The Microstrip (or strip line, which are not connected with cable,etc) RF coaxial connector’s VSWR test is always a difficult problem to solve,since the 1990’s, Shanghai 16^{th} Radio Factory, Chengdu High Frequency Connector Factory, 853 Factory(Huada), Shanghai 1423 Institute, etc., have tried many methods,but still was unable to form a unified way in the industry.
1. The methods proposed and used by the companies listed above at that time(Some other methods are not listed here for incomprehension)
a) Shanghai 16^{th} Radio Factory:setting one or more test frequency points,making the corresponding 1/2 wavelength Microstrip or strip line circuit(The Microstrip or strip line circuit defaults to a lossless transmission line and the contact impedance of each end continuous),connecting to a Microstrip or a strip line connector at both ends,in these case,break down the VSWR of the connector by the formula.
b) Chengdu High Frequency Connector Factory:according to the center frequency in the frequency of design a quarter-wavelength Microstrip or strip line circuits,one end connected with a ferrite load for continuous sliding,another end connected with the DUT,then make the ferrite load continuously sliding from the distal end to the proximal end,in the same time observe the VSWR or stop at a certain position to measure the VSWR of the corresponding frequency point of the electrical-length.
c) 853 Factory(Huada):in the early 1990’s, 853 Factory undertook the research project of VSWR test method for Microstrip RF connectors of the former Ministry of Electronics Industry，and by the solution that connect the Microstrip connector to a same-length connector with an opposite port shape(the connector defaulted is standard component,which isn’t replaced continuous testing) back to back(See figure 1 below).
(Hers is a fig, you lost)
The VSWR of a single connector could be obtained by taking the square root of the total VSWR of the pair of connectors described above.
2. Theoretical basic
a) Image method:Shanghai 16^{th} Radio Factory and 853 Factory all used the mirror method in electromagnetic field theory, and the paired connector t not only structurally equivalent, but also equally identical,in all manufacturing links,the electric length of the pair of connectors are also exactly the same.
b) Method by absorbing the load(Chengdu High Frequency Connector Factory):A quarter of the wavelength is designed to do theoretical full absorption at the wave antinodal point(The component is single).
3. Analysis of test methods
a) Shanghai 16^{th }Radio Factory chooses using electromagnetic field imaging theory: assuming that the structure of the paired connectors,to be tested,are the same without any difference,and other elements are exactly the same also.But in reality, there are differences between each of the products,and the Microstrip or strip line circuit,themselves,not only produce loss but also reflect.Therefore, even in the case of extremely accurate and excellent circuit design, it can only approximate measurement.In addition, for different frequency points, multiple Microstrip circuit or strip line circuit should be used for frequent replacement,and we cannot continuously sweep frequency measurement.This method is suitable for the process of design verification,but troublesome and not practical enough in production.
b) 853 Factory also chooses using electromagnetic field imaging theory:no transition section,assuming that the structure of the paired connectors,to be tested,are the same without any difference,and other elements are exactly the same also.This method can be used for continuous frequency sweep measurement and is suitable for design and production.However,as a matter of fact,In addition to each product itself will be different,there are structural differences in the inner conductors of the pair of connectors which is connected back to back inside.they use one of the connectors as a dedicated pair connector and assume it is a standard device,which is never replaced during testing.In doing so,the VSWR is not the same after every decomposition, which does not meet the basic specification of test requirements.
c) Chengdu High Frequency Contact Factory test connect with Using sliding ferrite loads with each one,which is the basic principle of testing.However, the ferrite load transmitted by 1/4 Microstrip or a strip line has full absorption characteristics,therefore, consideration should be given to whether the precision and bandwidth of the ferrite and Microstrip or strip line circuit itself can cover the standard frequency band of the connector;if it cannot be covered, the same problem exists as that of Shanghai 16^{th}^{ }Radio Factory.
