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How to judge whether the VNA calibration results are normal or not, many novices are not very clear about this. In order to help you solve this problem, the following editor will explain it in detail. People with this need can come and learn. I hope you can get something.

The network in the network analyzer can be understood as a microwave network, like a box, no matter how big or small, what is installed in the middle, we do not necessarily know, as long as it is externally connected with a coaxial connector, we call it a single-port network, and if it is equipped with two coaxial connectors, it is called a two-port network. The network mentioned here is not the same thing as the computer network, it mainly refers to a variety of simple radio frequency devices (network), not the Internet.

Single-port network is conventionally called load ZL. Because there is only one port, it is always connected at the end, which is also called terminal load. The most common ones are load, circuit breaker, etc., and the more complex ones are sliding load. The electrical parameters of single-port network are usually expressed by impedance or admittance, but it is more convenient to use reflection coefficient Γ (echo loss, standing wave ratio) in the field of radio frequency.

The most common and simplest two-port network is a radio frequency cable with connectors at both ends. The matching characteristic refers to the reflection coefficient measured at the other end of the two-port network after one termination of the standard load, which can be used to characterize the matching characteristic. Transmission coefficient and insertion loss for a two-port network, in addition to matching characteristics (reflection coefficient), there is also a transmission characteristic, that is, the ratio of voltage through the network to that not through the network is called transmission coefficient T.

Insertion loss (IL) = 20Log │ T │ dB, which is generally negative, but sometimes does not remember the minus sign, expressed by Φ phase shift.

The two-port network has four S parameters, namely S11, S21, S12, and S22.

S11 is the input reflection coefficient after the network output is connected to the matching load.

S21 is the ratio of the output voltage to the input voltage when the network output is matched, which is the transmission coefficient for the passive network.

T or insertion loss.

S12, that is, the influence of the network output on the input, is often called isolation for irreversible devices.

S22 is the reflection coefficient introduced by the network itself from the output side to the network.

The S-parameter measurement transmission is characterized by the following parameters:

Gain (dB or V)

Insertion loss (dB or V)

Insert phase (degrees)

Real part and imaginary part (R+jI)

Electric length (m)

Electrical delay (s)

Offset of linear phase (degrees)

Group delay (s)

The S-parameter measurement reflection is characterized by the following parameters:

Echo loss (dB)

SWR

Impedance (R + j X)

Reflection coefficient

Distance (measured in time domain)

Several terms related to instruments

Network analyzer: an instrument that can measure various parameters of a single-ended or two-port network is called a network analyzer. Can only measure the amplitude characteristics of various parameters of the network is called scalar network analyzer, referred to as the standard network. Those who can measure both amplitude and phase are called vector network analyzers, referred to as vector networks. The vector net can display the test data with Smith chart.

Connection cable: a radio frequency cable with connectors at both ends is called a connection cable (also known as a jumper), and a connection cable with very small reflection is called a test cable or a radio frequency cable.

Reflection bridge: in order to measure the reflection coefficient, a device that is directional (or directional) and maintains phase information, such as a directional coupler or reflection bridge, is usually used. Generally, the reflection bridge can only measure the single-ended feeder system such as coaxial line.

Dynamic range: the instrument is set to measure the insertion loss, one end of a good short cable is connected to the output port, and the other end is connected to the input port corresponding to the screen display, and the value displayed by the instrument is the dynamic range after straight-through calibration and then removing the cable. it will be indicated in the technical specifications of the normal instrument.

Isolation: the insertion loss that should not be connected is called isolation.

Error and calibration

Errors include: system error, random error (including drift error), random error is unpredictable with time and therefore can not be calibrated, such as noise in the instrument, repeatability of switches, repeatability of test cables and joints, natural environment, temperature drift and so on. The systematic error includes 6 forward errors and 6 reverse errors.

All 12 calibrations (Full 12-Term) can be measured as S11, S21, S12, S22.

S21&S11 or S12&S22 can be obtained in one direction (1Path, 2 Port).

S11 or S22 can be obtained by ReflectionOnly.

S21 or S12 can be obtained by FrequencyResponse.

Comparison of different calibration methods:

So after the calibration, how to call the calibration normal? It can be judged in the following ways:

1: after the calibration is completed, the Log values of S11 and S22 displayed by the vector network should be close to 0 in the calibration frequency band, otherwise the calibration is not good.

2: followed by the standard 50-euro load to the two ports, the values of S11 and S22 displayed by the vector network should be less than-40dB in the calibration frequency band, otherwise the calibration is not good.

3: the third step can be indirect another double negative transfer connector at both ports (not the straight-through used for calibration). The values of S21 and S12 shown by the vector network should be close to 0 in the calibration frequency band and the S11 S22 value should be less than-40dB (or-35dB) in the calibration frequency band, otherwise it means the calibration is not good.

4: add a transmission line in front of the straight-through or load, and test the straight-through or load again to see if S11 is better than 35dB. S11 at this time is called residual matching. Or connect other loads (not for on-time use) to test again to see if S11 is better than 35dB. If it does not meet the target, please replace the load of the calibrator and calibrate again.

Mixed use of calibration parts from different manufacturers

People often ask me if I want to use the calibration parts of Keysight on the instruments of Agilent or on the instruments of Agilent or 41, which involves the interoperability of calibration parts. In fact, the answer is yes, the method only needs to find the parameters of the calibration parts of the manufacturer (from the calibrator parameter menu of the instrument) and input them to the instruments of other companies.

The parameters of the OPEN calibrator are:

C0PercentC1C2C3offset length or time (time = length / speed of light in the air)

The parameters of the SHORT calibrator are:

L0 and L1, L2, L3 + offset length or time

The parameters of the LOAD calibrator are: 50 or 75ohm

For example, the following picture takes Keysight 85032F as an example, whose calibration parameters can be found on the official website. The author's file is an edited .ccf format document that can be loaded directly into the instrument.

The corresponding calibration parameters can be seen in the image above. You can load the calibration parameter document directly, or you can enter it manually.

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