Why use 50 ohms in RF?(1)

RF devices with various interfaces (SMA, SMP, BNC, etc.) are often used in RF circuits. Therefore, it is necessary to use RF cables or connectors of various interfaces to connect these RF devices and devices together. However, regardless of the connector type, did you know that they are all 50 ohms (Ω)? And the ports of the signal sources, spectrum analyzers, network analyzers, etc. we use are all 50 ohms. Do you still have some doubts here, why are the interfaces of so many devices or equipment all choosing 50 ohms? With these questions we start the study of this article.

Let's first understand the relevant knowledge of transmission lines. Transmission lines are used to propagate signal energy. There are many types of them, such as coaxial cables, microstrip lines, striplines, and so on. A common radio frequency cable is a coaxial cable, which generally consists of an inner conductor, a medium, an outer conductor and a sheath, but some have multiple layers of shielding and protective layers.

The following is a schematic diagram of the structure of a common coaxial cable:

The transmission of the maximum power of the signal and the reflection of the minimum signal are determined by the impedance of the transmission line and the impedance matching in the system. The impedance of the transmission line is called characteristic impedance or characteristic impedance. The characteristic impedance can be expressed by the following formula: 

D: inner diameter of outer conductor;
d: outer diameter of inner conductor;
ε: the dielectric constant of the filling medium;
ks: Inner conductor coefficient, which is related to the inner conductor structure.
From the formula, we can know that the characteristic impedance is related to the size of the inner and outer conductors of the transmission line and the dielectric constant of the filling medium. The characteristic impedance can be controlled by these three parameters.

The transmission line is used to transmit energy. It also has certain requirements for the maximum withstand power. The maximum withstand power is also related to the size of the inner and outer conductors, that is, the characteristic impedance.
E: electric field strength, when it reaches a certain strength, breakdown will occur;
D: inner diameter of outer conductor;
d: outer diameter of inner conductor.

Therefore, in order to ensure the maximum capacity transmission of the signal power and avoid the loss of the signal power, it is necessary to select an appropriate characteristic impedance.

Here I would like to mention the famous Bell Labs. Bell Labs has done a lot of experiments on coaxial cables with different parameters and found that the characteristic impedance of the coaxial cable bears the largest signal power at 30 ohms, while the signal power at 77 ohms is the largest. the least loss.

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1. Why use 50 ohms in RF?(2)