Calibration of vector network analyzer (vector network)

One 、 error analysis
The errors in the testing process of network analyzer are mainly divided into three categories ： System error 、 Random error 、 Drift error .
1、 The system error is caused by the unsatisfactory internal test device of the instrument , It is predictable and repeatable . Because it does not change with time , So it can be described quantitatively . The system error can be eliminated by calibration during the test .
2、 Random errors are unpredictable , Because it exists in random form , Will change over time , Therefore, it cannot be eliminated by calibration . The main source of random error is ： Instrument internal noise ( Such as ： Excitation source phase noise 、 Sampling noise 、 If receiver Lo noise ), The switch action repeatability and connector repeatability of the instrument also belong to random error .
3、 Drift error is the performance drift of the test device after the instrument is calibrated . Drift error is mainly caused by temperature change , Can be eliminated by further calibration . The length of time that the instrument can maintain stable accuracy after calibration depends on the drift speed of the instrument in the test environment .
Two 、 Specific analysis of systematic error
Since the calibration of network analyzer can eliminate the systematic error in the test process . The following is a specific analysis of the systematic error of the network analyzer in the reflection characteristic test process .

chart 2-1 The error content of network analyzer when testing the reflection characteristics of devices
1、 Frequency response error
The network analyzer works under the frequency scanning state , Whether it is the power divider inside the instrument , Directional coupler , Or external adapters and test cables , Within the working frequency band, its characteristics will change with frequency . The test error caused by these frequency response characteristics is called “ Frequency response error ”, Also known as “ Tracking error ”. Frequency response error includes reflection tracking error in reflection characteristic test and transmission tracking error in transmission characteristic test .
2、 Directional error
The error caused by the finite directivity of the directional coupler is the directivity error , The leakage signal caused by directional error will be superimposed on the real reflected signal , Cause test error . When the port matching performance of the tested part is good , The directional error has a great influence on the test .
3、 Port mismatch error
During the reflection index test , The reflected signal returns to the instrument port through the transmission path , There will be a mismatch between the impedance of the instrument port and the transmission line , This mismatch will cause secondary incidence of the signal , Finally, the multiple incidence of the signal in the transmission path , Accordingly, multiple reflections are formed , This error is called source mismatch error . The worse the matching performance of the tested part , The more obvious the influence of this error on the test .
Again , The transmission signal output by the device under test will also be reflected due to the impedance mismatch at the receiving end , The signal will be superimposed on the real reflected signal through the reverse transmission of the tested part . Thus, the load mismatch error is formed . If the reverse transmission isolation performance of the tested part is poor , The influence of load mismatch error is great .
4、 Isolation error
Inside the network analyzer R1、 A、 B The receiver should reflect the test input separately 、 Reflect and transmit signals . But there will be signal crosstalk between these receivers , For devices under test with high isolation ( switch 、 Isolators 、 Wide range attenuator ) Test of . The effect of this error is obvious .
Upper figure 2-1 in , There are common problems in the forward reflection characteristic test 6 Item error , There is symmetry in reverse testing 6 Item test error , Therefore, there are two port device tests 12 Item error .
3、 ... and 、 Calibration principle and method
The calibration principle is to measure the calibration device with known parameters , Store these measurement results in the memory of the analyzer , Use these data to calculate the error model . then , Use the error model to remove the influence of system error from the subsequent measurement .
The calibration process is the process of determining the instrument system error by testing the calibration pieces . According to the different calibration pieces , Calibration methods can be divided into mechanical calibration and electronic calibration . According to the difference of eliminating the error term , Mechanical calibration can be divided into frequency response calibration and vector calibration . Vector calibration can be divided into single port 、 Dual port 、 Multi port and TRL calibration .
Number of calibration pieces for each calibration method 、 The number of tests and the number of error elimination items are different . The accuracy of calibration from high to low is ： TRL calibration 、 Electronic calibration 、 Vector calibration 、 Frequency response calibration .
3.1 Frequency response calibration
Frequency response calibration (Response) Only test 1 Calibration pieces , Use only 1 Calibration test operation .
It is a total reflection calibrator during reflection test , Short circuit calibrators can be used (Short) Or open circuit calibrator (Open). Generally, use terminal short circuit (Short) Closer to the ideal total reflection state .
During transmission test , Use straight through calibrators (Through).
Frequency response calibration is relatively simple , Low precision , Only frequency response errors are eliminated . The frequency response calibration process is equivalent to the test normalization process . That is, first store the test results in the memory to get the reference line , Then compare with the test results of the tested piece . In this way, the influence of systematic errors in the reference line can be eliminated .
3.2 Vector calibration
Vector calibration requires the network analyzer to have the ability to test amplitude and phase , Simultaneous equations are needed in the process of calculating the error term . The vector calibration process is more complex , It is required to test multiple standard parts , Thus, more error terms can be eliminated , Ensure that the instrument has higher test accuracy .
3.2.1 Single port vector calibration
Single port calibration (1-Port Cal) Need to use 3 Calibration pieces （ Open、 Short、 Load）, Conduct 3 Calibration test operation .
When the calibration port is the port of the instrument 1 when , be called S11 Single port calibration ;
When the calibration port is the port of the instrument 2 when , be called S22 Single port calibration .
Single port calibration can eliminate the 3 Item systematic error ( Directional error 、 Source mismatch error 、 Reflection tracking error ).
3.2.2 Dual port vector calibration
Dual port calibration (2-Port Cal) Need to use 4 Calibration pieces （ Open、 Short、 Load、 Through）, Conduct 7 Calibration test
Trial operation . The dual port isolation calibration is only used to test high isolation （ Isolators 、 switch ）、 Large dynamic range （ filter ） Only when the device
be used .
When the network analyzer is used for the transmission performance test of the tested device , It is necessary to calibrate the test port and transmission connection line of the network analyzer . Dual port calibration eliminates all of the two test ports 12 Item systematic error .
3.2.3 Multi port vector calibration
Multi port calibration (3-Port Cal or 4-Port Cal) It is a pairwise combination of dual port calibration , Therefore, it is also necessary to use 4 Calibration pieces , But the calibration test operation will increase .
3.2.4 TRL calibration
TRL Calibration also belongs to vector calibration , Only for dual port and multi port calibration , However, it is different from the calibration devices and test methods used in the traditional dual port vector calibration described above .
It is not easy to accurately determine the parameters of calibration parts in traditional mechanical calibration , Because there will be parasitic inductance in the short circuit , There will be parasitic capacitance in the open circuit . and TRL The calibration uses transmission line devices , Its parameters are easier to establish , And the calibration accuracy is not completely determined by the calibration piece .
TRL Use three kinds of calibration pieces ： Straight through calibration piece (Through)、 Reflection calibration piece (Reflect)、 Transmission line calibration piece (Line).