4. Decomposition Formula and Calculation Formula
a) Chengdu High Frequency Contact Factory:measure them with single,needn’t to decompose resulting data.If the above precision and full frequency band problems as well as measurement errors caused by equipment, test fixtures and personnel, factors are not taken into account,then there will be no error in data processing.
b) 853 Factory(Huada):the VSWR of a single connector is obtained by taking the square root of the total VSWR measured.After comparing a large number of data, this data data is close to the true data only when the measured total VSWR is less than 2, due to ignoring these measurement errors caused by equipment, test fixtures and personal factors(Here the true data mentioned above refers to the data that can be repeatedly measured all points under the same meshing condition).This approach has a greater limitations,and if didn’t realize this, there will be serious misdirection caused by the data.
c) Shanghai 16^{th }Radio Factory:by virtue of the technical department with successive generations of microwave professional background,they adopted the decomposition formula which was derived from Professor Liao Chengen’s Fundamentals of Microwave Technology,Professor Wu Wanchun’s Theory of Electromagnetic Field,Professor Xie Chufang’s Electromagnetic Fields and Electromagnetic Waves, etc.
Formula 1 S_{单}=（1+3S_{双}）/（3+S_{双}）
(Here S2 refers to the total VSWR obtained by measurement)
In theory, it is calculated from Maxwell equation by differential and integral equations. Except for the error caused by continuous approximation and measurement error, the data processing itself is in accordance with the theoretical requirements in addition to the error caused by the continuous use of approximate values in the calculation process and the measurement error. The parameters involved in the reflection coefficient are all vector values, that is, according to the vibration principle of the wave, the superposition of the two waves is vector superposition rather than simple scalar addition and subtraction. At the same time, the rotation of electromagnetic wave should be taken into account in the calculation. As the following formula 2 / 5 shows:
(1) the formula and the formula indicate that the reflection coefficient is the ratio of the reflected wave voltage Ur (z) (or current at any point z on the transmission line to the incident wave voltage Ui (z) (or current). It's called the reflection coefficient,
G（z）=U_{r}（z）/U_{r}（z）=-I_{r}（z）/I_{i}（z）
for lossless transmission lines, γ = j β, end load ZL, It can be deduced that
G= A_{2}e^{-j}^{b}^{z}/ A_{1}e^{-j}^{b}^{z}=（（Z_{L}-Z_{o}）/（Z_{L}+Z_{o}））e^{-j2}^{b}^{z }3
=G_{L} e^{-j2}^{b}^{z}^{}
^{}
G=（（Z_{L}-Z_{o}）/（Z_{L}+Z_{o}））=|G_{L} | e^{j}^{w}_{L}
it is referred to as the terminal reflection coefficient. So the reflection coefficient of any point on the transmission line can be expressed as
G（z）=|G_{L} | e^{ j（}^{w}_{L}^{ -2}^{b}^{z）} 4
thus the characteristic impedance formula can be obtained:
Zo=√[（R+jvL）/（G+jvC）] 5
of which, Both the imaginary part j ω on the molecule and the denominator denote the rotation of the electromagnetic field.
Since the process of deducing the above formula by Maxwell equations is tedious and cumbersome,interested friends could refer to the above textbooks to derive the formulas themselves without further elaboration here.
d) Other methods of decomposition-(formula 6),
Γ =（1/2）*[（S_{双}-1）/（S_{双}+1）] 6
This method is also an approximate calculation based on the mirror method, but the like the square root method,it ignores rotation of electromagnetic field, and the data is not considered as a vector. In simple terms, even if the formula is revised to:
｜Γ｜ =（1/2）*[（S_{双}-1）/（S_{双}+1）] 7
T he angle of its projection on the number axis is still not taken into account.
5. Summary
Shanghai Junyou Radio Frequency Technology Co., Ltd. has a deep understanding of the history of research on test methods for series connectors, and combined with our testing experience over the years, we believe:
1, The theory of mirror method is a pure idealized theory and method. There are various differences in the application of mirror method in production practice, and the technicians and testers should have a clear understanding of this theory. Of course, the mirror method is also a convenient way to test. If the above three test methods and four calculation methods are used in production, 853 test method is recommended for Microstrip or strip line products, which is relatively convenient, but its theoretical defects should be recognized.
2, decomposing the calculation formula, it is suggested to adopt the formula: Pless insert the formula.
3. The method has the same effect for analyzing the VSWR of the connectors at both ends of the cable assembly. The combination of the two aspects will have both theoretical significance and practical value.
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