How to select RF connectors?

In the RF and microwave industries, many types of coaxial connectors are specially designed for specific purposes and applications, which are smaller in size and work at frequencies as high as GHz and millimeter wave. The Connector standard generally accepted by the industry is compatible with other RF/microwave components, so it can be used for interconnection with coaxial modules in the system while maintaining the coaxial characteristics of the corresponding transmission lines. There are many kinds of RF connectors with different classifications. How to choose the right RF connector?

I. Types of RF connectors

According to the connection interface structure, it is divided into:

1. threaded connection

The application series such as SMC,SMA,TNC,N,UHF,7/16 and 4.1/9.5 are suitable for test equipment, military and telecommunication equipment.

2. Bayonet type

Use series, such as BNC BNC,MHV, are used in reliable connection and quick plugging and unplugging, and are mostly used in test equipment and military equipment.

3. Push-in

It allows very fast joint and separation, and its structural characteristics are reliable. It is often used in small connectors, such as MCX and SMB series.

4. Push-in self-locking type

Unique self-locking mode, very reliable, usually on high-density small connectors, such as 1.0/2.3,1.6/5.6 series, it is quick to plug in and out and reliable to lock.

5. Slide-in type

It is used in different DIN multi-terminal connectors, and is often used for connection on PCB.

Classification by size:

1. normalized form

UHF,N,7/16,7mm

2. small-sized

BNC,TNC

3. subminiature

SMA,SMB,SMC,MCX,BMA,SAA,3.5mm

4. miniature

SSMA,SSMB,MMCX,2.4mm,K(2.92mm),1.85mm,1mm

Classification by frequency:

1. Audio: less than 20khz.

2. Video (VIDIO): 30 MHz ~ 500 MHz or less

3. Radio frequency: 500 MHz ~ 300 GHz

4. Optical fiber: 167 THz ~ 375 THz.

Connectors used in Radio band are called RF connectors.

According to the termination method is divided into

1. Connector MIL-C-39012(GJB681)

2. Adapter MIL-A-55339(GJB680)

3. Microstrip and stripline ML-C-83517(GJB976)

According to the function is divided into:

1. Universal type (level 2)

2. Precision type (level 0, level 1)

3. Special type (radiation resistance, high pressure resistance, waterproof, etc.)

4. Multifunctional type (including filtering, phase modulation, mixing, attenuation, detection, limiting, etc.)

RF/microwave coaxial connectors are divided into several categories. Like coaxial cable, the cutoff frequency is also a key characteristic for any coaxial cable connector, which indicates the highest frequency that can maintain the required single TEM propagation mode. The frequency range of any connector is limited by the propagation mode in the corresponding coaxial system. Millimeter-wave coaxial connector refers to a coaxial connector used for frequencies above 18GHz.

II. N-type, BNC and TNC connectors

The 50-ohm N-connector was developed in the 1940s and was originally used in military systems with operating frequency below 5GHz. N-type connector adopts internal gasket, and there is an air gap between the central conductor and the external conductor. Although its performance is improved to 18GHz by subsequent improvements, even the latest design will begin to spread and change at the frequency of about 20GHz. That is to say, when this type of connector is used at a frequency of 20GHz or higher, unpredictable consequences will occur. 75 ohm N connector is widely used in cable TV industry.

BNC connectors are used for video and RF applications within 2GHz, with slotted outer conductors, and both male and female connectors are equipped with plastic media. At the high frequency above 4GHz, the slot can emit a signal as high as 10GHz. Because the mating geometry of BNC connectors is compatible with N connectors, some BNC connectors can be temporarily mated with N connectors of corresponding polarity.

TNC connector is a threaded version of BNC connector, which helps to solve leakage and stability problems and allows it to be used at frequencies as high as 12GHz and 18GHz. TNC connectors are widely used for RF/antenna connection of cellular phones.

Three, SMA and SMB push connector

SMA (ultra-small A-type) connector adopts an outer coaxial conductor with a diameter of 4.2 mm and is filled with PTFE medium. According to different manufacturers, the upper limit frequency is 18~26GHz. SMA connectors are suitable for 5/16 inch (12.7/40.6 cm) wrenches, and can be mated with 3.5mm and 2.92mm connectors.

SMB (Subminiaturireb/Ultra-small B-type) connector is a push connector, which is usually used for frequencies from 4 GHz to 12.4 GHz. With the increasing frequency required by people, this kind of connector has the problem of being too large and unable to provide the bandwidth needed for high-frequency applications.

4. 3.5mm and 2.92mm connectors

These two connectors using air medium are not only compatible with each other, but also with SMA connectors.

The 3.5mm connector can work stably at frequencies within 26GHz, while the 2.92mm connector has a maximum working frequency of 40GHz.

V. 2.4mm and 1.85mm connectors

2.4mm and 1.85mm connectors are compatible with each other, but their designs are not compatible with SMA, 3.5mm and 2.92mm connectors. This is because these two types of connectors are more precise and more expensive, and if they are mated with connectors with lower precision, they may cause irreparable damage.

VI. 1mm and 0.8mm connectors

These two connectors are suitable for millimeter wave analysis, supporting transmission and repeatable interconnection applications in the range of DC~110GHz.

Seven, choose the appropriate RF connector.

How to choose an appropriate RF connector can be determined according to the actual application. If you choose only from the performance, you can judge it according to the electrical performance index.

The main indicators of RF connectors are:

impedance

Almost all RF connectors and cables are standardized with 50ohm impedance. The only exception is the cable TV connector of 75ohm TV system. It is also important that the RF Coaxial Cable connector has the characteristic impedance matching the cable.

VSWR (voltage standing wave ratio)

VSWR is an index commonly used by engineers to evaluate RF connectors, which is similar to impedance. In general, the VSWR is guaranteed to be less than 1.2 within the frequency range of interest. However, it is not necessary to reach 1.2, while others are all below 1.5 or 2. The lower the better, of course, and the higher the cost.

frequency range

The working frequency range of RF connectors must be paid attention to in high frequency and high speed fields. The frequency is divided into several grades, such as 1~6GHz, or 0~18GHz, 20GHz, 27GHz, 40GHz, 50GHz, 67GHz, 110GHz or even higher.

Insertion loss

Loss is an indicator that all connectors will pay attention to. It's just that the loss of RF connectors is relatively small (if it is too large, it means there is a problem). Generally, it is within 0.1~0.5dB within the frequency range of interest.

return loss

For some engineers who do digital circuits, VSWR is not so intuitive, so some will use return loss to characterize it.

Number of uses (plugging and unplugging)

Some engineers may not pay much attention to the number of uses of connectors. This number of uses is generally considered to be how many times after plugging and unplugging, the performance of the connector will change. Generally, the plugging and unplugging times of RF connectors are 500 or 1000 times, and there will be higher requirements. If you want to do some experiments on this, you must do it under a specific torque/torque, otherwise it will be difficult to ensure the requirements of this number of uses. Generally, the connectors I have used have not reached this number of times. It is estimated that I can use them for 100 or 200 times. The inner core has been deformed, because it is always possible to twist it by inserting it askew or exerting too much force.

In addition to the electrical performance of the connector mentioned above, the mechanical performance of the connector (fixity of the central contact, mechanical strength of the connector, vibration, impact, etc.), environmental performance (weather type, sealing, salt spray) and connector durability (mechanical durability, high temperature durability) are all considerations in selecting a suitable connector. In practical application, you can choose your suitable connector according to the above factors.

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