Vector Network Analyzer VNA Tutorial

- a tutorial, information and overview about the basics of the vector network analyser, VNA, detailing what is a vector network analyser, how it works and using a vector network analyzer.

The vector network analyzer, VNA is a form of RF network analyzer widely used for RF design applications. A vector network analyser is a test system that enables the RF performance of radio frequency (RF) and microwave devices to be characterised in terms of network scattering parameters, or S parameters. The information provided by the vector network analyser VNA is then used to ensure that the RF design of the circuit is optimised to provide the best performance. Using an RF network analyzer in any RF design provides the RF design engineer with a view of the components and circuits that would not be possible with any other form of test equipment. In this way the vector network analyzer, VNA is an essential tool that RF design engineers should be able to use.

Although the name of the vector network analyser VNA is the most widely used name for this item of test equipment, sometimes it may be called a gain phase meter in view of the fact that they are able to measure both the gain, i.e. amplitude and also the phase of the device or item under test. Another name that is occasionally used is the Automatic Network Analyzer. However by far the most widely used is the name vector network analyser, VNA.


Magnitude and phase

The key element of the vector network analyzer, VNA, is that it can measure both amplitude and phase. While an amplitude only measurement is much simpler to make, and can be undertaken by less complicated instruments. This may be quite sufficient for many instances. For example when the only consideration is the gain of an amplifier over a certain bandwidth, or the amplitude response of a filter is needed

However a measurement that includes phase as well as amplitude enables far more to be discovered about the device under test as phase is a critical element in network analysis. This is because a complete characterization of devices and networks involves measurement of phase as well as magnitude. Only with a knowledge of phase and magnitude from a vector network analyser can circuit models be developed that will enable complete simulation to be undertaken. This will enable matching circuits to be designed based on conjugate matching techniques. Time-domain characterization requires magnitude and phase information to perform the inverse-Fourier transform. Also, phase data is required to perform vector error correction.


Summary

Vector network analyzers are particularly useful items of RF test equipment. When used skilfully, they enable RF devices and networks to be characterised so that an RF design can be undertaken with a complete knowledge of the devices being used. This will provide a better understanding of how the circuit will operate. Vector network analyzers provide a much greater capability than their scalar counterparts, and as a result the vector network analyzers are more widely used, even though they tend to be more expensive.

By Ian Poole


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