Performance index of RF coaxial connector
Performance index of RF coaxial connector:
The electrical performance of RF coaxial connector shall be like the extension of RF coaxial cable, or the influence on the transmitted signal shall be minimized when the coaxial connector is connected with the coaxial cable, Therefore, the characteristic impedance and VSWR are important indexes of RF coaxial connector. The characteristic impedance of the connector determines the type of impedance of the cable connected with it. The VSWR reflects the matching level of the connector
A. Characteristic impedance: an inherent characteristic of a transmission line determined by the capacitance and inductance of the transmission line, reflecting the distribution of electric and magnetic fields in the transmission line. As long as the medium of the transmission line is uniform, the characteristic impedance is a constant. In the process of wave propagation, E / h is constant. The transmission line itself determines its characteristic impedance, and the characteristic impedance is the same everywhere in the transmission line. In coaxial cable or coaxial connector, the characteristic impedance is determined by the inner diameter of the outer conductor, the outer diameter of the inner conductor and the dielectric constant of the medium between the inner and outer conductors, which has the following quantitative relationship,
Characteristic impedance
|
特征阻抗 |
空气，ε=1 |
发泡PE, ε=1.5 |
PTFE, ε=2.05 |
PE,ε=2.28 |
|
50Ω |
2.30 |
2.78 |
3.30 |
3.52 |
|
75Ω |
3.49 |
4.63 |
6.00 |
6.60 |
B. Reflection coefficient: the ratio of reflection voltage to input voltage. The more the value is, the less the reflected energy is, the better the matching is, the closer the characteristic impedance is, and the better the continuity is
C. VSWR: there are two kinds of waves in the mismatched transmission line, one is incident wave and the other is reflected wave. In some places, the two waves produce superposition. The superimposed wave does not travel along the transmission line, but stagnates. In other words, there is always a maximum or minimum voltage on any reference plane. This kind of wave is called standing wave. The VSWR is the ratio of the sum of the input voltage and the reflected voltage to the difference between the input voltage and the reflected voltage, which is greater than or equal to 1. The smaller the VSWR is, the better the VSWR is. The VSWR has a quantitative relationship with the reflection coefficient.
D. Insertion loss: when a component or system is inserted and connected to a circuit, the circuit will generate energy loss. The energy lost is the insertion loss of the component or system, usually in dB. The insertion loss increases with the increase of frequency. This is the RF leakage due to skin effect. The insertion loss is mainly affected by the following factors: 1. Due to skin effect and medium loss, part of electric energy will be converted into heat energy; 2. Reflected energy will be lost in the transmission process; 3. RF leakage caused by surface bare leakage.
E. Third order intermodulation: nonlinear noise of two or more frequencies generated by passive components (such as connectors). The factors causing the third-order intermodulation are complex, which need professional design and production technology to reduce or prevent. Several typical factors affecting intermodulation are as follows:
——Oxidation of contact surface materials
Due to the oxidation of aluminum or other materials, silver can be used to improve performance;
——Magnetic material
Steel, stainless steel, etc., cause nonlinear characteristics;
——Current saturation
The relationship between current and voltage is no longer linear;
——High corona
Plasma effect;
——Small crack
Appears on the connection surface;
Grease
Direct connection between connecting elements is not allowed.
F. Cut off frequency: within the cut-off frequency, the signal propagates in the form of TEM wave. The mechanical size of transmission line determines the cut-off frequency. Generally speaking, the smaller the size (axial direction), the higher the transmission frequency of transmission line. There is no field in the direction of energy transmission (electric and magnetic fields are perpendicular to the cable axis). When the electromagnetic wave frequency is too high (the wavelength is too short), the dielectric space size of coaxial cable or coaxial connector is too large (relative to the wavelength) so that the electromagnetic wave can no longer propagate in TEM (the propagation direction of electromagnetic wave, the direction of electric field and the direction of magnetic field are perpendicular to each other), when the TEM wave reaches the cut-off frequency, it will become hybrid wave. The inner diameter of the outer conductor of coaxial cable or coaxial connector is smaller, and the cutoff frequency is higher. In the coaxial line, the voltage and current propagate in different ways, and the voltage wave propagates between the inner conductor surface and the inner surface of the outer conductor. The transmission of current along the coaxial line causes the ring field strength around the inner conductor, and the closer to the surface, the greater the field strength. The current causes a magnetic field, and the voltage causes an electric field.
G. Dielectric constant: it is used to measure the capability of the insulator to store electric energy, which represents the degree of polarization of the dielectric, that is, the binding capacity to the charge. The larger the dielectric constant is, the stronger the binding capacity to the charge is. When the dielectric is applied with an external electric field, it will generate an induced charge and weaken the electric field. The ratio of the original external electric field (in vacuum) to the electric field in the final medium is the dielectric constant, Also known as induced rate or relative permittivity. If a material with high dielectric constant is placed in an electric field, the intensity of the field will decrease significantly in the dielectric. The capacitance of a pair of capacitor plates increases by ε times when the dielectric constant is ε. Dielectrics have the property of making space larger or smaller than its actual size. For example, when a dielectric material is placed between two charges, it will reduce the force acting between them, just as they are moved away. When the electromagnetic wave passes through the dielectric, the speed of the wave is reduced, and the wavelength is shortened. The dielectric constant of the insulating material commonly used in connectors is generally between 2-5, such as Teflon is 2.1, FR4 is 4.6
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