RF coaxial cables are widely used as transmission lines in communication systems. Characteristic impedance is the first electrical parameter to consider when designing and selecting RF coaxial cables. This article will analyze the measurement of this parameter.
1, Definition of characteristic impedance
When electromagnetic waves propagate on cables, there are usually forward propagating incident waves and backward propagating reflected waves, which superimpose to form standing waves. The ratio of the total voltage to the total current at any point on the transmission line is defined as the input impedance of that point looking towards the load end. In general, the input impedance of a transmission line is not only related to the length of the line, but also to the frequency. However, when the transmission line is infinitely long, there are only forward traveling waves (traveling waves) on the transmission line. At this point, the input impedance at any point on the transmission line is independent of the line length and is equal to a constant value Zc, which is called the characteristic impedance of the transmission line.
In addition, when the transmission line terminal is connected to a constant value of pure resistive load, the input impedance at any point on the transmission line is also equal everywhere and independent of the line length. This constant resistance value is the characteristic impedance value of the transmission line. The characteristic impedance Zc of RF coaxial cable depends only on the diameter of the inner and outer conductors of the transmission line and the equivalent dielectric constant of the filling medium between them, and is independent of the line length.
2, Measurement method of characteristic impedance
The characteristic impedance of RF coaxial cables can be measured using frequency domain or time domain methods.
The frequency domain method generally uses a vector network analyzer to test the performance of cables. Due to the use of bandpass filters and digital filters, vector network analyzers have very low background noise and can accurately measure the characteristic impedance of cables. According to the different transmission directions of the test signal, the frequency domain method can be divided into two types: transmission measurement and reflection measurement.
Among the commonly used impedance measurement methods for RF coaxial cables, the transmission phase method, transmission phase difference method, open circuit or short-circuit resonance method, etc. belong to the transmission measurement in the frequency domain method, while the newer single connector measurement method belongs to the reflection measurement in the frequency domain method.
The voltage standing wave ratio of coaxial connectors is mainly caused by the unevenness of the internal impedance of the connector and the deviation from the characteristic impedance of the cable. Due to the easy control of the characteristic impedance of coaxial connectors (such as (50 ± 0.5) PSI), the internal impedance non-uniformity, including the standing wave ratio caused by discontinuous capacitance due to size changes, is extremely small. Moreover, below the low frequency range (such as 200MHz), the standing wave ratio of the connector is generally only around 1.005, which is much smaller than the voltage standing wave ratio of the cable component. Therefore, the voltage standing wave ratio of the connector can be ignored. However, the reflection caused by the impedance non-uniformity inside the tested cable cannot be ignored. This part of the influence should be eliminated during testing, so that the main reflection source in a single connector comes from the deviation between the cable impedance and the standard impedance. Finally, the characteristic impedance of the tested cable can be directly obtained by measuring the voltage standing wave ratio of a single connector.
In the radio frequency range, the characteristic impedance of coaxial cables is independent of frequency, so it only needs to be measured using the transmission phase method at any frequency within the 30-200MHz frequency range specified in the national standard GB 4098.3 "Measurement Method for Characteristic Impedance of Radio Frequency Cables".
It should be noted that due to the large error of the transmission phase difference method, it is recommended to use it with caution. The single connector method is easy to operate, provides accurate measurement data, and is directly linked to the voltage standing wave ratio. It has strong practicality and is a convenient and practical method for measuring the characteristic impedance of coaxial cables. It is recommended to use it as the preferred method.
