successful implementation. Recently, a satellite mission failed due to the on-board connectors breaking off. Therefore, it is important to select the right connector or the right Interconnect System for the required application and operating frequency.

3. Theory behind connector

As the signal frequency increases, say at microwave frequency, the properties and characteristics defining the signal and the transmission medium are different than those at lower frequency. In the high frequency domain of microwave and radio frequency, distributed circuit elements and their effect play an important role in analysing the selection of a proper interconnecting system. For example, a power amplifier circuit constructed on a microstrip line needs a special type of connector at its output end, which will allow it to be connected to the antenna port or another device. This connector should be able to handle high power and at the same time have a matched impedance of the transmission line. Hence, we talk of connectors, which are specially constructed for 50 ohms or 75 ohms characteristic impedance system.

4. Modes of propagation and connector

At microwave frequency we deal with the wave nature of the signal when describing its propagation through a transmission medium. These are so-called modes of propagation and result from the transverse nature of the electric and magnetic fields associated with the propagating microwave signal. Of the many possible modes of propagation, the Transverse Electro Magnetic or TEM mode of transmission represents a travelling electromagnetic energy with alternating electric and magnetic fields perpendicular to each other and to the direction of propagation, such as in a co-axial system. Other microwave systems constructed on transmission lines such as microstrip, stripline and waveguide supports different modes of propagation such as transverse electric (TE) or transverse magnetic (TM) mode of propagation. Hence, an interconnect system for microwave circuit should support the correct mode of propagation and enable signal to transit through it. The properties of a connector are specified by electrical parameters such as insertion loss, VSWR and return loss. The physical parameters include mechanical and chemical properties such as the conducting material used, the geometry, its dimension and the dielectric material used.

5. Co-axial transmission line

A co-axial transmission line has two conductors – inner and an outer one, separated by a gap consisting of a dielectric medium. The outer diameter determines the cut off frequency (or the highest frequency of operation possible supporting a single mode of transmission), smaller the diameter, higher the frequency. The dielectric media separating the two conductors, determines the signal attenuation: a dielectric material like PTFE will offer more attenuation and lower frequency of operation as compared to say, when the dielectric medium is air.

Much of the development work on connectors are due to defense and military related work and manufacturers of connectors, such as Amphenol, Omni Spectra, Agilent etc; involved in designing innovative interconnect systems for applications including various RF/Microwave and Millimetre wave systems and test and measurement equipment. A company called Nuvotronics LLC (www. nuvotronics.com) has evolved a new technology (PolyStrata™) integrating microwave circuit components with the coaxial transmission line. It is an amalgamation of semiconductor process technology and microwave micro-coax transmission line technique. The technology enables heterogeneous integration of the best of breed GaN, SiGe, Si, GaAs, and MEMS die into a “parasitic free” 3D stacked circuit module yielding unprecedented performance, size, integration, and thermal management.

6. Types of connectors

One of the prime considerations for a connector is the number of times it is connected and disconnected or its shelf life.

Connectors are classified into three categories:

• Primary interconnects are the ones which are widely used in RF industry
• Less commonly used interconnects
• Precision interconnects used on T&M equipments such as Network Analyzer etc

a. Some common terminology – Male/Plug and Female/jack connectors

Similar to plug and socket concept of a power cable system, the connectors used in RF and microwave applications, have been termed either ‘male’ or ‘female’. Making an analogy with the power cable plug and socket, the male connector corresponds to the plug and the female connector to a socket. The center conductor of the male connector is a probe and that on the female connector is a receptacle.

Typical components of a crimp-type connector are shown in the diagram below:

 

As seen from the above diagram, a crimp type connector consists of an outer body, center conductor, and a ferrule. The one shown in the above figure is a male connector. A female connector will have receptacle instead of a centre pin and the thread will be on the outer side of the body as shown in the diagram below:

N-type connector is shown for illustration, and typically most of the rf connectors will have similar architectures, with different geometries and mounting mechanism. A connector may broadly be categorized as panel mount, crimp, bulkhead etc. Each of these will be explained in the later sections of the article.

An rf cable with N-type crimp connector is shown in the diagram above. It consists of a suitable coaxial cable crimped to the connector using a ferrule. The joint is then covered with a thermally sensitive sleeve that contracts over the ferrule and the coax. The purpose of the sleeve is to provide mechanical strength to the joint, as well as to seal the joint from environment.

b. Connectors which are Gender Neutral

Connectors such as HP/Amphenol APC-7 and the General Radio GR874 and GR900BT are neither male nor female. Another example of this type of connector is that of the 7 mm precision connector available on the front panel of the older HP 8510 Network Analyzer. Users of this equipment would have observed the uniqueness of the mating sequence. Each connector has an outside rotating sleeve. On one connector, one has to rotate the outer sleeve so that the threaded connector sleeve extends completely out from the outer sleeve. This is done on any fixed-mounted connectors such as those on the test ports of the network analyzer. On the other connector, one has to rotate the outer sleeve so that the threaded connector sleeve recedes completely into the outer sleeve. Next, one has to carefully mate the surfaces flush and rotate the forward sleeve to engage the threads of the other connector. Complete connection is made when the forward rotating sleeve is tight and the other sleeve is loose.

