What is a Vector Network Analyzer, VNA

The RF Vector Network Analyzer, VNA is an RF test instrument that is able to measure the response of a network as vector or real and imaginary parameters.

RF vector network analyzer includes:
What is a VNA     Scalar analyzer    

RF network analyzers are vital items of test instrumentation for RF design laboratories as well as many manufacturing and service areas.

Although mainly focussed on research and development, RF network analyzers are able to provide vital insights into the operation and performance of RF networks of all types.

The RF network analyzer provides a stimulus for the network and then monitors the response. In this way the operation and performance can be seen and assessed for its suitability.

RF network analyzers can be used for all RF and microwave frequencies - some network analyzers can operate well into the microwave region.

Types of RF network analyzer

Within the broad scope of RF network analyzers, there are various types of instrument which can be bought and used. These types of RF network analyzer are very different, but they are all able to measure the parameters of RF components and devices but in different ways:

  • Scalar network analyzer (SNA):   The scalar network analyzer, SNA is a form of RF network analyzer that only measures the amplitude properties of the device under test - i.e. its scalar properties. In view of this it is the simpler of the various types of analyser.
  • Vector network analyzer (VNA):   The vector network analyzer, VNA is a more useful form of RF network analyzer than the SNA as it is able to measure more parameters about the device under test. Not only does it measure the amplitude response, but it also looks at the phase as well. As a result vector network analyzer, VNA may also be called a gain-phase meter or an Automatic Network Analyzer.
  • Large Signal Network Analyzer (LSNA):   The large signal network analyzer, LSNA is a highly specialised for of RF network analyser that is able to investigate the characteristics of devices under large signal conditions. It is able to look at the harmonics and non-linearities of a network under these conditions, providing a full analysis of its operation. A previous version of the Large Signal Network Analyser, LSNA was known as the Microwave Transition Analyzer, MTA.

Difference between RF network analyzers and spectrum analyzers

Although there are many similarities between network analyzers and spectrum analyzers, there are also several major differences, especially in the types of measurements that are made. In particular they make very different types of measurement. In the first instance a spectrum analyzer is intended for analysing the nature of signals that are fed into them. A network analyzer, on the other hand generates a signal and uses this to analyze a network or device.

RF Network analyzers are used to measure components, devices, circuits, and sub-assemblies. An RF network analyzer will contain both a source and multiple receivers. It will display amplitude and often phase information (frequency or power sweeps) and normally in a ratio format. An RF network analyzer looks for a known signal, i.e. a known frequency, at the output of the device under test, since it is a stimulus response system. With vector-error correction, network analyzers provide much higher measurement accuracy than spectrum analyzers.

By contrast to RF network analyzers, spectrum analyzers are normally used to measure the characteristics of a signal rather than a device. The parameters measured may include: signal or carrier level, sidebands, harmonics, phase noise, etc. They are most commonly configured as a single channel receiver, without a source. Because of the flexibility needed to analyze signals, spectrum analyzers generally have a much wider range of IF bandwidths available than most RF network analyzers.

Spectrum analyzers can be used for testing networks such as filters. To achieve this they need tracking generator. When used in this way, spectrum analyzers can be used for scalar component testing (magnitude versus frequency, but no phase measurements). With spectrum analyzers, it is easy to get a trace on the display, but interpreting the results can be much more difficult than with a network analyzer.

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.

RF network analyzers can be very expensive items of test equipment. However they are invaluable in the design of RF networks - particularly filters and any other items where the RF performance of the item needs to be accurately assessed. In this way it is possible to fully understand what is happening and ensure that the overall design operates correctly.

More Test Topics:
Analogue Multimeter     Digital Multimeter     Oscilloscope     Signal generators     Spectrum analyzer     Frequency counter     LCR meter / bridge     Dip meter, GDO     Logic analyzer     Power meter (RF & microwave)     RF signal generator     Logic probe     Time domain reflectometer, TDR     LabVIEW     PXI     GPIB / IEEE 488     Boundary scan / JTAG    
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