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Introduction to VNA Basics
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PRIMER

PRIMERIntroduction to VNA Basics

The Vector network analyzer or VNA is an important test

instrument that has helped make countless modern wireless

technologies possible. Today, VNAs are used in a wide range

of RF and high frequency applications. In design applications,

simulations are used to accelerate time-to-market by reducing

physical prototype iterations. VNAs are used to validate

these design simulations. In manufacturing applications, RF

components or devices are assembled and tested based on

a certain set of specifications. VNAs are used to quickly and

accurately validate the performance of these RF components

and devices.

This paper discusses why VNAs are used and how they are

unique compared to other RF test equipment. We'll define

S-Parameters, the fundamental VNA measurement, and how

best to use them when evaluating your Device-Under-Test or

DUT. We'll review various VNA calibration techniques and show

how VNA user calibrations help achieve the best accuracy

possible. Finally, we'll review typical VNA measurements

such as swept frequency measurements, time domain

measurements, and swept power measurements and how

they're used and why they are important.

Contents

Vector Network Analyzer Overview .................................3

Who Needs a VNA ................................................................ 4

Basic VNA Operation

Key Specifications

VNA vs. Spectrum Analyzer

Understanding S-Parameters ..........................................9

Types of Measurement Error .............................................. 11

Calibration Techniques .................................................. 12

What is User Calibration ..................................................... 12

VNA Calibration Methods

Calibration Standards

Typical VNA Measurements ...........................................15

Swept Frequency Measurements ....................................... 15

Time Domain Measurements

Swept Power Measurements

Testing Multiport Components

............................................................ 6

................................................................ 6

.................................................. 8

................................................... 13

......................................................... 14

.............................................. 16

............................................. 16

........................................... 17

Summary ....................................................................... 18

PRIMERIntroduction to VNA Basics

Not for measuring

WiFi networks

FIGURE 1. Today there are a wide variety of networks, each with its own network analyzer. The vector network analyzer, discussed in this document, is used for a
different kind of network and was defined long before any of these networks existed.

Vector Network Analyzer Overview

Today, the term “network analyzer”, is used to describe tools

for a variety of “networks” (Figure 1). For instance, most people

today have a cellular or mobile phone that runs on a 3G or

4G “network”. In addition, most of our homes, offices and

commercial venues all have Wi-Fi, or wireless LAN “networks”.

Furthermore, many computers and servers are setup in

“networks” that are all linked together to the cloud. For each of

these “networks”, there exists a certain network analyzer tool

used to verify performance, map coverage zones and identify

problem areas.

Not for drive testing

mobile phone networks

Tektronix 2016

FIGURE 2. Vector Network Analyzers or VNAs were invented in the 1950s and
are actively used around the world today.

Not for computer

networks or clouds

However, the network analyzer of interest in this paper is used

for a different kind of network and was defined long before any

of these networks existed. The first VNA was invented around

1950 and was defined as an instrument that measures the

network parameters of electrical networks (Figure 2). In fact, it

can be said that the VNA has been used over the years to help

make all the networks mentioned above possible. From mobile

phone networks, to Wi-Fi networks, to computer networks

and the to the cloud, all of the most common technological

networks of today were made possible using the VNA that was

first invented over 60 years ago.

FIGURE 3. VNAs are used to make most modern technologies possible.

PRIMERIntroduction to VNA Basics

WHO NEEDS A VNA

All wireless solutions have transmitters and receivers, and each

contains many RF and microwave components. This includes

not only smartphones and WiFi networks, but also connected

cars and IoT (Internet of Things) devices. Additionally, computer

networks today operate at such high frequencies that they are

passing signals at RF and microwave frequencies. Figure 3

shows a range of example applications that exist today with the

help of VNAs.

