Network Analyzer

Introduction

The popularity and application of the Internet has changed our work, study and life. The network failure also affects our work, study and life, and may even cause huge losses. With the continuous expansion of the network scale and the continuous updating of network technologies, network maintenance and fault diagnosis are even more difficult and challenging. How to improve the efficiency of network management, how to fully understand the performance of the network to prevent failures, how to predict the network failure in advance, and how to quickly resolve the network failure, these are the problems faced by network management personnel. Then the network comprehensive agreement analyzer can help the network management personnel to solve these thorny problems that they face daily. The network integrated protocol analyzer integrates the network monitoring and fault diagnosis functions that customers have come to expect in a hand-held instrument. It can complete the cable test, network traffic testing, network equipment search function. At the same time, it also has the function of a protocol analyzer, which can capture, decode, and filter packets. You can also use this device to set up network devices such as switches or routers directly through Telnet, Terminal Emulation. The tester itself is also a device that supports RMON2. You can use it as a network data acquisition instrument to monitor the status of your network using any general-purpose RMON2-enabled network management system. You can even remotely control the tester directly, just as you would use it in front of the tester. In the past, you may have to carry a box of instruments to complete all of the above tests. The vector network analyzer, which itself comes with a signal generator, can scan the frequency of a frequency band. If it is a single-port measurement, the excitation signal is applied to the port. By measuring the amplitude and phase of the reflected signal, the impedance or reflection can be determined. For dual-port measurements, the transmission parameters can also be measured. Due to the significant influence of distribution parameters, etc., the network analyzer must be calibrated before use.

development process

The network analyzer was developed on the basis of a four-port microwave reflectometer (see standing wave and reflection measurements). Automated in the mid-1960s, using a computer to correct errors caused by directional coupler directional imperfections, mismatches, and leaks at each frequency point according to a certain error model, thereby greatly improving the measurement accuracy. The measurement accuracy of the most precise measurement line technology in the metrology room can be achieved, while the measurement speed is increased by tens of times.

principle

When the ports of an arbitrary multi-port network are all matched, the incident traveling wave an inputted by the n-th port will be scattered to all other ports and emerged. If the outgoing traveling wave of the m-th port is bm, then the scattering parameter between the n-port and the m-port is Smn=bm/an. A dual-port network has a total of four scattering parameters S11, S21, S12, and S22. When the two terminals are When both are matched, S11 and S22 are the reflection coefficients of ports 1 and 2, respectively, S21 is the transmission coefficient from 1 port to 2 ports, and S12 is the transmission coefficient in the opposite direction. When a port m terminal is mismatched, the traveling wave reflected by the terminal re-enters the m port. This is equivalent to seeing that the m port is still matching, but there is a traveling wave am incident on the m port. In this way, it is possible to list the simultaneous equations of the relationship between the equivalent incident and emergent traveling wave and scattering parameters of each port in any case. According to this, all the characteristic parameters of the network can be solved, such as input terminal reflection coefficient, VSWR, input impedance and various forward reverse transmission coefficients when the terminal is mismatched. This is the most basic working principle of the network analyzer. A single-port network can be regarded as a special case of a dual-port network, in which in addition to S11, constant S21=S12=S22.

For a multi-port network, in addition to one input and one output port, all other ports can be connected to a matching load, which is equivalent to a two-port network. Each pair of ports is selected in turn as the input and output ports of the equivalent dual-port network. After a series of measurements and corresponding equations are listed, all n2 scattering parameters of the n-port network can be solved to find out all the n-port network. Characteristic parameters. The left side of the figure shows the principle of the test unit when the four-port network analyzer measures S11, and the arrow shows the path of each traveling wave. The output signal of the signal source u is input to the port 1 of the network under test via the switch S1 and the directional coupler D2. This is the incident wave a1. The reflected wave of the port 1 (that is, the outgoing wave b1 of the port 1) is transmitted through the directional coupler D2 and the switch. The measurement channel to the receiver. The output of the signal source u is passed through the directional coupler D1 to the reference channel of the receiver. This signal is proportional to a1. The two-channel amplitude-phase receiver then measures b1/a1, ie measures S11, including its amplitude and phase (or real and imaginary parts). During the measurement, the 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 also terminates the matching load R2 in order to avoid adverse effects. The remaining three S-parameters have the same measuring principle. The right side of the figure shows where each switch should be placed when measuring different Smn parameters.

Before the actual measurement, a series of measurements, called calibration measurements, are made by the instrument with three standard instruments of known impedance (for example, a short circuit, an open circuit, and a matching load). Comparing the results obtained from the actual measurement result with the ideal (without instrument error), the error factors in the error model can be calculated and stored in the computer so that the measurement results of the tested part can be corrected for errors. Calibration and correction are performed at each frequency point. The measurement steps and calculations are very complex and non-manually competent.

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.

new development

In 1973 it developed a six-port network analyzer. It uses a six-port network consisting of a directional coupler and a hybrid connector (Magic T) as the measurement unit. In addition to the two ports connecting the signal source and the device under test respectively, the other four ports are connected to the amplitude detector or dynamometer. Through the appropriate combination of the four detected amplitudes, the mode and phase of the measured scattering parameters of the network can be determined. It does not have to use complex two-channel receivers to obtain phase information, which greatly simplifies the hardware of the measurement system. In addition, it has more than the necessary number of redundant measurement ports and can use the redundancy check to check the credibility of the measurement results. But its computational work is much more complicated than the four-port network analyzer. A two-port network analyzer is used to measure the dual-port network, that is, a six-port network analyzer is connected to port 1 of the tested network, and the other is connected to port 2 to avoid switchover or manual reversal of the measured network during the measurement process. The input and output terminals further improve the measurement accuracy.