An arbitrary multi port network each port terminal are matched, the incident wave an input by a n port to port and the rest will scatter all emitted out. If the M ports of exit wave is BM, between the N and m scattering parameters of Smn=bm/an. A two port network consists of four scattering parameters S11, S21, S12 and S22. When the two terminal are matched, S11 and S22 are respectively 1 and 2 Port reflection system

Network analyzer

Number, S21 is composed of transmission coefficient of 1 port to the 2 port, S12 is the transmission coefficient in the opposite direction. When a port m terminal mismatch, traveling back from the terminal reflection back into the m port. This can be equivalently as m port, but still, there is a traveling wave is incident to the M port of am. Thus, in any case can list all export equivalent incident, a simultaneous equations the relationship between shoot traveling wave and scattering parameters. This can work out all the characteristic parameters of network, such as a terminal mismatch when the input reflection coefficient, input impedance, voltage in Bobbi and a variety of reverse transmission coefficient. The basic principle of this is the network analyzer. A special case of a single port network can be regarded as a two port network, in which in addition to S11, the constant S21=S12=S22. For the multi port network, in addition to one input and one output port, can be matched load in all the rest ports are connected, which is equivalent to a two port network. Take turns to choose the port as the input, the output end of the equivalent two port network, a series of measurements and a list of the corresponding equations can be obtained, all of the N2 N port network scattering parameters, so that we can find all the characteristic parameters of N port network. Left for the measurement of S11 four port network analyzer test unit schematic, the arrow indicates the path of the traveling wave. The signal source of u output signal through a switch S1 and a directional coupler D2 input to the measured network port 1, this is the incident wave a1. Port reflection wave of 1 (i.e. 1 export exit wave B1) measuring channel through a directional coupler D2 and switch to the receiver. The reference channel output signal source u simultaneously with a directional coupler to the D1 receiver, the signal is proportional to the a1. Then the dual channel receiver detects amplitude and phase of b1/a1, namely, measure S11, including its amplitude and phase (or the real and imaginary parts). When measuring, network port 2 connected to a matched load R1, to meet the provisions of the conditions of the scattering parameters. Another directional coupler in D3 system is terminating matching load R2, so as to avoid the adverse effects. The remaining three S parameters measurement principle and similar. Right graph measurements for different Smn parameters of each switch should be placed.

In the actual measurements before, first with the standard three impedance known (such as a short circuit, an open circuit and a matched load) for a series of measuring instrument, called a calibration measurement. The experimental results and the ideal (no instrument error when) alignment due results, can be obtained by calculating the error factor of error in the model and stored in a computer, in order to measure the part to be measured results for error correction. At each frequency point according to the calibration and correction. The measuring steps and calculation are very complex, non artificial skill.

The network analyzer is referred to as the four port network analyzer, because the instrument has four ports, are respectively connected to the signal source, the part to be measured, the measuring channel and the reference channel measuring. It is the shortcomings of the receiver structure is complex, error model does not include the error generated by the receiver.