Calibration of Vector Network Analyzer (VNA)

1. Error Analysis

The errors in the network analyzer testing process are mainly divided into three categories: systematic error, random error, and drift error.

1. Systematic error is caused by the imperfection of the instrument's internal test device. It is predictable and repeatable. Since it does not change over time, it can be described quantitatively. Systematic error can be eliminated through calibration during the test process.

2. Random error is unpredictable because it exists in a random form and changes over time, so it cannot be eliminated by calibration. The main sources of random error are: internal noise of the instrument (such as: excitation source phase noise, sampling noise, intermediate frequency receiver local oscillator noise, etc.), instrument switch action repeatability and connector repeatability are also random errors.

3. Drift error is the drift of the instrument's performance after calibration. Drift error is mainly caused by temperature changes and can be eliminated through further calibration. The length of time the instrument can maintain stable accuracy after calibration depends on the drift rate of the instrument in the test environment.

2. Specific analysis of system errors

Since network analyzer calibration can eliminate the systematic errors that occur during the test, the following specifically analyzes the systematic errors that exist in the network analyzer during the reflection characteristic test.

Figure 2-1 Errors of a network analyzer when testing the reflection characteristics of a device

1. Frequency response error

When the network analyzer works in the frequency sweep state, the characteristics of the power divider, directional coupler, external adapter and test cable, etc., will change with the frequency within the working frequency band. 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 limited directivity of the directional coupler is a directivity error. The leakage signal caused by the directivity error will be superimposed on the real reflected signal, causing test errors. When the port matching performance of the device under test is good, the directivity error has a greater impact 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 the signal to be incident twice, and finally multiple incidents of the signal in the transmission path will form multiple reflections accordingly. This error is called source mismatch error. The worse the matching performance of the DUT, the more obvious the impact of this error on the test.

Similarly, the transmission signal output by the device under test will also be reflected due to impedance mismatch at the receiving end. This signal will be superimposed on the real reflected signal through the reverse transmission of the device under test, thus forming a load mismatch error. If the reverse transmission isolation performance of the device under test is poor, the impact of the load mismatch error is greater.

4. Isolation error

In the network analyzer, the R1, A, and B receivers should reflect the input, reflected, and transmitted signals of the test respectively. However, there will be signal crosstalk between these receivers, and this error will have a significant impact on the test of high-isolation devices under test (switches, isolators, and wide-range attenuators).

In Figure 2-1 above, there are 6 errors in the forward reflection characteristic test, and 6 symmetrical test errors in the reverse test, so there are 12 errors in the dual-port device test.

3. Calibration Principle and Method

The principle of calibration is to measure a calibration device with known parameters, store these measurement results in the analyzer's memory, and use these data to calculate an error model. The error model is then used to remove the effects of systematic errors from subsequent measurements.

The calibration process is the process of clarifying the instrument system error by testing the calibration parts. According to the different calibration parts, the calibration method can be divided into mechanical calibration and electronic calibration. According to the different error terms to be eliminated, mechanical calibration can be divided into frequency response calibration and vector calibration. Among them, vector calibration can be divided into single-port, dual-port, multi-port and TRL calibration.

The number of calibration parts, the number of tests and the number of error elimination items for each calibration method are different. The calibration accuracy from high to low are: TRL calibration, electronic calibration, vector calibration, and frequency response calibration.

3.1 Frequency response calibration

Frequency response calibration (Response) only tests one calibration component and only requires one calibration test operation.

When the reflection test is performed, the full reflection calibration component can be a short circuit calibration component (Short) or an open circuit calibration component (Open). Generally, the terminal short circuit (Short) is used to get closer to the ideal full reflection state.

When performing transmission testing, use a through calibration kit (Through).

Frequency response calibration is relatively simple, with low accuracy, and only eliminates frequency response errors. The frequency response calibration process is equivalent to the test normalization process. That is, the test results are first stored in the memory to obtain the reference line, and then the test results of the device under test are compared with it. This can eliminate the influence of systematic errors in the reference line.

3.2 Vector calibration

Vector calibration requires the network analyzer to have the ability to test amplitude and phase, and a set of simultaneous equations is required in the process of calculating the error terms. The vector calibration process is more complicated and requires testing multiple standards, which can eliminate more error terms and ensure that the instrument has higher test accuracy.

3.2.1 One-port vector calibration

One-port calibration (1-Port Cal) requires three calibration pieces (Open, Short, Load) and three calibration test operations.

When the calibration port is port 1 of the instrument, it is called S11 single-port calibration;

When the calibration port is port 2 of the instrument, it is called S22 single-port calibration.

Single-port calibration can eliminate three systematic errors (directivity error, source mismatch error, and reflection tracking error) of the calibrated port.

3.2.2 Two-port vector calibration

Two-port calibration (2-Port Cal) requires four calibration components (Open, Short, Load, Through) and seven calibration tests.

The dual-port isolation calibration is only used when testing high isolation (isolators, switches) and large dynamic range (filters) devices.

Used.

When a network analyzer is used to test the transmission performance of a device under test, it is necessary to perform a two-port calibration on the test port and transmission connection line of the network analyzer. The two-port calibration can eliminate all 12 systematic errors of the two test ports.

3.2.3 Multi-port vector calibration

Multi-port calibration (3-Port Cal or 4-Port Cal) is a combination of two-port calibrations, so four calibration components are also required, but the calibration test operations are increased.

3.2.4 TRL Calibration

TRL calibration is also a type of vector calibration and is only used for two-port and multi-port calibration. However, it is different from the calibration devices and test methods used in the traditional two-port vector calibration described above.

The parameters of the calibration parts of traditional mechanical calibration are not easy to determine accurately, because short-circuit parts have parasitic inductance and open-circuit parts have parasitic capacitance. However, TRL calibration uses transmission line devices, whose parameters are easier to establish, and the calibration accuracy is not completely determined by the calibration parts.

TRL uses three types of calibration components: through calibration components (Through), reflection calibration components (Reflect), and transmission line calibration components (Line).

Figure 3-1 TRL Calibration

3.3 Electronic Calibration

In addition to the traditional mechanical calibration kits, the Agilent ENA series network analyzers can also use electronic calibration kits (E-Cal). The communication control between the Agilent ENA network analyzer and the electronic calibration kit uses a USB interface. Compared with mechanical calibration kits, electronic calibration kits have the following characteristics:

1. The calibration process is simple. The electronic calibration component only needs to be connected to the vector network once to complete the test items required for the dual-port calibration. There is no need to connect the calibration component multiple times.

2. Fast calibration speed. It only takes a few seconds to complete the dual-port calibration using electronic calibration, which greatly improves the efficiency of the entire test process.

3. There are fewer uncertainties in the calibration process. Since multiple connection processes are not required, the probability of electronic calibration being affected by misoperation is reduced.

4. Support mixed-port calibration. Agilent can provide electronic calibration components in the form of mixed ports to ensure the accuracy of testing many non-insertion devices.

Figure 3-2 Electronic calibration