Network Analyzer
What is a network analyzer?
A network analyzer is a new type of instrument for measuring network parameters.
The network analyzer can directly measure the RF complex scattering parameters of active or passive, reversible or irreversible dual-port and single-port networks, and gives the amplitude and phase of each scattering parameter Frequency characteristics.
Network analyzers are based on four-port microwave reflectometers. In the mid-1960s automation, the use by computers by a certain error model at each frequency point to correct the directional coupler imperfection, mismatch, and blowby caused by the error so that the measurement accuracy is greatly improved.
The network analyzer measurement accuracy of the most accurate measurement line technology in the metering chamber can be achieved, while the measurement speed is increased by several tens of times.
Network analyzer Introduction
Vector network analyzer, which comes with a signal generator that can scan a frequency band frequency. If the single-port measurement, the excitation signal is added to the port, by measuring the amplitude and phase of the reflected signal back, you can determine the impedance or reflex situation.
For the two-port measurement, you can also measure the transmission parameters. Because of the distribution of parameters such as obvious, the network analyzer must be calibrated before use.
Parameter
The parameters (scattering parameters) are used to evaluate the performance of the DUT reflected signal and the transmitted signal. The parameter is defined by the ratio of two complex numbers and contains information about the amplitude and phase of the signal. The parameters are usually expressed as:
Output input
Output: DUT port number of the output signal
Input: DUT port number of the input signal
For example, parameter S21 is the ratio of the output signal from port 2 on the DUT to the input signal to port 1 on the DUT. Both the output signal and the input signal are represented by a complex number.
When starting the Balancing-Unbalance conversion function, you can choose to mix the S parameters.
Network analyzer principle
When an interface of each port of an arbitrary multi-port network matches, the incident traveling wave n input by the first port is scattered to all other ports and transmitted.
If the first port of the outgoing traveling wave m, then the mouth of the scattering parameters mn = m / n. A dual-ported network has a total of four scattering parameters 11, 21, 12, and 22.
When both terminals match, 11 and 22 are the reflection coefficients for ports 1 and 2 respectively, 21 is the transmission coefficient from 1 to 2, and 12 is the inverse transmission coefficient.
When a port terminal mismatches, the terminal is reflected by the traveling wave again into the mouth. This can be considered equivalently if the port is still matched, but a traveling wave m is incident on the port.
Thus, in any case, simultaneous equations of the equivalent incident and outgoing traveling waves and scattering parameters for each port can be listed.
Accordingly, all the network characteristic parameters can be solved, such as input reflection coefficient, voltage standing wave ratio, input impedance, and various forward reverse transmission coefficients when the terminal is mismatched. This is the most basic working principle of a network analyzer.
A single-port network can be considered a special case of a dual-port network in which, except 11, constant 21 = 12 = 22.
For a multi-port network, except for one input and one output port, a matching load can be connected to all the remaining ports, which is equivalent to a two-port network.
Taking each pair of ports as the input and output of the equivalent dual-port network, in turn, taking a series of measurements and listing the corresponding equations, all the two scattering parameters of the port network can be solved to obtain all the characteristics of the n-port network parameter.
Signal source u output signal through the switch S1 and directional coupler D2 input to the measured network port 1, which is the incident wave a1. The reflected wave of port 1 (that is, the outgoing wave b1 of one port) is transmitted to the measuring channel of the receiver via the directional coupler D2 and the switch.
The output of the signal source u is simultaneously transmitted to the reference channel of the receiver via the directional coupler D1, which is proportional to a1. Thus, the dual-channel amplitude-phase receiver measures b1 / a1, that is, S11 is measured, including its amplitude and phase (or real and imaginary parts).
When measured, the network port 2 connected to the matching load R1, to meet the conditions specified by the scattering parameters. The other directional coupler D3 in the system also terminates the matching load R2 to avoid adverse effects. The remaining three S-parameter measurement principles and the same.
Before the actual measurement, a series of measurements are made by the instrument with three standard instruments of known impedance (for example, a short circuit, an open circuit, and a matched load), called calibration measurements.
The measured results and the ideal (no instrument error) should be compared to the results obtained by calculating the error model of the error factor and stored in the computer, in order to test the measured error correction.
Click here for calibration and correction at each frequency point. Measurement procedures and calculations are very complex, and non-manual competence.
The above-mentioned network analyzer is called a four-port network analyzer because the instrument has four ports, which are respectively connected to the signal source, the DUT, the measurement channel, and the measured reference channel. Its disadvantage is the complex receiver structure, the error model does not include the error generated by the receiver.
New development
1973 also developed a six-port network analyzer. It uses a directional coupler and hybrid connector (magic) composed of the six-port network as a measurement unit, in addition to the two ports connected to the signal source and the DUT, the remaining four ports are connected to the amplitude detector or power meter.
By detecting the appropriate combination of the four amplitudes, the model and phase of the measured network scattering parameters can be obtained. It eliminates the need to use sophisticated dual-channel receivers for phase information, making the measurement system’s hardware significantly simplified.
In addition, it has more than the required number of redundant measurement ports, which can be used to check the reliability of measurements with redundant data. But it’s computationally more complex than a four-port network analyzer.
Dual six-port network analyzer to measure dual-port network, which uses a six-port network meter connected to the measured network port 1, the other connected to port 2 in the measurement process to avoid switching or artificial inversion of the measured network inputs and outputs further improve the measurement accuracy.
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