Network analyzer (electrical)

A network analyzer ( American: Network Analyzer or Vector Network Analyzer, british: Network Analyser, in short: NWA, VNA or NA ) is used in the electronics, communications engineering, and especially in the high frequency technique to the scattering parameters (S- parameters ), ie reflection and transmission of electrical measurement objects (short: MO or DUT Device Under Test after engl. ) to measure as a function of frequency.

• 5.1 ... of the reflected signal
• 5.2 ... of the transmitted signal

Principle

The network sends a signal ( forward wave ) on the (DUT ). The frequency, amplitude and phase are known. The sample reflects a portion of this signal ( wave running off at the entrance). The rest goes into the measurement object is changed there ( damped, amplified, phase-shifted ) and makes its appearance at the output of the DUT as a transmitted signal ( wave running off at the output ).

• The reflection of the object is measured from the ratio of reflected to transmitted signal and

• From the ratio of transmitted to the transmitted signal, the transmission of the measurement object is measured.

Nomenclature

One distinguishes the previously widespread scalar network analyzers (English Scalar Network Analyzer, simply: SNA ), which can only measure the amount of reflection and transmission, of the now almost only encountered complex network analyzers, which except for the magnitude and phase of the S- can measure parameters. Based on the common in the English language designation Vector Network Analyzer ( VNA ), they are often imprecise and referred to Germans as vector network analyzers or as Vector Network. However, since they do not measure vectors but complex variables ( pointers, also see phasor ), the term complex network analyzers is more precise. Meanwhile, sufficient simply the name " network analyzer " without any addition, as scalar analyzers are almost gone and the conceptual distinction is no longer necessary.

Confusion may also arise if the network analyzers are placed in error with the analysis of computer networks such as the World Wide Web in context. However, they are not used for their analysis but for measuring electrical networks, components and circuits, such as attenuators, filters, directional couplers, amplifiers or mixers. In this respect, a name like " S-parameter measuring instrument" for network analyzers would be less misleading.

Construction

The most important interfaces of a network is its test ports (English ports). Ordinary network analyzers ( " two-port network analyzers " ) have two test ports in a coaxial design, for example N-Type connectors. They are well suited for the measurement of specimens with one or two goals. The examination of targets with more than two goals, such as from directional couplers designed thus cumbersome. Meanwhile, however, are also network analyzers with more than two goals ( " multi-port network analyzers " ) available. Here devices with up to eight test ports and 16 parallel receiving channels of the German manufacturers of measuring instruments, Rohde & Schwarz and Ballmann available. For achieving even higher Torzahlen external switch ( switching matrices ) may be used.

Other interfaces ( GPIB, Serial, LAN ( LXI - compliant part ) or USB) used for remote control and data output. Thus, the network can be controlled by a computer and the measurement results are stored. Modern network analyzers already contain a computer, so that the entire measurement process can be handled automatically by the device itself. To save the measured data a floppy drive, a memory card reader or a USB port is often present.

Devices with built-in screen show the measured parameters as amplitude or phase transition or in a complex representation in a Smith chart. The representation of the Smith chart is only at the input and output reflection (S11 and S22) of interest. It serves, for example, to determine the appropriate impedance matching (matching ) for transmission of maximum power. Today's network analyzers make it possible in addition to the S- parameters to measure other parameters such as the group delay of the device under test and displaying.

Calibration and system error correction

The individual components of a network are faulty, which means that they themselves have a frequency and phase response. These so-called system error change the values ​​obtained from a measurement object particularly to higher frequencies so strong, that is to make a precise statement about this any more.

By calibration of the analyzer system error can be compensated and the measurement accuracy can be greatly increased. For this, different calibration standards are connected successively with known electrical properties at the test ports and measures values ​​. By offsetting the values ​​of the different calibration taking into account the known electrical properties of the standards, the error coefficients can be calculated that describe the system error of the analyzer.

