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Take you to understand why the impedance of MCX RF Connectors is 50Ω
 Dec 31, 2022|View:402

With the development of society, MCX RF Connectors, due to their lightweight, small size, simple structure, easy to use and other characteristics, is widely used in fixed communication instruments RF circuit to space. Careful friends in the use of all will notice that no matter how the MCX RF Connectors are, their impedance is 50Ω. this is why it?

MCX RF Connectors

"Characteristic impedance" is the RF transmission line that affects the high-frequency wave voltage, current amplitude, and phase changes in the inherent characteristics, equal to the ratio of voltage and current at each location, the unit of characteristic impedance is the ohm.

(1) From the perspective of the production process.

From the PCB production and processing process perspective, for most of the existing PCB manufacturers to consider the equipment, the production of 50Ω impedance PCB is relatively easy to achieve.

From the impedance calculation process, too low impedance requires a wider line width and thin media or a larger dielectric constant, which is more difficult to meet the space for the current high-density board.

Too high impedance requires a thin line width and thicker dielectric or smaller dielectric constant, which is not conducive to EMI and crosstalk suppression, and for multilayer boards and from the point of view of mass production, the reliability of processing will be poor.

Control 50Ω impedance in the use of common plates (FR4, RO4350B, etc.), common core board environment, the production of common board thickness of the product (such as 0.508mm, 1mm, 1.2mm, etc.), the common line width can be designed (4 ~ 10mil), so the board factory processing is very convenient, the processing of the use of equipment is not very high requirements.

(2) from the PCB design considerations.

50Ω is also the choice after careful consideration. The impedance calculation process can be seen for the width of the alignment. The thinner the plate media, the lower the impedance. Three main factors will affect the impedance of the PCB alignment.

First is the PCB alignment near the area of the field EMI (electromagnetic interference), and the height of this alignment from the reference plane (i.e., the distance from the microstrip line to the reference ground) is a certain proportion of the relationship, the lower the height means that the radiation is smaller.

Secondly, crosstalk will significantly change with the height of the line, and the height is reduced by half, and crosstalk will be reduced to nearly a quarter.

Finally, the lower the height, the lower the impedance, less susceptible to capacitive loads.

(3) Consider the full path of the signal.

The design also needs to consider one of the most critical factors, that is, the driving ability of the chip, in the early most chips can not drive the impedance of less than 50Ω transmission line, and higher impedance transmission lines due to the inconvenience of implementation, so the compromise of 50Ω impedance.

(4) From the perspective of electrical performance.

Here's another look at the loss perspective. The high-frequency, high-speed lines have a skin effect. The industry has proved that 50Ω for the skin effect, its loss is the smallest. Usually, the skin effect loss L of the cable (in decibels) is proportional to the total skin effect resistance R (unit length) divided by the characteristic impedance Z0.

The total skin effect resistance R is the sum of the shield and intermediate conductor resistance. The skin effect resistance of the shield is inversely proportional to its diameter d2 at high frequencies. The skin effect resistance of the inner conductor of the coaxial cable is inversely proportional to its diameter d1 at high frequencies. The total series resistance, R, is therefore proportional to (1/d2+1/d1).

Combining these factors, given d2 and the corresponding dielectric constant Er of the isolation material, the ratio d2/d1 can be calculated with minimal skin effect losses. Assuming a dielectric constant of 2.25 for solid polyethene and a minimum skin effect loss, d2/d1=3.5911 yields a characteristic impedance of exactly 50Ω.

(5) From the historical point of view.

A widely circulated version of the story from HarmonBanning's "Cable: There may be many stories about the origin of 50Ω". In the early days of microwave applications, during World War II, the choice of impedance depended entirely on the need for use. For high power handling, 30Ω and 44Ω were often used.

On the other hand, the lowest loss air-filled wire impedance was 93 Ω. In those years, there were no easily bendable, flexible cables for higher frequencies, which were rarely used, but simply rigid conduits filled with air media. Semi-rigid cable was born in the early 50s, and the real microwave flexible cable was about 10 years later.

As technology progressed, impedance standards needed to be given to striking a balance between economy and convenience. In the United States, 50Ω was a compromise; to unite the Army and Navy to solve these problems, an organization called JAN was formed, later DESC, and 60Ω was chosen for Europe, developed specifically by MIL.

The most used conduit in the United States is made from existing scale rods, and water pipes connected into 51.5Ω is very common. Seeing and using 50Ω to 51.5Ω adapters/converters, it felt strange that eventually, 50Ω won out, and special conduits were made (or maybe the renovators changed the diameter of their tubes slightly). Soon after, the Europeans were forced to change under the influence of dominant industry players like Hewlett-Packard.

The above is a story, but it reflects that 50Ω is a compromised product after much practice. Early microwave RF systems commonly used coaxial cable as a connection line. From the design point of view, I certainly, hope that after the coaxial cable transmission of the microwave RF signal, transmission power can be as large as possible during the transmission loss as small as possible.

The above is why the impedance of MCX RF Connectors is 50Ω. If you need more detailed information, welcome to contact us!