A Simple Method for Measuring the Standing Wave Ratio and Return Loss of Series Homogeneous Transmission Lines

Abstract

The present invention discloses a simple method for measuring the standing wave ratio and return loss of serially connected homogeneous transmission lines in the field of radio frequency and microwave circuit design and testing. The test method for a homogeneous long transmission line to be tested is as follows: step 1: measuring and obtaining the length of the long transmission line to be tested; step 2: making a short transmission line with the same structure and material as the long transmission line to be tested, wherein the length of the short transmission line is 1 / n of the long transmission line to be tested, where n is greater than 1; step 3: measuring and obtaining a return loss value RL1 and a standing wave ratio value VSWR1 of the short transmission line; step 4: using formula 1 to obtain a return loss value RLn of the long transmission line to be tested, where RLn=10logn+RL1; and using formula 2 to obtain a standing wave ratio value VSWRn of the long transmission line to be tested. The present invention can simply and conveniently measure the return loss and standing wave ratio of a long transmission line formed by connecting multiple homogeneous short transmission lines in series, and when the return loss and standing wave ratio of the long transmission line are known, the return loss and standing wave ratio of the short transmission line can be measured.

Description

Technical Field

[0001] The present invention relates to the field of radio frequency and microwave circuit design and testing, and in particular to a simple method for measuring the standing wave ratio and return loss of a series homogeneous transmission line. Background Art

[0002] In RF and microwave circuit engineering, it's sometimes necessary to test the return loss and standing wave ratio (SWR) of a long transmission line. Existing testing methods use a long RF cable and connectors to connect the two ends of the transmission line to the input / output ports of a vector network analyzer or other suitable instrument. However, the test cables of typical testing instruments are relatively short, while the transmission line to be tested is much longer, necessitating an additional extension of the RF cable. Furthermore, the return loss and SWR values of the extended RF cable must be de-embedded in the test results. This results in inconvenience, additional errors, and inaccurate test results.

[0003] In RF and microwave circuit engineering, it's sometimes necessary to measure the return loss and standing wave ratio (SWR) of a short transmission line. However, for various reasons, measuring these values can be challenging. For example, with a QFN-type ceramic housing containing an RF I / O coplanar waveguide transmission line, one end of the line rests on the top surface of the ceramic substrate inside the housing, while the other end rests on the bottom surface of the ceramic substrate outside. Existing testing methods make it difficult to measure these values. Summary of the Invention

[0004] The present invention proposes a simple measurement method. It only needs to measure the return loss or standing wave ratio value of a single short transmission line. There is no need to extend the RF cable or perform "de-embedding" processing on the test results. The return loss and standing wave ratio values of a long transmission line formed by connecting multiple homogeneous short transmission lines in series can be easily obtained.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A simple method for measuring the standing wave ratio and return loss of a series homogeneous transmission line. The test method for the homogeneous long transmission line to be tested is as follows:

[0007] Step 1: Measure the length of the homogeneous long transmission line to be tested;

[0008] Step 2: Make a short transmission line with the same structure and material as the long transmission line to be measured. The length of the short transmission line is 1 / n of the long transmission line to be measured, where n is greater than 1.

[0009] Step 3: Measure the return loss value RL1 and the standing wave ratio value VSWR1 of the homogeneous short transmission line using a testing instrument;

[0010] Step 4: Use Formula 1 to obtain the return loss value RLn of the long transmission line to be measured. Formula 1 is:

[0011] RLn=10logn+RL1;

[0012] The standing wave ratio value VSWRn of the long transmission line to be measured is obtained using Formula 2, which is:

[0013]

[0014] In some embodiments, a test method for a short transmission line to be tested is as follows:

[0015] Step 1: Measure the length of the short transmission line to be tested;

[0016] Step 2: Prepare a long homogeneous transmission line with the same structure and material as the short transmission line to be tested as a test sample. The length of the long homogeneous transmission line is n times the length of the short transmission line to be tested, where n is an integer and n>1;

[0017] Step 3: Obtain the return loss value RLn and standing wave ratio value VSWRn of the long transmission line through a testing instrument;

[0018] Step 4: Use Formula 3 to obtain the return loss value RL1 of the short transmission line to be tested. Formula 3 is:

[0019] RL1=RLn-10logn;

[0020] The standing wave ratio value VSWRn of the short transmission line to be tested is obtained using Formula 4, which is:

[0021]

[0022] Beneficial effects: The present invention utilizes the theory of multiple reflections of high-frequency signals between the port interfaces of two or more short transmission lines. It only needs to measure the return loss or standing wave ratio of a single short transmission line to obtain the return loss and standing wave ratio of a long transmission line formed by connecting multiple homogeneous short transmission lines in series; it only needs to measure the return loss or standing wave ratio of a long transmission line formed by multiple homogeneous short transmission lines to obtain the return loss and standing wave ratio of a single short transmission line. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a flow chart of Example 1 of the present invention;