As mentioned earlier, there are different types of connectors for different applications, frequency of operation, power-handling capacity etc; we now describe some of these connectors in detail. The first connector on our list is the N type connector; perhaps the most commonly used connector after the SMA connector.

c. Reverse Polarity Connectors

Reverse polarity connectors have the normal "male" and "female" inner connector parts swapped over so that female contacts are used in the plug connector and male contacts in the jack. The primary objective of using reverse polarity connectors is to prevent the user from inadvertently plugging in equipment (such as a radio antenna) that differs from the type specified by the manufacturer. Reverse polarity connectors are available for various types of connectors including SMA; TNC, and BNC.

The figure displays an SMA male connector and SMA male connector with reverse polarity. Note that the connector with reverse polarity has a female centre pin, whereas normally it should be a protruded pin.

Similarly, an SMA female reverse polarity connector with have normal female outer shell (with threads outside the shell), but has a male center conductor.

d. N Connector

The Navy connector or the N connector was designed for the military RF systems during World War II. It has undergone many improvisations since then, and is available as normal and precision versions. When connected in the form of a N-cable, it is widely used in RF and microwave equipments, field-testing and installations, Laboratories etc.

N connector is constructed with an outer and an inner conductor separated by a dielectric medium. It has a sealing mechanism wherein a gasket is inserted between the outer and the inner conductor. This protects it from the outside environment. Typically the cut-off frequency of operation for N type connector is 11 GHz beyond which multi-modes come into the picture, producing unpredictable results. It uses PTFE as the dielectric material. N connectors conform to a military standard called MIL-C-39012.

It is rugged and can be used many times leading to its popularity.

                                       
Improvement in the technology has led to the construction of Precision N type of connector, which supports a single TEM mode of operation upto 18 GHz. Different names (usually of the manufacturer) are associated with N type connector, such as OSN (Omni Spectra N) and APC-N (Amphenol Precision Connector-N) but basically represent the same N type family of connector. N type of connector is available in both 50 ohms and 75 ohms characteristic impedance. The 75-ohm version of N connector is widely used in CATV industry.

Important Connector Features:

•  The connector is primarily used with medium to miniature size coaxial cable, including RG-8, RG-58, RG-141, and RG-225.
•  Frequency of operation is up to 18 GHz with precision connectors.
•  The military standards as specified in MIL-C-39012 are applicable for N type connectors

e. BNC Connector

BNC stands for “Bayonet Navy Connector”. BNC connector was designed for military application during World War II and conforms to MIL-C-39012 standard. The construction of BNC connector is such that it gives satisfactory performance below 4 GHz of operation. The BNC uses a slotted outer conductor separated from the inner gender conductor by a plastic PTFE dielectric.  

                       
The slots on the outer conductor are found to radiate beyond 4 GHz. The dielectric also causes increasing losses with increasing frequency. The BNC connector is widely used in video and RF applications upto 2 GHz of operation. BNC type of connector is available in both 50 ohms and 75 ohms characteristic impedance version.

The 75 ohm version of BNC connector is widely used in Telecommunication and satellite industry at IF frequency level of operation. It is used on-board of a printed circuit board, or panel mounted or as a back-to-back cable interconnecting system. The BNC connector when assembled with a flexible cable, can accept cable with diameter upto 6.3 mm or 0.25 inches. It is a commonly used connector in applications below 1 GHz frequency of operation.

Important Connector Features:

• The connector can typically be used up to   2 GHz. Above 2 GHz, the reflection coefficient becomes significant, and usage is limited to applications that do not require very good VSWR.
• Impedance: The connector is widely available in both 50 Ohm and 75 Ohm versions.
• Cable Suitability: BNC’s are suited for cable termination for miniature to subminiature coaxial cable (RG-58, 59, to RG-179, RG-316, etc.)
• The military standards as specified in MIL-C-39012are applicable for BNC type connectors

f. SMA Connector

SMA stands for “Sub Miniature A” connector. It is the most widely connector in the RF industry from being used on RF modules to being used with a semi-rigid cable to interconnect two RF modules. A ‘launcher’ SMA connector can be found soldered directly onto a RF PCB board or fastened to a RF housing with its center pin soldered to the microstrip line. SMA connector is available in both SMA-Male and Female versions. The SMA male connector has a hexagonal outer body and mating threads underneath. The SMA female connector has a threaded outer body.

