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Vector Netwok Analyzer FAQ

As a VNA vendor we get many questions from our customers. This page shares some of the frequently asked questions for your information.

What does a Vector Network Analyzer do?

A Vector Network Analyzer (VNA) is much like a multimeter that measures resistance. But where a multimeter measures a resistor at DC (0 Hz), a VNA measures resistance of an RF or microwave circuit at their operating frequency, measuring both the resistance and the phase (they make up a Vector) of the circuit. In addition, a VNA can also measure the gain or loss between two ports of a circuit, again by magnitude and phase. And by measuring a range of frequencies they provide a graph of the impedance and gain over a frequency range.

What is a Vector Network Analyzer used for?

A VNA is used to characterize RF and microwave devices in order to make them work together efficiently. An RF system works best when its components are matched to each other, with the output impedance matching the input impedance of the next component. Especially antennas can have a wide range of impedances and they need to be measured and matched to the devices that they connect to.

How does a Vector Network Analyzer work?

Much like a multimeter, a VNA connects a known signal source to the device under test and measures the voltage across its terminals (usually signal and ground of a coax connection). From the known source and the measured signal it can calculate the (complex) impedance. But where a multimeter uses DC (from a battery), the VNA has a built-in RF signal generator to measure at RF frequencies. And the measurement circuit compares both the amplitude and the phase of the measured signal to that of the source. For gain or loss measurement the VNA connects the same source signal to one port and measures the output signal at the other port of the device under test.

Why calibrating a Vector Network Analyzer?

A VNA usually connects to a device under test through some cables and connectors. At high frequencies these connections affect the source signal and the measured signal so that the VNA can not determine the impedance properly. For this reason a calibration of the measurement setup is necessary so that the VNA can normalize the measurement setup.

How to calibrate a Vector Network Analyzer?

There are different schemes to calibrate a VNA setup but one of the most commonly used schemes is done with an Open, Short and Load (50 Ohm) element. These elements are connected in turn at the point of measurement and the VNA will measure the setup. From these three measurements the VNA has enough information to normalize the test setup away and show the impedance at the calibration point. For gain or loss calibration, a known Through element is connected between the two measurement points that are to connect to the input and output of the device under test.

What is the difference between a Vector Network Analyzer vs Network Analyzer?

The term ‘Network Analyzer’ is used loosely by many people. In some cases it can refer to a Vector Network Analyzer, but it is most commonly used for hardware or software that analyzes data traffic on computer networks like LAN and WLAN.

What is the difference between a Vector Network Analyzer vs Scalar Network Analyzer?

A Scalar Analyzer is a simpler (and usually cheaper) device than a VNA. It also measures impedance (and/or gain) of a device but only the magnitude, not the phase. While these results can be used to evaluate the matching of the device, they can not be used to calculate any kind of matching network to get a correct impedance match so the application is very limited. A VSWR meter is an example of a scalar measurement device.

What is the difference between a Vector Network Analyzer vs Spectrum Analyzer?

While a VNA consists of a signal generator and a (narrow band) receiver that scan over a frequency range, a Spectrum Analyzer is only a receiver that scans over frequencies. So a Spectrum Analyzer is a passive device that measures external signals that are present at a device or at an antenna, and can be used to measure (transmitter output) levels and harmonics and spurious signals. The receiver part of a Spectrum Analyzer is optimized for these measuring functions with wider bandwidth, demodulation options etcetera.

What is the difference between a Vector Network Analyzer vs Spectrum Analyzer with Tracking Generator?

Some Spectrum Analyzers are provided with a Tracking Generator that generates a signal that is sweeping with the frequency of the Spectrum Analyzer. Together they can be used as a Scalar Network Analyzer, with the same shortcomings of not measuring the phase of the impedance or gain.

What is the difference between a Vector Network Analyzer vs Oscilloscope?

A VNA and an Oscilloscope are totally dissimilar devices. An oscilloscope measures an external signal on a device and shows the waveform as a function of time. There is a similarity between an oscilloscope and a Spectrum Analyzer where both measure and analyze external signals. With frequency transform functions as FFT that modern oscilloscopes are offering their functionality is blending with that of a Spectrum Analyzer.

 

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