Basic Introduction to RF Connector

Catalog

Ⅰ Introduction

RF connectors are defined as detachable components that are usually attached to cables or equipment for the electrical connection of transmission line systems. It can be seen from this definition that it has the common feature of the connector "detachable element". "Transmission line system" refers to the microwave transmission system. The common transmission line structure is shown in the following figure:

Take the commonly used coaxial line as an example, the main mode of the coaxial line is TEM wave, and the field distribution is shown in the figure below:

field distribution

The electromagnetic wave transmitted by the RF connector is the oscillating particle-wave derived from the in-phase and mutually perpendicular electric field and magnetic field in space. It is an electromagnetic field that propagates in the form of waves and has the duality of wave and particle.

RF connectors

Electromagnetic waves are caused by in-phase oscillating and perpendicular electric and magnetic fields moving in space in the form of waves. The electric field direction of electromagnetic waves, the direction of magnetic fields, and the direction of propagation are perpendicular to each other. Electromagnetic waves have no longitudinal component and are transverse electromagnetic waves.

Ⅱ Types of RF connector

RF connectors are mainly divided into three categories: RF coaxial connectors, RF triaxial connectors, and dual-core symmetric RF connectors. The main uses are as follows:

1. RF coaxial connector: It is mainly used to transmit transverse electromagnetic waves (TEM waves).

2. RF triaxial connector: It is mainly used for occasions with higher requirements on shielding efficiency, transmitting transverse electromagnetic waves (TEM waves) or transmitting pulse waves;

3. Dual-core symmetrical RF connector: It is mainly used to transmit digital signals with a low transmission rate.

1) The main performance and parameters of RF coaxial connectors include characteristic impedance, frequency of use, return loss, insertion loss, isolation, RF leakage, phase consistency, third-order intermodulation, etc.

2) Common materials and plating of RF coaxial connectors

The RF connector is mainly composed of outer conductors, inner conductors, and insulating support media and other components. Its commonly used materials are as follows:

Outer conductor: stainless steel passivation, copper alloy gold plating, copper alloy nickel plating, copper alloy plating ternary alloy, etc.;

Inner conductor: copper alloy gold plating, copper alloy silver plating, etc.;

Insulation support media: PTFE, PEI, LCP, etc.

Ⅲ Selection of RF coaxial connectors

The selection of RF coaxial connectors must consider both performance requirements and economic factors. The performance must meet the requirements of the system electrical equipment. In principle, the following four aspects should be considered in the selection of connectors.

• Connector interface (SMA, SMB, BNC, etc.)

• Electrical performance, cables, and cable connection

• Termination form (PC board, cable, panel, etc.)

• Mechanical structure and plating (military, commercial)

1. Connector interface

The connector interface is usually determined by its application, but at the same time, it must meet electrical and mechanical performance requirements.

BMA type connectors are used for blind-mate connection of low-power microwave systems up to 18 GHz.

The BNC connector uses a bayonet-type connection, which is mostly used for radiofrequency connection with a frequency lower than 4GHz and is widely used in the field of the network system, instrumentation, and computer interconnection.

The interface of TNC is similar to that of BNC except for the screw port. It can still be used at 11 GHz and has excellent performance under vibration conditions.

SMA screw connectors are widely used in military and civil fields such as aviation, radar, microwave communication, and digital communication. Its impedance is 50Ω and the use frequency is lower than 12.4GHz when using a flexible cable. The maximum use frequency is 26.5GHz when using a semi-rigid cable, and 75Ω has a broad application prospect in digital communication.

SMB volume is smaller than SMA. It is inserted into a self-locking structure to facilitate a quick connection.

SMC is similar to SMB because it has screw holes to ensure stronger mechanical properties and a wider frequency range. It is mainly used in military or high vibration environments.

The N-type screw connector uses air as an insulating material with low cost, an impedance of 50Ω and 75Ω, and a frequency of up to 11GHz. It is usually used in regional networks, media transmission, and test instruments.