                      
SMA connector was designed by Bendix Scintilla Corporation. The Omni Spectra Corporation version of SMA connector is known as OSM connector. SMA connector   is constructed by an outer and an inner conductor separated by a dielectric material such as PTFE. The threads on the SMA connectors are 0.250 x 36; the male connector has a 0.312 hex body, tightened with a 5/16-inch wrench. While assembling a semi-rigid cable with SMA connector at its end, the connector accepts the cable dielectric directly upto the interface leaving no air gaps.

SMA connector has a wide range of frequency of operation, from 18 GHz to 26 GHz, depending upon its make and handling. Generally, SMA connectors are used where frequent connects and disconnects are not performed. However, a standard SMA connector is designed for 12.4 GHz frequency of operation extendable to 18 GHz with precision make. Most well constructed semi-rigid cables with SMA connectors can be used in applications, which can tolerate higher insertion loss and lower return loss up to 24 GHz. SMA connectors exhibit higher reflection coefficients and hence lower return loss upto 24 GHz of operation, as compared to other connectors. Another difficulty with SMA connector is that of anchoring the dielectric support.

Manufacturers such as M/A-Com and Johnson have designed and manufactured high quality version of a SMA connector. The Johnson field replaceable SMA performs to 26.5 GHz, and the M/A-Com OSM extended frequency series to 27 GHz. These high performing SMA connectors rated at 26.5 GHz are widely used in test equipment and component applications.

Precision SMA connectors are those, which have been designed to operate beyond 27 GHz, such as 3.5 mm or APC-3.5, WSMA, 2.92 mm, K connectors. These connectors follow different standard and are designed in a way to be compatible with the SMA geometries to allow mating with an SMA connector.

When handled carefully, SMA connectors can support a few hundred interconnects. Proper handling precautions, if taken, can prolong the SMA shelf life. Care should be taken to align the SMA connectors straight on and then join. Prior to making a connection it is a good practice to inspect the female end (receptacle) and the male end (probe) of the SMA connector to ensure that it is not damaged or broken. It is also a good practice to clean the inner dielectric surface of the connector and the inner conductor, with cleaning agent or a solution.

Connections made between two SMA connectors of different frequency ratings such as between a normal SMA and a Precision SMA can be avoided by using a SMA Connector Gauge to gauge the SMA male before mating. If not, the performance will deteriorate and it will be that of the lesser connector’s performance. Care should be taken to ensure that SMA male mates with the Precision SMA female connector straight on, and that it is from a professional manufacturer. Similar care should be taken while mating a SMA female connector with a Precision SMA male connector such as 3.5 mm or 2.9 mm connectors.

Important Features:

• Applicable Standards: MIL-C-39012 and CECC 22110/111. Any connectors that meet these standards can mate with SMA connector.
• Normal connectors can support up to 12.4GHz, and precision connectors can be used up to 18 GHz. Significant deterioration in reflection coefficient takes place if used above 18GHz.
• Impedance: 50 Ohms

g. SMB Connector

SMB stands for “Subminiature B” connector. Unlike SMA connectors, which use a threaded connection, the SMB uses a quick connect-disconnect snap-on mechanism for its connections.  

                 
The SMB connector falls into the category of lesser-used connector. The Subminiature B connector is a snap-mount connector with a frequency of operation up to 4 GHz and is available in both 50 ohms and 75 ohms version. However, SMBs can be used up to 10 GHz without having multi mode issues. The SMB connector conforms to the military standard called the MIL-STD-348. SMB is not useful in low noise microwave interconnects, such as, in a low noise amplifier module in a receiver system, as it has higher insertion loss (typically 0.3 dB at 4 GHz), which directly adds to the receive chain noise figure. But it can be used where low noise is not a criteria, for example, for signal delivery in the transmit chain, distribution system etc.

SMBs are designed for around 500 interconnect cycles. SMB connector has an inherent disadvantage in systems where vibration is a concern, because of its snap on nature of quick connect and disconnect.

Important Features:

• Unlike SMA (which is a screw type coupling), SMB connector uses snap-on coupling.
• Applicable standards: MIL-C-39012 and MIL-STD-348
• Cheaper, and used widely with video interface
• Frequency range: Typically used up to 4 GHz, and up to 10 GHz with degradation in return loss/reflection coefficient.
• Impedance: 50 Ohms and 75 Ohms

h. SMC Connector

The SMC or SubMiniature C connector was developed in the 1960's and is a threaded type interface connector. In dimension, it is much smaller than an SMA connector and accepts flexible cables with diameters up to 3.17 mm or 0.125 inch. 

