How to choose high-speed and high-frequency rf test cables（2）

How to choose rf test cable, generally choose according to the following parameters.

Characteristic impedance: The characteristic impedance of the rf test cable is generally 50ohm, and there are other special-purpose cables, such as 75ohm cable TV and video cables.

VSWR and impedance consistency: VSWR reflects the problem of consistency. In high-speed signal integrity, we usually use the reflection coefficient. The standing wave ratio of the rf test cable is generally very small, and the high-speed and high-frequency rf test cable is generally within 1.05.

Loss: Attenuation, also known as attenuation, refers to an indicator of the energy attenuation of the signal as the length and frequency of the rf test cable increase. This loss is mainly determined by conductivity and dielectric loss. In general, the larger the rf test cable, the smaller the conductor loss and the smaller the loss. However, the dielectric loss has nothing to do with the size of the rf test cable, and the dielectric loss plays a major role at high speed and high frequency.

Insertion loss of rf test cable: In the loss, there is another point that needs to be paid attention to, which is the stability of the loss. This stability does not mean that the loss remains constant, but that the loss needs to remain constant at a certain frequency. Generally, the loss of the rf test cable with the braided structure will increase with time or the number of bends. So when we buy rf test cable, we usually ask the supplier if it is a stable amplitude and stable rf test cable, in order to prevent the fluctuation and change of the value during the test.

Return loss of rf test cable
Transmission rate: The transmission rate of high-speed and high-frequency rf test cables is mainly determined by the dielectric constant of the insulating material between the inner and outer conductors. This has the same meaning as the well-known signal propagation rate on a PCB. This is different from the transmission rate of the signal.
 

Shielding: The shielding of the rf test cable mainly means that its signal should not be interfered by the outside world, nor should the signal interfere with the outside world. We also call the shielding of the rf test cable isolation.

 
Cut-off frequency: The common understanding is the highest bandwidth frequency that the rf test cable can transmit without affecting the signal transmission.

 
Flexibility: Although this has little to do with electrical performance, it is still worth talking about. Flexibility is very important for our testing, because if the rf test cable is too stiff, it will make our test system not easy to stabilize, and it will easily damage the test fixture. But the high-speed rf test cable I have seen so far is not very soft.

In short, the choice of high frequency rf test cable must be satisfied: do not let our test results be affected by the rf test cable.

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1. How to choose high-speed and high-frequency rf test cables(1)