[0024] Figure 2 This is a flow chart of Example 2 of the present invention. DETAILED DESCRIPTION

[0025] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0026] Example 1, as Figure 1 As shown, the test method for the homogeneous long transmission line to be tested is as follows:

[0027] Step 1: Measure the length of the homogeneous long transmission line to be tested using an instrument such as a ruler to confirm its proportional relationship with the short transmission line;

[0028] Step 2: Fabricate a short transmission line with the same structure and material as the long homogeneous transmission line to be tested. The length of the short transmission line is 1 / n of the long transmission line to be tested, where n is greater than 1 but not necessarily an integer. For example, if the long homogeneous transmission line to be tested is 3 meters long, the short transmission line can be 1 meter (corresponding to n=3) or 2 meters (corresponding to n=3 / 2), without affecting the subsequent steps.

[0029] Step 3: Use RF cables and connectors to connect a short transmission line to a vector network analyzer or other suitable testing instrument to measure the return loss RL1 and standing wave ratio VSWR1 of the short transmission line.

[0030] Step 4: Using the theory of multiple reflections of high-frequency signals between the interfaces of multiple short transmission line ports, by superimposing and summing the accumulated reflected power of the normalized signal multiple reflections at the input port of the long transmission line, we can obtain Formula 1 and Formula 2. Formula 1 is used to obtain the return loss value RLn of the long transmission line to be measured, which is:

[0031] RLn=10logn+RL1;

[0032] The standing wave ratio value VSWRn of the long transmission line to be measured is obtained using Formula 2, which is:

[0033]

[0034] For example, to measure the return loss and standing wave ratio of a 2mm-long copper-core air coaxial transmission line with a characteristic impedance of 50Ω and a core diameter of 0.2mm at 3GHz, first fabricate a short test line sample of 1mm-long copper-core air coaxial transmission line with a characteristic impedance of 50Ω and a core diameter of 0.2mm. This short test line sample has the same material and structure as the long transmission line to be measured.

[0035] Then, a short transmission line test sample and a vector network analyzer were connected using RF cables, connectors, etc., and the return loss at 3 GHz was obtained to be -15.56 dB, and the standing wave ratio was 1.4, that is, RL1 = -15.56 dB, and VSWR1 = 1.4.

[0036] Since the length of the short transmission line test sample is 1 / 2 of the long transmission line to be measured, n=2. Finally, using formula 1 to calculate the return loss value of the long transmission line to be measured, RL2=10log2+(-15.56)≈-12.55(dB) can be obtained. Using formula 2 to calculate the standing wave ratio value of the long transmission line to be measured,

[0037] As a comparative verification example, a simulation model of a copper-core air coaxial transmission line with a length of 1 mm, a characteristic impedance of 50 Ω, and a core diameter of 0.2 mm was established in the three-dimensional electromagnetic design simulation software HFSS. Using finite element analysis, the return loss RL1' = -15.56 dB and the standing wave ratio VSWR1' = 1.4 of the model at a frequency of 3 GHz were obtained.

[0038] Then, in the 3D electromagnetic design simulation software HFSS, the length of the coaxial transmission line in the transmission line model was changed to 2 mm. This is equivalent to connecting two short 1 mm transmission lines in series to form a long 2 mm transmission line. Finite element analysis was then used to simulate and calculate the return loss RL2' of this model at 3 GHz, which was -12.55 dB, and the standing wave ratio VSWR2' was 1.62. These results are identical to those of Example 1, verifying the accuracy of the measurement method of the present invention.

[0039] Example 2: Formula 3 can be obtained by transforming the above formula 1, and Formula 4 can be obtained by transforming the above formula 2. When it is inconvenient to use a detection instrument to measure the return loss and standing wave ratio of a short transmission line, such as Figure 2 As shown, the detection can be carried out by the following steps:

[0040] Step 1: Measure the length of the short transmission line to be tested. Generally speaking, short transmission lines are difficult to measure because their test ports are blocked. For example, in one embodiment, one end of the RF I / O transmission line in a QFN-type ceramic housing is located on the upper surface of the ceramic substrate inside the housing cavity, while the other end is located on the lower surface of the ceramic substrate outside the housing cavity. Existing testing methods make it difficult to measure the return loss and standing wave ratio of this short transmission line with ports distributed on both the upper and lower surfaces of the ceramic substrate.

[0041] Since the I / O transmission line is a three-section structure of "horizontal coplanar waveguide-vertical coaxial-horizontal coplanar waveguide", the length of the transmission line to be tested can be obtained by adding the lengths of each section. In this embodiment, its length is 2 mm.

[0042] Step 2: Prepare a long transmission line with the same structure and material as the short transmission line to be tested as a test sample. The length of the long transmission line is n times the length of the short transmission line to be tested, where n is an integer and n>1.