            
Because of its small size, it is suitable for applications with size constraints. Construction wise SMC has 10-32 threads and does not have a gasket. It is similar to a small version of the TNC connector wherein the dielectric material is present in both male and female connectors. The SMC is designed to be used up to 4 GHz frequency of operation. SMC designed in conformance with military standard MIL STD-348 can be used up to 10 GHz frequency of operation but with increased loss. The SMC connector can substitute a SMB connector in applications where vibration is a concern.

Important Features:

• Threaded Connector with frequency of operation up to 4 GHz
• MIL-C-39012 (Interface dimensions) and MIL-STD-348
• Frequency: 50 and 75 Ohms

i. TNC Connector

TNC stands for Threaded Navy Connector and is similar to a BNC connector except that it has a threaded interconnect mechanism. Having threads in a TNC connector solves the problem of radiation from the slot on a BNC connector.

   
TNC Male                   TNC  Female                                                                                           

The connector exhibits useful performance upto a frequency of operation of 12 GHz, which is further extended to 18 GHz when using with a semi-rigid cable. The N, BNC and TNC conform to MIL-C-39012 specification. Extended frequency of operation up to 18 GHz is possible for TNC connectors made to IEC 169-26 specification. However, this connector exhibits higher insertion loss.

The TNC connector is used widely in cellular telephone RF/antenna connections because the mating geometries are compatible with the N connector. It is possible to temporarily mate some gender combinations of BNC and N but this is not recommended as the connection is not mechanically stable, and there will be significant impedance changes at the interface. TNC connector is available in 50 ohm and 75 ohm version.

j. 7/16 DIN connector

DIN connector was originally standardized by the Deutsches Institute for Normung, Germany, a national standards organization, hence the name, DIN. “7/16” in its name represents the size of the inner and outer contacts in metric measurements. The inner contact on the 7/16 DIN connector measures 7mm while the outer contact on the connector measures 16 mm. 7/16 DIN connector is designed to handle high power up to 100 watts and is used in high power communications systems, antennas, base stations, and satellite communication applications.

7/16 DIN Male             7/16 DIN Female                

k. GR874 Connector

GR874 or General Radio G874, a gender-less 50 ohms connector, was designed by General Radio Corporation. It has a slide-on interface that has been an industry standard on a wide variety of test and measurement equipments.  

             GR874 genderless connector

In some variation to the design, these connectors have a locking mechanism added for applications requiring mechanical security. These locking and non-locking interfaces are inter-mate-able.

As shown in the connector picture, both inner and the outer conductors of this connector were made from four leaves, two of which were displaced slightly outwards and two of which were displaced inwards, giving the appearance like that of a flower petal. Aligning one connector with another one and rotating it by 90 degrees, the inner leaves of the first connector would mate with the outer leaves of the second connector and vice-versa. This allowed one GR connector to mate with any another GR connector irrespective of its gender; hence, it was rendered as a gender less connector. The GR874 exhibited flat electrical impedance across a wide range of frequency (DC to 8.5 GHz) and along with its reliable mating mechanism, made it a widely used connector for test and measurement applications.

With technological advancement, General Radio designed GR900 connector, as a successor to GR874, to address the higher performance expectations from the industry. GR900 was designed to be incompatible with the GR874 connector. However, with the advent of newer and cheaper connectors such as the BNC connector; SMA and N connectors for high precision applications, the GR874 was phased out of the industry.

There are three categories of 7/16 DIN connector cables: the corrugated cable, which is manufactured as Annular or Superflex; the standard cable connector; and the custom cable connector (www.amphenolrf.com).

7/16 DIN connector is manufactured as per the DIN 47223 specification and performs up to 7.5 GHz frequency of operation. It offers lower VSWR as compared to other audio and communication cable. These connectors are useful where low Intermodulation Distortion or IMD product is a prime requirement (www.amphenolrf.com). It has gained popularity in the cellular wireless tower application, replacing N connector. The design uses rubber gasket to prevent dust or other debris from interfering with the contacts. 7/16 DIN connectors come in a variety of material, including silver and gold plated contacts, providing improved conductivity, performance, and corrosion resistance.

The DIN connector is a popular connector used in Europe, especially in Germany , for audio and video devices, such as radios in Mercedes-Benz to the speaker set ups made by Bang & Olufsen. Huber & Suhner (www.hubersuhner.com), Pasternack (www.pasternack.com), Amphenol are some of the well-known manufacturers of 7/16 DIN connectors.

l. GR900BT Connector

GR900BT is a 14 mm genderless connector, designed by the General Radio Corporation.

             14 mm MPC14 or GR900 connector

This connector was used in highly critical laboratory applications with performance up to 8.5 GHz frequency of operation.

To be continued in RF and Microwave Connectors Part II /articles/11.html