The selection of connectors must consider both performance requirements and economic factors. The performance must meet the requirements of the electrical equipment of the system, and the economics must meet the requirements of value engineering. In principle, the following four aspects should be considered:
Connector excuses (SMA, SMB, BNC) electrical performance, cable and cable connection, connection form (PC board, cable, panel, etc.), mechanical structure and plating (military, commercial)
1. Connector interface
The interface of the device is usually determined by its application, but at the same time it must meet the requirements of electrical and mechanical performance. BMA type connectors are used for blind-mate connection of low-power microwave systems with frequencies up to 18 GHz. BNC type connector adopts bayonet connection, which is mostly used for radio frequency connection with frequency lower than 4GHz, and is widely used in network system, instrumentation and computer interconnection field. Except for the screw port, the interface of TNC is similar to that of BNC, which can still be used at 11GHz and has excellent performance under vibration conditions. SMA screw connectors are widely used in military and civilian fields such as aviation, radar, microwave communications, and digital communications. Its impedance is 50Ω, the frequency is lower than 12.4GHz when equipped with flexible cables, and the highest frequency is 2635GHz when equipped with semi-rigid cables; 75Ω has a broad application prospect in digital communication. The size of SMB is smaller than SMA. It is inserted into a self-locking structure for quick connection. The most typical application is digital communication. It is a replacement product of L9. Commercial 50Ω meets 4GHz and 75Ω is used for 2GHz. SMC is similar to SMB, because it has a screw port to ensure stronger mechanical performance 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, has a low cost, and has an impedance of 50Ω and 75Ω, and a frequency of up to 11GHz. It is usually used in local area networks, media propagation and test instruments.
Impedance: The connector should match the impedance of the system and the cable. It should be noted that not all the interfaces of the butter connector meet the impedance of 50Ω or 75Ω. The impedance mismatch will cause the system performance to decrease.
Voltage: Ensure that the maximum withstand voltage of the connector cannot be exceeded during use.
Maximum operating frequency: Each type of connector has a maximum operating frequency limit, and some commercial or 75Ω designs have a minimum operating frequency limit. In addition to electrical performance, each type of interface has its own unique features, such as: BNC is a bayonet connection, which is easy to install and low in price, and is widely used in low-performance electrical connections; SMA and TNC series are connected by nuts to meet high requirements. The vibration environment requires the connector, SMB has a fast connection and disconnect function, so it is more and more favored by users.
2. Gas performance, cable and cable assembly
Because of its low shielding performance, cables are usually used in systems that only consider impedance. A typical application is TV antennas.
The flexible TV cable is a variant of the TV cable. It has relatively continuous impedance and good shielding effect. It can be bent and is low in price. It is widely used in the computer industry but cannot be used in systems that require high shielding performance.
Flexible cables eliminate inductance and capacitance, and are mainly used in instruments and buildings. Flexible coaxial cables have become the most common enclosed transmission cables due to their special properties. Coaxial means that the signal and ground conductors are on the same axis, and the outer conductor is also called braided coaxial cable because of the fine braided wire. This cable is also suitable for use under high frequency and high temperature conditions.
The flexible coaxial cable replaces the braided layer with a tubular shell, which effectively compensates for the shortcomings of the braided cable with poor shielding effect at high frequencies. Semi-rigid cables are usually used at high frequencies.
There are two main ways to install the device: welding the center conductor and screwing the shielding layer. Crimping the center conductor and crimping the shielding layer. Other methods are derived from the above two methods, such as: welding the center conductor, crimping the shielding layer. Method one is used for occasions where there is no special installation tool; because the crimping method has high work efficiency and reliable termination performance, and the design of the special crimping tool can ensure that every cable assembly that is assembled is the same, so With the development of low-cost assembly tools, welding center conductors and crimping shielding layers will become increasingly popular.
The device can be used for radio frequency 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, cables with small outer diameters are connected to small coaxial connectors such as SMA, SMB, and SMC. Refer to the cable size list to select the appropriate connector.
Mechanical structure and coating:
The structure of the device will greatly affect its price. The design of each connector includes military standard and commercial standard. The military standard is manufactured in accordance with MIL-C-39012, all copper parts, PTFE insulation, and inner and outer gold plating, the performance is the most reliable. Commercial standard designs use cheap materials such as brass castings, polypropylene insulation, silver plating, etc.
The materials used for the device are brass, beryllium bronze, tin bronze, aluminum and stainless steel. The center conductor is generally plated with gold because of its low resistance, corrosion resistance and excellent airtightness. The military standard requires gold plating on SMA and SMB, and silver plating on N, TNC and BNC. However, because silver is easy to oxidize, many users prefer nickel plating.
The connector insulators are PTFE, polypropylene and toughened polystyrene, of which PTFE has the best insulation performance, but the production cost is higher.
The material and structure of the connector affect the processing difficulty and processing efficiency of the connector, so users should choose a connector with a good performance and price ratio according to their application environment.