Microwave Electronics - the Network Analyzer experiment. In this lab, we have used the Hewlett Packard HP8720 Network analyser to measure the S-Parameters for some simple devices.

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Electronics Engineering and Computer Science

ELE 569: MICROWAVE ELECTRONICS

Experiment 1: The Network Analyser

Contents

Abstract........................................................................3

Introduction...................................................................3

Background Theory..........................................................4

The S-parameters.....................................................4

The network analyzer...............................................5

Measurement of S-parameters..............................................7

Reflection and Transmission test set........................................14

Transmission Measurement.................................................14

Reflection Measurements 1.................................................18

Reflection Measurements 2.................................................21

Conclusion....................................................................23

References....................................................................24

2

ABSTRACT [1]

The main objective of this experiment is

??To provide a hands-on experience with the Microwave Network Analysis using

HP8720 Vector network analyser.

??To be familiarise in real life experience with the concepts of ELE569 course.

??To show the principles of swept frequency measurement

??Show how S-parameters are measured and reinforce the theory of S-

parameters in microwave circuit design

??Demonstrate principles of stub matching and show that it is a narrow

bandwidth method.

INTRODUCTION [1][3][6]

It is often convenient to measure the complete performance of a microwave

device. Just as h parameters are used in transistor design, S-parameters are used

in microwave design. S-parameters can be easily measured using an S-parameter

test set. In this lab, we have used the Hewlett Packard HP8720 Network analyser

to measure the S-Parameters for some simple devices.

The experiment is broken down into three parts. The first part is basically the

familiarisation with the various complexities of the equipment. Therefore, the

second part investigates the variation of the amplitude and the phase of S-

parameters. Different scenarios have been analysed, where either only Port 1 has

been used, or both Port 1 and Port 2 have been used.

In microwave engineering we frequently need to measure the phase response of

a component as well as its amplitude response. For this we use a Vector Network

Analyser, often referred to, without qualification, as a Network Analyser. This

instrument has two microwave inputs, the REFERENCE input and the TEST input,

and gives as output the relative amplitude (dB) and phase (deg.) between the two

inputs. By connecting the instrument to various microwave networks a wide

variety of precision measurements can be made [1].

And the final part is use of the reflection/transmission test set to observe

variations in the amplitude and the phase of the test device over a range of

frequencies . Various devices have been thus tested, including stub tuner, horn,

and micro strip patch antenna. The procedures are given in details in the next

sections of the report. The explanations given do not go into too much depth of

the theories of the subject because the experiments were aimed at only giving a

brief introduction of the functions of the equipment (the Network Analyser).

3

BACKGROUND THEORY

The concepts of S-parameters and Network Analysers are essential to

understanding this experiment and are briefly explained in relation to the

information required for this experiment.

Scattering Parameters [2][3][6]

This property is used to describe electrical behaviours of linear high frequency

electrical networks. It can be used to determine many electrical properties such

as gain, return loss, VSWR, reflection coefficient and amplifier stability. It uses

matched and un-matched loads to characterize a linear electrical network. The S-

matrix for an N-port contains a N2 coefficients (S-parameters), each one

representing a possible input-output path. Let us consider an N port network.

Such that the Vn is the amplitude of the incident wave and V'n is the amplitude of

the reflected wave. To calculate the output of the incident wave we need to

multiply it with the appropriate scattering parameter to get the amplitude of the

reflected signal. If the value of N is 2 and then the definitions of S-parameters are

like below:

2

Figure 1: Basic Definition of S-Parameters [2]

The no of ports might be higher than 2. Suppose if the value of N is 4 and is

represented like the following diagram,

4

Figure 2 : Measuring scattering parameters (4 ports)

Then,

S11 = V'n1 / Vn1 (nk = 0; nk =n1) S22 = V'n2 / Vn2 (nk = 0; nk =n2)
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S21 = V'n2 / Vn1 (nk = 0; nk =n1) S12 = V'n1 / Vn2 (nk = 0; nk =n2)

Similarly the S-parameters for other ports can be calculated. The S- parameter is

also the gain of that port. For example S21 can be found by driving port 1 with the

voltage 1 and then calculating the output voltage at port 2 provided that, the

incident voltage is zero at all ports except port 1.

We will be investigating four S parameters in the first part which are as follows:

• S11 ...

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