# What is a Vector Network Analyzer, VNA: the basics

### The RF Vector Network Analyzer, VNA is a test instrument that measures the response of a network as vector: real & imaginary parameters so that its performance can be characterised.

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

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

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 VNA network analyzer 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 the it measure the amplitude response, but it also looks at the phase as well. As a result VNA network analyzer 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.

the various types of rf network analyzer are quite different in their make-up and the way in which they are able to make measurements. the scalar network analyzer is the least expensive, although not cheap, but it also provides the least information. the vna network analyzer is able to provide considerably more information, but these rf network analyzers are also considerably more expensive.

## Difference between RF network analyzers and spectrum 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.

## Concept of vector network analyzer

the vector network analyzer utilises the concept of measuring the transmitted and reflected waves as a signal passes through a device under test.

measuring the transmitted and reflected signals across the band of interest, and often beyond, enables the characteristics of a device to be determined. if both transmitted and reflected signals are used to characterise the input and also the output then the device can be fully characterised. this can form a key part of any design or test for an rf circuit.

#### • Reflection parameters

When power enters an RF network, some enters the network, but dependent upon the impedance match, some of the power is reflected back to thee source.

#### Note on Standing Wave Ratio, SWR & VSWR:

the reflection coefficient, γ is is one of the key parameters.

$\Gamma =\frac{{V}_{\mathrm{reflected}}}{{V}_{\mathrm{incident}}}=\frac{{Z}_{L}-{Z}_{0}}{{Z}_{L}+{Z}_{0}}$

Where:
Γ = reflection coefficient
Vreflected = voltage of the reflected wave
Vincident = voltage of the forward or incident wave
Z0 = feeder characteristic impedance

return loss is another way of expressing the reflection characteristics:

#### • Transmission characteristics

the transmission characteristics of the rf network are equally important in the vector network analyzer as these determine how power passes through the network.

the transmission coefficient, t can be expressed:

$T=\frac{{V}_{\mathrm{transmitted}}}{{V}_{\mathrm{iincident}}}$

$G=20\mathrm{log}\left(\frac{{V}_{\mathrm{transmitted}}}{{V}_{\mathrm{incident}}}\right)=20\mathrm{log}\left(\tau \right)$

## 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.

## Vector network analyzer block diagram

the diagram shows the very basic blocks of the vna including the signal ports, the signal separation blocks, the receiver detector, and finally the processor and display.

the block diagram of a basic two port vector network analyser is shown above. this shows the high level blocks needed for a typical vna.

the vna has precision connectors on the front panel of the unit itself and then precision cables are used to connect these to the device under test. precision cables are required because the phase and loss of a standard cable would vary too much with even slight movement, etc.

although this very simplified example of an rf network analyzer shows two ports, some vector network analyzers may use more ports for systems where many different signal paths exist.