Development process:

network analyzerIt is developed on the basis of four port microwave reflectometer (see standing wave and reflection measurement). In the mid-1960s, the automation was realized. The computer was used to correct the errors caused by the imperfect directivity, mismatch and channeling of the directional coupler at each frequency point according to a certain error model, so that the measurement accuracy was greatly improved, which could reach the measurement accuracy of the precision measuring line technology in the metering room, and the measurement speed was increased by tens of times.

Principle:

When all port terminals of an arbitrary multiport network are matched, the incident traveling wave an input from the nth port will scatter to all other ports and emit. If the outgoing traveling wave of port m is bm, the scattering parameter Smn=bm/an between port n and port m. A two port network has four scattering parameters S11, S21, S12 and S22. When the two terminals are matched, S11 and S22 are the reflection coefficients of ports 1 and 2, S21 is the transmission coefficient from port 1 to port 2, and S12 is the transmission coefficient in the opposite direction. When the terminal m of a port is mismatched, the traveling wave reflected from the terminal enters the port m again. This can be equivalent to the fact that the m-port is still matched, but there is a traveling wave am incident to the m-port. In this way, in any case, the simultaneous equations of the relationship between the equivalent incident and outgoing traveling waves and the scattering parameters can be listed. All characteristic parameters of the network, such as input reflection coefficient, voltage standing wave ratio, input impedance and various forward and reverse transmission coefficients when terminals are mismatched, can be solved accordingly. This is the basic working principle of network analyzer. A single port network can be regarded as a special case of a dual port network. Except for S11, S21=S12=S22. For a multi port network, except for one input port and one output port, all other ports can be connected with matching loads, which is equivalent to a dual port network. Select each pair of ports in turn as the input and output terminals of the equivalent two port network, make a series of measurements and list the corresponding equations, then all n2 scattering parameters of the n-port network can be solved, and all characteristic parameters of the n-port network can be obtained. The left side of the figure shows the principle of the test unit when the four port network analyzer measures S11, and the arrows show the paths of each traveling wave. The output signal of signal source u is input to port 1 of the network under test through switch S1 and directional coupler D2, which is called 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 directional coupler D2 and switch. The output of signal source u is simultaneously transmitted to the reference channel of receiver through directional coupler D1, and this signal is proportional to a1. Then the dual channel amplitude phase receiver will measure b1/a1, that is, S11, including its amplitude and phase (or real part and imaginary part). 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 also terminates the matching load R2 to avoid adverse effects. The measurement principle of the other three S parameters is the same as that of this type. The right side of the figure shows the position of each switch when measuring different Smn parameters.

Before actual measurement, first use three standards with known impedance (such as a short circuit, an open circuit and a matching load) for the instrument to conduct a series of measurements, which is called calibration measurement. By comparing the measured results with the ideal (no instrument error) results, the error factors in the error model can be calculated and stored in the computer, so as to correct the error of the measured results. Calibrate and correct at each frequency point according to this. The measurement steps and calculation are very complex, and it is not competent for manual work.

The above network analyzer is called a four port network analyzer, because the instrument has four ports, which are respectively connected to the signal source, the object under test, the measurement channel and the measurement reference channel. Its disadvantage is that the structure of the receiver is complex, and the error generated by the receiver is not included in the error model.

Parameters:

Parameters (scattering parameters) are used to evaluate the performance of DUT reflected signals and transmitted signals. The parameter is defined by the ratio of two complex numbers, which contains information about the amplitude and phase of the signal. The parameters are usually expressed as:

Output Input

Output: DUT port number of output signal

Input: DUT port number of input signal

For example, parameter S21 is the ratio of the output signal of port 2 on the DUT to the input signal of port 1 on the DUT. Both the output signal and the input signal are expressed in complex numbers.

When the balance imbalance conversion function is started, the mixed mode S parameter can be selected.

New developments:

In 1973, a six port network analyzer was developed. It uses a six port network consisting of a directional coupler and a hybrid connector (magic T) as the measuring unit. Except for two ports connected to the signal source and the object under test, the other four ports are connected to the amplitude detector or power meter. The mode and phase of the measured network scattering parameters can be obtained by properly combining the detected four amplitudes. It does not need to use a complex dual channel receiver 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, which can improve the reliability of the measurement results by checking redundant data with each other. But its calculation is much more complicated than that of four port network analyzer. The dual six 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 network under test, and the other is connected to port 2, which can avoid switching or manually reversing the input and output ends of the network under test during the measurement process, further improving the measurement accuracy.

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