There are many kinds of micro miniature coaxial connectors: SMA, SMB, SMC, APC-7, K connector, etc. This article will give you an interpretation.
Whatever connector you use, you need to pay attention to its applicable frequency range before using it. The frequency range of the connector is limited by the first circular waveguide propagation mode in the coaxial structure. Reducing the diameter of the outer conductor will increase the maximum usable frequency. Filling the space with insulators reduces the maximum usable frequency and increases system losses. The performance of all connectors is affected by the quality of the connector interface. If the diameter of inner and outer conductors deviates from the design requirements, the quality of electroplating is poor, or the gap between the joints is large, the reflection coefficient and resistance loss of the interface will be reduced. This is why in the same kind of connector, the one with good quality, can be used at higher frequencies and has smaller standing wave coefficients.
If the circuit you need to measure and test is below 18GHz, you can choose SMA connector, N connector and APC-7 connector. You can choose the specific type, according to your circuit type, cost and so on. If you have enough money, you can also use K connector, etc. If you measure and test in millimeter wave band, you can choose K connector or 2.4mm connector. Why not choose a 3.5mm connector? Because compared with K-connector, it has neither price advantage nor performance advantage (the applicable frequency is not as high as K-connector).
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