Network Analyzer How to use the network analyzer to debug the matrix

The network analyzer was developed on the basis of the four-port microwave reflectometer (see standing wave and reflection measurement). In the mid-1960s, automation was achieved, and the computer was used to correct the errors caused by the imperfect directivity, mismatch and leakage of the directional coupler at each frequency point according to a certain error model, thereby greatly improving the measurement accuracy, which can reach the measurement accuracy of the most precise measurement line technology in the measurement room, and the measurement speed is increased by dozens of times.

　　Principle

　　When the terminals of each port of an arbitrary multi-port network are matched, the incident traveling wave an input from the nth port will be scattered to all other ports and emitted. If the emitted traveling wave of the mth port is bm, the scattering parameter Smn between the nth port and the mth port is Smn=bm/an. A two-port network has four scattering parameters S11, S21, S12 and S22. When both terminals are matched, S11 and S22 are the reflection coefficients of ports 1 and 2 respectively, S21 is the transmission coefficient from port 1 to port 2, and S12 is the transmission coefficient in the opposite direction. When the terminal of a port m is mismatched, the traveling wave reflected by the terminal re-enters the m port. This can be equivalently regarded as the m port is still matched, but there is a traveling wave am incident on the m port. In this way, in any case, the simultaneous equations of the relationship between the equivalent incident and outgoing traveling waves and scattering parameters of each port can be listed. Based on this, all the characteristic parameters of the network can be solved, such as the input reflection coefficient, voltage standing wave ratio, input impedance and various forward and reverse transmission coefficients when the terminal is mismatched. This is the most basic working principle of the network analyzer. The single-port network can be regarded as a special case of the two-port network, in which S21=S12=S22 is always true except for S11. For a multi-port network, except for one input and one output port, matching loads can be connected to all other ports, which is equivalent to a two-port network. By selecting each pair of ports as the input and output of the equivalent two-port network in turn, performing a series of measurements and listing the corresponding equations, all n2 scattering parameters of the n-port network can be solved, thereby finding all the characteristic parameters of the n-port network. The left figure shows the principle of the test unit when the four-port network analyzer measures S11. The arrows indicate the paths of each traveling wave. The output signal of the signal source u is input to port 1 of the network under test through the switch S1 and the directional coupler D2. This is the incident wave a1. The reflected wave of port 1 (i.e. the outgoing wave b1 of port 1) is transmitted to the measurement channel of the receiver through the directional coupler D2 and the switch. The output of the signal source u is also transmitted to the reference channel of the receiver through the directional coupler D1. This signal is proportional to a1. Therefore, the dual-channel amplitude-phase receiver measures b1/a1, that is, S11, including its amplitude and phase (or real and imaginary parts). During measurement, port 2 of the network is connected to the matching load R1 to meet the conditions specified by the scattering parameters. Another directional coupler D3 in the system is also terminated with the matching load R2 to avoid adverse effects. The measurement principles of the remaining three S parameters are similar.

　　Before the actual measurement, three known impedance standards (such as a short circuit, an open circuit and a matched load) are used for the instrument to perform a series of measurements, which is called calibration measurement. By comparing the actual measured results with the ideal results (when there is no instrument error), the error factors in the error model can be calculated and stored in the computer to correct the error of the measured results of the device under test. Calibration and correction are performed at each frequency point. The measurement steps and calculations are very complicated and cannot be done manually. The

　　above network analyzer is called a four-port network analyzer because the instrument has four ports, which are connected to the signal source, the device under test, the measurement channel and the reference channel for measurement. Its disadvantage is that the structure of the receiver is complex, and the error model does not include the error generated by the receiver.

　　Network analyzers are widely used in antenna and feed system testing in FM broadcasting, television, CATV, communication (Xun), radar and other equipment, as well as RF microwave teaching experiments in colleges and universities. After selection, it can test different characteristic impedance systems such as 50Ω, 75Ω, 100, 150Ω, 230Ω, 300Ω, etc. The time domain fault location function can quickly determine the fault location of the coaxial cable in the antenna feed system . The measurement range is 0-120 meters, and the positioning is accurate. The positioning accuracy of 10m is ±3mm, and the positioning accuracy of 30m is ±1cm. Equipped with corresponding test accessories (impedance converter, parallel bridge...), it can meet the characteristic impedance, insertion loss, delay, phase shift and other parameter tests of coaxial cables, twisted pairs, coaxial connectors and other transmission lines, and can detect the leakage and shielding performance of RF cables. Equipped with corresponding probes, it can measure the dielectric constant of liquids, flat solids, powders and other forms of materials.