During the subsequent examination of measurement objects, the measurement data obtained will be charged during the system error correction with the error coefficients and thus compensates for the error caused by the analyzer. There are a number of calibration with different benefits, which are usually named after the initials of the calibration standards used, for example:

• OSL or MSO: Open- Short -Load or match- Short - Open
• SOLT: Short - Open -Load -Through
• TAN: Through Attenuation Network
• TRL: Through - Reflect -Line

Any change in the measured frequency range ( to higher or lower frequencies ) or the change of the test leads to make a new calibration is required. Depending on the method and number of test ports, several measurements must be performed with the appropriate standards for calibration. Thus, it may take several minutes to complete a Kalibirierungsvorgang. Time additionally depends on the measurement configuration of the network analyzer. The more measuring points within the desired frequency range is, and the longer the waiting time for a measurement point, the longer the measurement of each calibration standard.

For some time the various measuring instruments manufacturers also offer automatic calibration devices that combine the different necessary for calibration standards in a compact housing. The standards can be connected to the various test ports of the analyzer within the device via RF switches that were connected before starting the calibration with the calibration. During the calibration process, wherein the automatic ( controlled by the analyzer ) the standards are on or switched a rewiring is no longer necessary. So also measurement configurations with a larger number can be calibrated easily and quickly from test ports.

Open

In an Open ( German: Open) is the measuring line defines open, that is not connected to anything. An open end of the line causes a total reflection of the transmitted signal. Considering the complex data of a reflection measurement of a target of the network in the Smith chart, the Open defines the point at infinity in the X- axis.

Short

In a short ( German: Short ) is the measuring line defined with the cable shield (ground), ie shorted. A short-circuited end of the cable also causes a total reflection of the transmitted signal, but the phase of the signal is rotated with respect to the open through 180 °. In the Smith chart of the Short defines the zero point on the X- axis.

Match

In the match- calibration ( German: Adapted ) the measuring line is terminated with the characteristic impedance, in communications technology usually 50 Ω, ie a defined resistance between the line core and shielding connected. If the measuring line is terminated with the characteristic impedance, there are no signal reflections. In the Smith chart of the match one defines the point on the X- axis, that is the center of the diagram. This point is related to network analyzers also often referred to by the term system impedance.

Through

When through measurement ( German: Continuous ) the test leads of the network are defined together. Since the through standard has two ports, it is attributed to the Zweitorstandards.

Reflect

Reflect the standard is a general form of short or open, in which the exact characteristics need not be known. Therefore, it is only in the calibration, which have at least one Selbstkalibrierstandard ( not fully known standard ) used. Suitable calibration procedures are, for example, TRL or TRM.

Attenuation

The attenuation standard provides as well as the Reflect a Selbstkalibrierstandard represents is unlike him, but (as the through or the line ) is a two-port, between the two to be calibrated measurement ports of the analyzer is switched. The attenuation standard should have a constant as possible insertion loss and its exact value is not known must be - and must have reciprocal behavior, ie have no directional properties.

Line

The Line standard is similar to the non-interacting through a possible two-port whose impedance must be known. The electrical length need only be known exactly, if it replaces a standard line -through. Is TRL completely programmed, the length of L must be known only to ± 90 °.

Frequency- NWA measurements

It is also possible by means of a network analyzer to perform an additional software ( such as with calibration Without Thru ) as well as two additional calibration standards ( comb generator and power sensor ) frequency- NWA measurements. In addition to vectorial harmonics measurements also calibrated vector intermodulation and mixer measurements are thus possible. The vectorial information allows a modeling of nonlinear effects and their localization. In addition, this NWA is used as a highly accurate sampling oscilloscopes, since mismatches in contrast to oscilloscopes eliminated by the system error correction.

Important parameters

The reflected signal ...

• Damping
• Impedance
• Reflection coefficient
• S- parameters
• Standing wave ratio

Of the transmitted signal ...

• Group delay
• Phase shift
• S- parameters
• Gain / attenuation