Network Analyser

Low Cost Narrowband

Network Analyzer

Bachelor Degree Project Report

Submitted by:

Kaja Ameeruddin Mohammad (09007035)

Under the guidance of

Prof. Girish Kumar

Department of Electrical Engineering

Indian Institute of Technology Bombay

November 2012

i

Abstract

Network analyzers have become one of the most important measurement tools for

characterizing the performance of high-frequency components and devices. They can

provide a wealth of knowledge about a device under test (DUT), including its magnitude,

phase and group-delay response etc. But high costs of such instruments makes them

unaffordable, especially in engineering institutes, where the use will only be limited to

testing of devices for a specific application range. This project aims to design a low cost

computer interfaced network analyzer to make it more affordable and portable, operating in

the frequency range 800MHz 1000MHz.

The design has been started with the idea of keeping the cost low without significantly

affecting the performance of the system as a whole. Since laptops, computer desktops are

extremely common nowadays, their computation power can be used for analysis. A

microcontroller is used for system control and the laptop/computer desktop is used for

computation and display purposes. The performance of both hardware and software has

been followed very closely so as to keep the performance optimal. The following report

covers not only the logic involved but also the detailed working, results and prototype

hardware and software used.

ii

Table of Contents

Abstract .. i

List of Figures .... iii

List of Tables iv

Nomenclature . iv

Chapter 1 Introduction 1

1.1 Networks and their Properties 1

1.2 Scattering Parameters .. 2

1.3 Measurement of S-parameters .. 3

1.4 Network Analyzers Background and Cost Survey .. 5

1.5 Motivation . 6

1.6 Outline of Report .. 6

Chapter 2 Network Analyzer Overview . 7

2.1 Network Analyzer Architecture .. 7

2.1.1 Transmission/Reflection (T/R) Test Set .. 8

2.1.2 S-parameter Test Set 9

2.2 Proposed Design Technique . 10

Chapter 3 Hardware Specifications of SNA 12

3.1 Computer Microcontroller Interface 12

3.1.1 Microcontroller Unit .. 12

3.2 Voltage Controlled Oscillator (VCO) . 14

3.3 Power Divider .. 15

3.3.1 Wilkinson Power Divider . 15

3.4 Directional Coupler . 17

3.5 Power Detectors 19

Chapter 4 Software Specifications of SNA 21

4.1 PC Software and Graphical User Interface (GUI) . 21

Chapter 5 Conclusions and Future Work .. 23

Bibliography . 24

iii

List of Figures

Figure 1.1 Two port network showing incident and reflected waves . 2

Figure 2.1 Transmission/Reflection (T/R) Test Set Network Analyzer . 8

Figure 2.2 S-parameter Test Set Network Analyzer . 9

Figure 2.3 Proposed PC-based Network Analyzer Architecture .. 11

Figure 3.1 PIC18F4550 pin Configuration . 13

Figure 3.2 Functional Block Diagram of ADF4350 . 14

Figure 3.3 T-junction Power Divider 15

Figure 3.4 Wilkinson Power Divider 16

Figure 3.5 Directional Coupler 18

Figure 3.6 Power Detector - MAX4003 pin configuration 19

Figure 3.7 MAX4003 Output Voltage vs Input Power . 20

iv

List of Tables

Table 1.1 Cost Survey of Network Analyzers 5

Table 4.1 Advantages and disadvantages of different Coding Platforms .. 21

Nomenclature

VSWR Voltage Standing Wave Ratio

Vmax, Vmin Maximum and Minimum Voltage Values

Reflection Coefficient

T Transmission Coefficient

ZL Impedance of Load

ZO Impedance of the Transmission Medium

IL Insertion Loss

RL Return Loss

Efficiency

S11 Input port Voltage Reflection Coefficient

S12 Reverse Voltage Gain

S21 Forward Voltage Gain

S22 Output port Voltage Reflection Coefficient

1

Chapter 1

Introduction

Microwave and RF Networks are used in a large variety of applications today, and their uses

will keep increasing in the future with the coming of next generation of networks. It is

important to understand the properties of these networks as well as the transmission and

receiving devices. Each of these would have parameters we can theoretically calculate, but

their practical applications can only be understood once their parameters are measured.

