RF coaxial connector

Jacktang

RF coaxial connector [Copy link]

As an RF engineer and tester, in my daily work, in addition to test instruments, calibration parts, and connecting cables, I am most exposed to adapters. During the maintenance process, we found that many instruments were damaged or test indicators were unstable due to damage to the adapters, and some connectors were not connected and fixed in the right way. Every time the instrument was repaired, the customer tightened it in the original way. Especially in some production-oriented enterprises, due to the high mobility of operators, many employees may not know the various adapters, and do not understand why the adapters are damaged. Based on the above reasons, combined with my own maintenance over the years, I have introduced some common adapters, mainly from the following 6 aspects.

1. Adapter type and structure

2. Adapter grade

3. Matching between adapters

4. Damage to the adapter

5. Check the adapter

6. Connection of adapter

These things are all very basic, but if we pay attention to these details, our instruments will have one less reason to be damaged, and our test results will be more stable and reliable.

1. Adapter type and structure

Coaxial adapters are used to transmit radio frequency signals. The transmission frequency range is very wide, up to 50GHZ or higher. They are mainly used in radar, communication, data transmission and aerospace equipment. The basic structure of a coaxial adapter includes: center conductor, dielectric material (or insulator), outer conductor (this part plays the same role as the outer shielding layer of the coaxial cable). Coaxial adapters are mainly divided into SMA, SMB, N type, 3.5MM, 2.4MM, 1.85MM, 1.0MM, APC-7MM, etc.

Common knowledge about coaxial adapters

The basics of describing an adapter:

1: Connector type --- such as SMA, 3.5MM, N type

2: Characteristic impedance --- The impedance of the connector is generally divided into 50 ohms and 75 ohms 3: Contact method --- male, female

When you ask for an adapter, as long as you meet these three basic requirements, you will know what you need:

For example, if you need an N-type to 3.5MM adapter, you can describe it like this: you need a 50 ohm impedance adapter with an N-type male connector on one end and a 3.5MM female connector on the other end. Then there is no problem.

2. Adapter Grade

Usually each type of adapter has three grades of products, and a suitable adapter is provided for each type while taking into account cost and performance. Some special adapters (connectors) may not have all three grades of products.

Metrology Grade

Calibration standards

Highest performance slotless connection design

Smallest tolerances

Air medium

Long life

High production costs

Among all connector grades, metrology grade connectors have the best performance, but also the highest cost. They are generally used as calibration standards, performance verification standards, and some applications that require precision connections. This grade of connector has the most accurate dimensions, the smallest tolerances, and can withstand repeated connections, so it also has the longest service life among all grades.

Metrology-grade adapters are the closest in material and size. To provide the highest level of performance and traceability, metrology-grade adapters use an air dielectric interface and a slotless female connection.

Although metrology-grade adapters have good repeatability and provide good accuracy after repeated use, this does not mean that metrology-grade adapters are very strong and do not require caution when using them. On the contrary, since the tools and processes used in the production process of this grade of adapters are of the highest precision, using a lower-grade adapter to match the metrology-grade adapter will cause dust and other small debris to invalidate a high-precision measurement and may damage the metrology-grade adapter.

NOTE: Do not use the adapters from your metrology standards (metrology grade) onto production grade adapters.

Instrument Grade

Used on the input and output interfaces of instruments and some economical calibration parts

Better performance

Smaller tolerances

Support non-conductive media interface

Long service life

Instrument-grade connectors are generally considered to be "intermediate-grade" connectors. This grade of connector is mainly used for connecting wires and connectors inside instruments. Some economical calibration parts also use this grade of adapter. It can provide better test performance and smaller tolerance. Compared with metrology-grade adapters, because some instrument-grade adapters may use interfaces for non-conductive media, they may not be perfectly matched with metrology-grade connectors.

Note: Although it is called instrument grade in academic terms, the test connectors on some instruments use very precise connectors (very close to the metrology grade), so try to add appropriate adapters to the test ports of the instruments during use.

Production Level

Used in products

Low performance

Large tolerance

Support non-conductive media interface

Limited number of connections

Low cost

This is the lowest grade connector, generally used in testing products. Due to its large tolerance, the performance of this grade connector is the worst among all grades. Every time you use this grade connector to connect to an instrument, you need to check the connector carefully. Do not use production-grade connectors to connect your metrology-grade connectors. Production-grade connectors are cheaper and have a shorter service life. If the connector grade of the device under test is lower, please use an appropriate adapter to protect the connector of the instrument when connecting it to the instrument.

3. Matching between adapters

The outer conductor sizes of the adapters are different to prevent the mixing of incompatible adapters. The outer conductor sizes of the adapters with the same background color in the table are the same, so they can be safely matched.

However, in the process of daily use, wear, lack of cleaning, wrong connection method, and poor storage method will cause damage to the adapter. Using a damaged or defective adapter will cause damage to the connector that matches it, so please clean and inspect all connectors before using them.

In addition, not all adapters have three grades. When different adapters are matched and used, the production-grade adapter may cause damage to the metrology-grade adapter. Refer to the grade of the adapter and use the appropriate adapter to make the transition.

The first five connectors in the table below all use air as the medium, and the names of the connectors are also determined by the diameter of the air medium. Together with the following notes, we can easily identify these adapters.

The APC-7MM connector is not a typical millimeter wave connector because its maximum frequency can only reach 18G. However, the APC-7MM connector is very durable and has an excellent standing wave ratio. It has no grade difference, and there is no difference between male and female connectors. Its appearance is the same, so when using the APC-7MM connector, you need to pay attention to some matters.

In the daily instrument maintenance process, we often find that customers tighten both nuts of Agilent's 8753E series network analyzers, which makes the instrument test unstable and easily damages the connector.

4. Damage of the adapter

1. There is a deviation in the size of the conductor inside the male and female connectors

This type of damage is most likely to occur when different grades of connectors are mixed. Because the tolerance of the production level is relatively large, the center conductor diameter of some inferior male connectors is too large, and when using this connector, the female connector is directly damaged. In addition, on some 75-ohm network instruments, due to the carelessness of the user, the 50-ohm connecting wire is used, which will directly damage the N-type 75-ohm female connector.

2. The PIN depth of the male adapter is too long

3. The inner conductor of the male connector is deformed

5. Check the adapter

The millimeter wave connector is very small and has very precise mechanical tolerances. A small defect, damage, or dirtiness will affect its accuracy and repeatability. Therefore, we need to check it when using it to avoid using defective adapters that affect the indicators and stability of our tests.

When checking the adapter, please wear an anti-static wrist strap, use at least a 10x optical magnifier, and check both the male and female ends.

1. Slotless female connector

2. Slotted female connector

a: Check the contact integrity of the inner conductor reed

b: Check the concentricity of the male and female connectors

3. Check the PIN Depth of the adapter

This requires a special measuring tool to test the PIN Depth of each adapter. Generally, we don't use it, so I won't introduce it here. If you are interested, you can check the relevant information and testing methods yourself.

6. Connection of the adapter

When connecting various test cables and adapters, we need to pay attention to some basic matters, so as to minimize the wear between the connectors and extend the service life of the connectors while ensuring a good connection.

1. First, check all the adapters that need to be connected to see if there are any problems described in Section 5.

2. Align the center conductors between the two connectors and push the connectors in gently until you can feel the center conductors of the male and female connectors making contact.

3. Fix one end and rotate the other end until it cannot be twisted by hand.

4. Use a suitable torque wrench to secure the connection.