chart 3-1 TRL calibration
3.3 Electronic calibration
Agilent ENA The series network analyzer can be calibrated with traditional mechanical calibrators , Electronic calibrators can also be used （ E-Cal). Agilent ENA The communication control between the network analyzer and the electronic calibrator adopts USB Interface . Compared with mechanical calibration pieces , The electronic calibration piece has the following characteristics ：
1、 The calibration process is simple , The electronic calibrator only needs to be connected to the vector network once , The test items required for dual port calibration can be completed . There is no need to connect the calibration pieces many times .
2、 Fast calibration , Using electronic calibration to complete dual port calibration takes only a few seconds , The efficiency of the whole testing process is greatly improved .
3、 There are few uncertainties in the calibration process , Because there is no need to connect multiple times , Therefore, the probability of electronic calibration affected by misoperation will be reduced .
4、 Support mixed port calibration , Agilent Electronic calibrators in the form of mixed ports are available , It can ensure the accuracy of testing many non inserted devices .

chart 3-2 Electronic calibration
Four 、 Calibration method
The operation of calibration mainly includes the following steps ：
1、 First , Select the port to be calibrated and tested according to the tested devices and test items （ Single port 、 Dual port ）;
2、 secondly , Determine the model of calibration piece （ Model of electronic calibration or specific model of mechanical calibration piece ） ;
3、 Again , Determine the way of calibration （ Electronics 、 Frequency response 、 Vectors, etc ） ;
4、 then , Enter the specific calibration test process . Such as , If the machine is calibrated , After connecting the Colonel approval , Then click the option corresponding to the network partition ;
5、 Finally, the storage and call of test status and calibration parameters .