2. Electrical performance, cable, and cable connection

A. Impedance

The connector should match the impedance of the system and the cable. It should be noted that not all connector interfaces conform to the impedance of 50Ω or 75Ω. Impedance mismatch will cause the system performance to deteriorate.

B. Voltage

Make sure that the maximum withstand voltage of the connector cannot be exceeded during use.

C. Maximum working frequency

Each connector has a maximum operating frequency limit, and some commercial or 75n designs have a minimum operating frequency limit. In addition to electrical performance, each interface form has its unique features, such as BNC is a bayonet connection, which is easy to install and low in price. It is widely used in low-performance electrical connections; SMA and TNC series are nut connections to meet high vibration environments. SMB has a quick connection and disconnection function, so it is more and more popular with users.

D. Cable

Because of its low shielding performance, TV cables are usually used in systems where the only impedance is considered. A typical application is TV antennas.

TV flexible cable is a variant of the TV cable. It has a relatively continuous impedance and a good shielding effect. It can be bent and has a low price. It is widely used in the computer industry, but it cannot be used in systems that require higher shielding performance.

Flexible coaxial cable has become the most popular closed transmission cable due to its special properties. Coaxial means that the signal and ground conductors are on the same axis. The outer conductor is composed of fine braided wire, so it is also called braided coaxial cable. This cable has a good shielding effect on the center conductor, and its shielding effect depends on the braided wire type and braid thickness. In addition to its high voltage resistance, this cable is also suitable for use in high frequency and high-temperature conditions.

The semi-rigid coaxial cable replaces the braid with a tubular shell, which effectively compensates for the shortcomings of the braided cable's poor shielding effect at high frequencies. Semi-rigid cables are usually used at high frequencies.

E. Cable connection

There are two main installation methods for connectors: (1) Welding the center conductor and rotating the shield. (2) Crimp the center conductor and crimp the shield. Other methods are derived from the above two methods, such as welding the center conductor and crimping the shield. Method (1) is used when there is no special installation tool; due to the high efficiency of the crimping method, the reliable termination performance, and the design of the special crimping tool can ensure that each cable maggot part is the same. So with the development of low-cost attachment tools, the welding center conductor crimp shield layer will be increasingly popular.

3. Termination form

The connector can be used for RF coaxial cables, printed circuit boards and other connection interfaces. Practice has proved that a certain form of connector matches a certain type of cable. Generally, a small-diameter cable is connected to small coaxial connectors such as SMA, SMB, and SMC. We can refer to the cable size chart to select the appropriate connector.

4. Mechanical structure and a plating layer

The structure of the connector will greatly affect its price. The design of each connector includes military standards and commercial standards. The military standard is manufactured according to MIL-C-39012 with all-copper parts, Teflon insulation, and gold-plated inside and outside. The performance is the most reliable. Commercial standard designs use inexpensive materials such as brass castings, polypropylene insulation, silver plating, etc.

The connectors use brass, beryllium copper, and stainless steel. The center conductor is generally plated with gold because of its low resistance, corrosion resistance, and excellent tightness. The military standard requires gold plating on SMA and SMB, and silver plating on N, TNC, and BNC. However, because silver is easily oxidized, many users prefer nickel plating.

Commonly used connector insulators include PTFE, polypropylene, and toughened polystyrene, among which PTFE has the best insulation performance but higher production cost.

The material and structure of the connector affect the processing difficulty and processing efficiency of the connector, so the user should choose a connector with good performance and price according to his application environment.

Ⅳ The future development of RF coaxial connectors

A connector is an electromechanical component that connects a conductor (wire) to an appropriate mating component to achieve circuit connection and disconnection. Consumer electronics connectors include JST, AMP, MOLEX, JAM, I-PEX. Automotive electronics connectors: JST, SUMOTOMO, YAZAKI, FCI, KET/KUM. Since the appearance of UHF series connectors in 1930, the history of the development of RF coaxial connectors has been only a few decades, but because of its good broadband transmission characteristics and a variety of convenient connection methods, it has been used in communication equipment, weapon systems, instrumentation, and home appliances.