1.1 Networks and their properties

The main properties of a network you would capture is the power at any point along with its

reflection coefficient, transmission coefficient, insertion loss, gain which are best described

by finding its scattering parameters. As for a high frequency network, there is no proper

definition for current voltage in the circuit. (They can even be used at lower frequencies

but since we are dealing with microwave frequency range we limit our calculations to the

S-parameters)

Power at a given point is a measurable quantity. The other parameters are described in the

equations below.

Reflection coefficient is given by

=

+

Transmission Coefficient T

T = 1 +

Efficiency

=

=

|2|

1| |2

Return Loss

RL = -20log | |

2

Insertion Loss

IL = 20log |T|

Voltage Standing Wave Ration: Measures the level of mismatch

VSWR =

=

1+| |

1| |

The above equations exhaustively describe the quantities of a given network we would want

to analyse. In order to simplify the calculation and measurement of these quantities we use

the two-port method calculating the scattering parameters for the signal. We use matched

and unmatched loads unlike a normal electric circuit. Quantities are measured in terms of

power of voltage or travelling waves.

1.2 Scattering Parameters

Two port method

Figure 1.1: Two Port network showing incident and reflected waves

For a case like the above two-port, the relations between the input and the output ports can

be shown as the following equation

Scalar linear gain

|G| = |S21|

3

Scalar logarithmic gain

g = 20 log|S21| dB

Insertion Loss

IL = -20 log |S21| dB

Input Return Loss

RLin = |20 log|S11|| dB

Output Return Loss

RLout = |20 log|S22|| dB

Reverse Gain (for when we invert the circuit)

grev = 20 log|S12| dB

VSWR

VSWRin = 1+|11|

1|11|

1.3 Measurement of S-parameters

The measurement in case of a given network circuit is the power given by source, power

reflected and power transmitted of a two-port device. The Device Under Test (DUT) is

considered a two-port black box and by obtaining its scattering parameters, we can

characterize it.

In order to connect the scattering parameters with the power measurements, we can

observe the following

From Figure 1.1 of the two-port network we can see

a1, a2 are normalized incident waves on port 1 and 2 respectively

b1, b2 are normalized reflected waves on port 1 and 2 respectively

4

Hence,

|a1|2 = Power incident on input of the network; Power available from the source

|a2|2 = Power incident on the output of the network; Power reflected from load

|b1|2 = Power reflected from the input port of the network; Power available from source

minus the power delivered to the network

|b2|2 = Power reflected from the output port of the network; Power incident on the load

|S11|2 =

S11 = 1

1 (if a2 = 0)

|S22|2 =

S22 = 2

2 (if a1 = 0)

|S21|2 =

S21 = 2

1 (if a2 = 0)

|S12|2 = Reverse Transducer power gain

S12 = 1

2 (given a1 = 0)

Considering we have found the modulus of the S-parameters, we know the values of most of

the properties of the given device under test (DUT).

Since we are working on a scalar network analyzer, we plot only the magnitudes of the

reflection and transmission coefficients and VSWR.

5

1.4 Network Analyzers Background and Cost Survey

A network analyzer is an instrument that measure the network parameters of electrical

networks. It commonly measure S-parameters because reflection and transmission of

electrical networks are easy to measure at high frequencies. A modern vector network

analyzer can measure a components magnitude, phase, and group delay, show port

impedances on a Smith chart, and, with time-domain capability, show the distance from a

test port to an impedance mismatch or circuit fault.

Network analyzers commonly measure S-parameters because reflection and transmission of

electrical networks are easy to measure at high frequencies. Understanding a network

analyzers capabilities and operation can help an operator derive optimum performance

from the instrument.

There are two kinds of network analyzers, depending on the measured s-parameters

1. Scalar network analyzer (SNA): Measures only the magnitude of the S-parame