[0043] In this embodiment, in order to produce a long transmission line test sample, the RF I / O transmission line is replicated twice (n=2) on the QFN-type ceramic housing processing layout and mirrored in series, thereby producing a long transmission line test sample with a length of 4 mm and a structure of "horizontal coplanar waveguide-vertical coaxial-horizontal coplanar waveguide-horizontal coplanar waveguide-vertical coaxial-horizontal coplanar waveguide". The two ports of this long transmission line are all on the upper surface of the ceramic substrate, and the material and structure are the same as those of the short transmission line to be tested.

[0044] Step 3: Use RF cables, microwave probes, etc. to connect the long transmission line test sample and the vector network analyzer to test the return loss value RL2 = -12.55dB and the standing wave ratio value VSWR2 = 1.62 of the long transmission line at a frequency of 3GHz.

[0045] Step 4: Use Formula 3 to obtain the return loss value RL1 of the short transmission line to be tested. Formula 3 is:

[0046] RL1=RLn-10logn;

[0047] The standing wave ratio value VSWRn of the short transmission line to be tested is obtained using Formula 4, which is:

[0048]

[0049] Based on Formula 3, the return loss value RL1 of the QFN ceramic housing RF I / O short transmission line can be obtained:

[0050] RL1=RL2-10log2=-12.55-3.01=-15.56dB.

[0051] Based on Formula 4, the VSWR1 of the short RF I / O transmission line in a QFN ceramic housing can be obtained:

[0052]

[0053] As a comparative verification example, a transmission line model with a total length of 2 mm and identical materials and structure (a three-section structure of "horizontal coplanar waveguide-vertical coaxial-horizontal coplanar waveguide") to the short RF I / O transmission line in the QFN ceramic housing was established in the 3D electromagnetic design simulation software HFSS. Finite element analysis was used to simulate and calculate the return loss RL1 = S11 = -15.56 dB and the standing wave ratio VSWR1 = 1.4 of this model at a frequency of 3 GHz. The test results were identical to those obtained in Example 2, thus verifying the accuracy of the measurement results of the present invention's measurement method.

[0054] It should be noted that the terms "long" and "short" in the context of the long transmission line to be tested in Example 1 and the short transmission line to be tested in this example are relative terms and do not have an absolute standard. If it is inconvenient to test the return loss and standing wave ratio of a "long" transmission line, the measurement method in Example 1 can be used. If it is inconvenient to test the return loss and standing wave ratio of a "short" transmission line, the measurement method in Example 2 can be used.

[0055] The present invention has the following advantages:

[0056] 1. The measurement method is simple. When measuring long transmission lines, there is no need to extend the test RF cable or de-embed the test results. Instead, a short transmission line is simply prepared and used as a test sample for testing. The measurement results can then be obtained by calculating according to the calculation formula in the present invention.

[0057] 2. The calculation formula is simple. The calculation formula proposed in the present invention does not involve advanced mathematical knowledge, and people with relatively little mathematical foundation can calculate the results.

[0058] 3. Wide range of applications. The measurement method of the present invention can be widely used to test various types of transmission lines such as coaxial lines, microstrip lines, strip lines, and coplanar waveguides.

[0059] 4. Accurate measurement results. Since no extra length of RF cable is introduced, no additional cable loss error will be generated. The measurement results are consistent with those calculated by professional software simulation.

[0060] Although this specification is described according to implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

[0061] Therefore, the above description is only a preferred embodiment of the present application and is not intended to limit the scope of implementation of the present application; that is, all equivalent modifications made according to the scope of the claims of the present application are within the scope of protection of the claims of the present application.

Claims

1. A simple method for measuring the standing wave ratio and return loss of series-connected homogeneous transmission lines, characterized in that: The test method for the homogeneous long transmission line to be tested is as follows: Step 1: Measure the length of the homogeneous long transmission line to be tested; Step 2: Make a short transmission line with the same structure and material as the homogeneous long transmission line to be tested. The length of the short transmission line is 1 / n of the homogeneous long transmission line to be tested, where n is greater than 1. Step 3: Measure the return loss value RL1 and the standing wave ratio value VSWR1 of the short transmission line using a detection instrument; Step 4: Use Formula 1 to obtain the return loss value RLn of the homogeneous long transmission line to be tested. Formula 1 is: RLn=10log 10 n+RL1; The standing wave ratio value VSWRn of the long transmission line to be measured is obtained using Formula 2, which is:

2. A simple method for measuring the standing wave ratio and return loss of series-connected homogeneous transmission lines, characterized in that: The test method for the short transmission line to be tested is as follows: Step 1: Measure the length of the short transmission line to be tested; Step 2: Prepare a long transmission line with the same structure and material as the short transmission line to be tested as a test sample. The length of the long transmission line is n times the length of the short transmission line to be tested, where n is an integer and is greater than 1. Step 3: Obtain the return loss value RLn and standing wave ratio value VSWRn of the long transmission line through measuring instruments; Step 4: Use Formula 3 to obtain the return loss value RL1 of the short transmission line to be tested. Formula 3 is: RL1=RLn-10log 10 n; The standing wave ratio value VSWRn of the short transmission line to be tested is obtained using Formula 4, which is:

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