With the continuous development of the whole machine system and the continuous progress of production technology, the RF coaxial connectors are also constantly developing, and new varieties are emerging. Through the analysis and sorting of relevant information in some professional magazines, we believe that in the future, RF coaxial connectors will develop in the following directions:

1. Miniaturization

The miniaturization of the whole machine system can not only realize the multi-function and portability of the whole machine but also greatly reduce the material cost, transportation cost, and energy consumption. The miniaturization of components is the prerequisites for miniaturization of the whole system. Only by using miniaturized components can the high-density installation be achieved and more space can be saved.

The SSMB series RF coaxial connector products that appeared in the past few years have been widely used in military products such as portable radios and missiles because of their small and compact structure and fast plug-in connection characteristics. In recent years, 1.0/ 2.3 (SAA) self-locking connectors and new connector varieties such as MMCX and SMP (2.4mm plug-in connectors) are all miniaturized products, which are widely used in new-generation communication equipment. These products can achieve a high installation density.

2. High frequency

To obtain a wider channel space and achieve a higher data transmission rate, the operating frequency of the whole system is constantly increasing. In the new-generation optical transceiver developed by Wuhan Institute of Posts and Telecommunications, the frequency of some coaxial transmission microwave signals has reached above 12GHz. The working frequency of military communication systems has already entered the millimeter-wave band. The research of weapon systems has also shifted from 8mm to 3mm.

In the early 1990s, the American HP company introduced 1.0mm RF coaxial connectors with a frequency of up to 110GHz, and it has been used in small batches in its microwave test instruments. Other large internationally renowned companies also have different types of millimeter wave coaxial connectors, such as AMP's APC2.4, APC3.5 series, OMNI's OSSP series, and SMP (common type 0~26.5GHz, precision type 0~40GHz) series, K series, V series ( 1.85mm), etc. Among them, 3.5mm, 2.4mm, K type, and SMP series have formed mass production, and are widely used in communication equipment, test instruments, and weapon systems.

3. Surface mount

In the short ten years since SMT (surface mount) technology appeared, the assembly automation of the whole machine industry has been significantly improved, and the product cost has been greatly reduced. This has also prompted the component industry to switch from traditional pin-type packaging to chip surface-mount devices (SMD). The emergence of SMD is also known as the fourth revolution in electronics. According to Fleck Research statistics, in 2000 the global production of chip surface-mount components reached 700 billion, accounting for 70% of the total output of components.

At present, many series of low-frequency surface mount printed board connectors have begun mass production. Surface mount RF coaxial connectors are special due to their structure and working conditions. They are used only in batches of user terminal products such as mobile phones, manufacturers are quite limited. However, with the continuous development of SMT technology, the surface mount will become the mainstream way for small connectors to connect with microstrips and printed boards.

4. Multi-function

Multi-function is a direction of components. In addition to the role of electrical connection, the new connector also has functions such as filtering, phase shifting, attenuation, detection, and mixing. DC Block RF coaxial connectors with filtering function have been widely used in many complete machine systems. SMD series attenuation and detection connectors have been widely used in distribution frame equipment.

The use of multi-functional RF coaxial connectors can greatly simplify the structure of the whole machine and improve the anti-interference ability of the system. In the next few years, more varieties of multi-functional RF coaxial connectors will be developed and used.

5. High performance and high power

To meet the development needs of the information highway, communication equipment requires a high transmission rate and high signal-to-noise ratio, which requires various components in the system to achieve very high electrical performance indicators. The characteristics of high-power, multi-channel transmission of the new generation communication system have put forward new requirements on the performance indicators of the RF coaxial connector EMC (Electromagnetic Interference). The International Electrotechnical Commission (IEC) has formulated a test standard of performance index "passive intermodulation for coaxial connectors", which will become the basic electrical performance index of high-power RF coaxial connectors.

In short, the RF coaxial connector will develop rapidly with the development of the whole system, and replace waveguides and other microwave devices in more fields. It will become an indispensable key component in the field of microwave transmission.

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