VNAs are used to test component specifications and verify

design simulations to make sure systems and their components

work properly together. R&D engineers and manufacturing

test engineers commonly use VNAs at various stages of

product development. Component designers need to verify the

performance of their components such as amplifiers, filters,

antennas, cables, mixers, etc. The system designer needs

to verify their component specs to ensure that the system

performance they're counting on meets their subsystem

and system specifications. Manufacturing lines use VNAs to

make sure that all products meet specifications before they're

shipped out for use by their customers. In some cases, VNAs

are even used in field operations to verify and troubleshoot

deployed RF and microwave systems.

PRIMERIntroduction to VNA Basics

Up-Mixer

Duplexer VCO

Down-Mixer

RF Front-End

Antenna

Filter PA

LNA Filter

IF

How efficient is the
antenna for transitioning
the signal to/from the air?

How well is the transmit
signal isolated from the
receive signal?

How well is the signal
being converted to a new
frequency and are any
unwanted signals being
generated?

How well are unwanted signals
going to be filtered out?

How much stronger will a
signal be after the amplifier?

How much signal is
getting to the antenna?

FIGURE 4. VNAs may be used to verify component, subsystem and system level performance.

As an example, Figure 4 shows an RF system front end and

how different components and parts of the system are tested

with a VNA. For the antenna, it is important to understand how

efficient the antenna is at transitioning the signal to and from

the air. As we’ll explain later, this is determined by using a VNA

From a system design point of view, how much signal goes

through the RF board and out of the antenna? On the receive

side, how effective is the duplexer in providing isolation

between the transmit and the receive signal? All of these

questions can be answered using a VNA.

to measure the return loss or VSWR of the antenna.

Looking at the right side of Figure 4, the up-mixer takes the

IF signal and mixes it with an oscillator (VCO) to produce the

RF signal. How well is the signal being converted to a new

frequency? Are any unwanted signals being generated? What

power levels are the most efficient at driving the mixer? VNAs

are used to answer these questions.

FIGURE 5. VNAs contain both a stimulus source and receivers to provide a very accurate closed loop for evaluating DUTs.

PRIMERIntroduction to VNA Basics

BASIC VNA OPERATION

One unique feature of a VNA is that it contains both a source,

used to generate a known stimulus signal, and a set of

receivers, used to determine changes to this stimulus caused

by the device-under-test or DUT. Figure 5 highlights the basic

operation of a VNA. For the sake of simplicity, it shows the

source coming from Port 1, but most VNAs today are multipath

instruments and can provide the stimulus signal to either port.

The stimulus signal is injected into the DUT and the VNA

measures both the signal that's reflected from the input side,

as well as the signal that passes through to the output side of

the DUT. The VNA receivers measure the resulting signals and

compare them to the known stimulus signal. The measured

results are then processed by either an internal or external PC

and sent to a display.

There are a variety of different VNAs available on the market,

each with a different number of ports and paths for which

the stimulus signal flows. In the case of a 1-port VNA, the

DUT is connected to the input side of Figure 5 and only the

reflected signals can be measured. For a 2-port 1-path VNA,

both the reflected and transmitted signal (S11 and S21) can

be measured, however, the DUT must be physically reversed

to measure the reverse parameters (S22 and S12). As regards

to a 2-port 2-path VNA, the DUT can be connected to either

port in either direction because the instrument has the

capability of reversing the signal flow so that the reflections at

both ports (S11 and S22), as well as the forward and reverse

transmissions (S21 and S12), can be measured.

KEY SPECIFICATIONS

When determining your needs for a VNA, there are several

key specifications to consider. While there are many VNA

specifications, there are four top level specs which can be

used to guide your selection process – frequency range,

dynamic range, trace noise, and measurement speed.

Frequency range is the first and most critical specification to

consider (Figure 6a). For this, it is often good to consider not

only your immediate needs but also potential future needs. In

addition, while all DUTs have a given operational frequency, for

some DUTs you may need to consider harmonic frequencies

as well. Active components, such as amplifiers, converters and

mixers may need to be tested at their harmonic frequencies

which are 2 to 5 times operational frequency. Filters and

duplexers may also need to be tested at harmonics of their

passband. Although a higher frequency range may be desired,

maximum frequency range can be a major cost driver for

VNAs.