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Teknikit Digital Communication

Student’s Workbook

53-002S

Feedback

Feedback Instruments Ltd, Park Road, Crowborough, E. Sussex, TN6 2QR, UK. Telephone: +44 (0) 1892 653322, Fax: +44 (0) 1892 663719.

email: [email protected] website: http://www.fbk.com

Manual: 53-002S Ed04 072003 Printed in England by Fl Ltd, Crowborough

Feedback Part No. 1160–53002S

DIGITAL COMMUNICATIONS - STUDENT’S WORKBOOK Preface

53-002S i

THE HEALTH AND SAFETY AT WORK ACT 1974

We are required under the Health and Safety at Work Act 1974, to make available to users of this equipment certain information regarding its safe use.+

The equipment, when used in normal or prescribed applications within the parameters set for its mechanical and electrical performance, should not cause any danger or hazard to health or safety if normal engineering practices are observed and they are used in accordance with the instructions supplied.

If, in specific cases, circumstances exist in which a potential hazard may be brought about by careless or improper use, these will be pointed out and the necessary precautions emphasised.

While we provide the fullest possible user information relating to the proper use of this equipment, if there is any doubt whatsoever about any aspect, the user should contact the Product Safety Officer at Feedback Instruments Limited, Crowborough.

This equipment should not be used by inexperienced users unless they are under supervision.

We are required by European Directives to indicate on our equipment panels certain areas and warnings that require attention by the user. These have been indicated in the specified way by yellow labels with black printing, the meaning of any labels that may be fixed to the instrument are shown below:

CAUTION - RISK OF DANGER

CAUTION - RISK OF

ELECTRIC SHOCK

CAUTION - ELECTROSTATIC

SENSITIVE DEVICE

Refer to accompanying documents

PRODUCT IMPROVEMENTS

We maintain a policy of continuous product improvement by incorporating the latest developments and components into our equipment, even up to the time of dispatch.

All major changes are incorporated into up-dated editions of our manuals and this manual was believed to be correct at the time of printing. However, some product changes which do not affect the instructional capability of the equipment, may not be included until it is necessary to incorporate other significant changes.

COMPONENT REPLACEMENT

Where components are of a ‘Safety Critical’ nature, i.e. all components involved with the supply or carrying of voltages at supply potential or higher, these must be replaced with components of equal international safety approval in order to maintain full equipment safety.

In order to maintain compliance with international directives, all replacement components should be identical to those originally supplied.

Any component may be ordered direct from Feedback or its agents by quoting the following information:

1. Equipment type

3. Component reference

2. Component value

4. Equipment serial number Components can often be replaced by alternatives available locally, however we cannot therefore guarantee continued performance either to published specification or compliance with international standards.

DIGITAL COMMUNICATIONS - STUDENT’S WORKBOOK Preface

ii 53-002S

DECLARATION CONCERNING ELECTROMAGNETIC COMPATIBILITY

Should this equipment be used outside the classroom, laboratory study area or similar such place for which it is designed and sold then Feedback Instruments Ltd hereby states that conformity with the protection requirements of the European Community Electromagnetic Compatibility Directive (89/336/EEC) may be invalidated and could lead to prosecution.

This equipment, when operated in accordance with the supplied documentation, does not cause electromagnetic disturbance outside its immediate electromagnetic environment.

COPYRIGHT NOTICE

© Feedback Instruments Limited

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of Feedback Instruments Limited.

ACKNOWLEDGEMENTS

Feedback Instruments Ltd acknowledge all trademarks.

IBM, IBM - PC are registered trademarks of International Business Machines.

MICROSOFT, WINDOWS 2000, WINDOWS 98, WINDOWS 95, WINDOWS 3.1 are registered trademarks of Microsoft Corporation.

DIGITAL COMMUNICATIONS - STUDENT’S WORKBOOK Contents

53-002S TOC-1

TABLE OF CONTENTS

1 Introduction 1-1

2 Assignments using the PCM and Link Analysis Workboard 2-1

2.1 Familiarisation with the PCM & Link Analysis Board Assignment 2-1

2.1.1 Objectives 2-1

2.1.2 Practicals 2-1

2.1.3 Workboard Required 2-1

2.1.4 Theory 2-2

2.1.5 Practical : Picture Transmission 2-2

2.2 Introduction to Digital Communications Assignment 2-7




2.2.4 Theory 2-8

2.2.5 Practical 1 : Information 2-8

2.2.6 Practical 2: t & f Domain 2-11

2.2.7 Practical 3: Pulse Train Spectrum 2-15

2.3 PCM Encoding Assignment 2-18




2.3.4 Theory 2-19

2.3.5 Practical 1 : Codes Simulation 2-20

2.3.6 Practical 2: Asynchronous Link 2-23

2.3.7 Practical 3: Synchronous Link 2-27

2.4 Introduction to Noise Assignment 2-32



TOC-2 53-002S



2.4.4 Theory 2-33

2.4.5 Practical 1: Noise 2-35

2.4.6 Practical 2: Noise on a Link 2-38

2.4.7 Practical 3: Signal to Noise Ratio 2-41

2.5 Noise on a Digital Link Assignment 2-44




2.5.4 Theory 2-45

2.5.5 Practical 1 : Data Format 2-46

2.5.6 Practical 2: Keying 2-49

2.5.7 Practical 3: Picture Transmission 2-53

2.6 Error Detection Assignment 2-56




2.6.4 Theory 2-57

2.6.5 Practical 1: Parity Checking 2-63

2.6.6 Practical 2: Block Parity Check 2-68

2.6.7 Practical 3: Cyclic Redundancy Check 2-73

2.7 Bit Error Rate Assignment 2-76




2.7.4 Theory 2-77

2.7.5 Practical 1: Bit Error Rate 2-78

2.7.6 Practical 2: Effects of Format 2-82

2.7.7 Practical 3: Effect of Keying 2-88


53-002S TOC-3

2.8 Error Correction Assignment 2-91




2.8.4 Theory 2-92

2.8.5 Practical 1 : Hamming Code 2-101

2.9 Time Division Multiplexing Assignment 2-109




2.9.4 Theory 2-110

2.9.5 Practical 1 : Analogue TDM 2-115

2.9.6 Practical 2: A/D Conversion 2-119

2.9.7 Practical 3: Digital TDM 2-122

2.9.8 Practical 4: D/A Conversion 2-125

2.9.9 Practical 5: Demultiplexing 2-128

2.10 Fibre Optic Link Assignment 2-131




2.10.4 Theory 2-132

2.10.5 Practical 1: Attenuation Losses 2-136

2.10.6 Practical 2: Attenuation Measurement 2-139

2.10.7 Practical 3: Effect on Error Rates 2-142

3 Assignments using the Modulation and Keying Workboard 3-1

3.1 Amplitude Shift Keying Assignment 3-1




3.1.4 Theory 3-2


TOC-4 53-002S

3.1.5 Practical 1: Production of ASK 3-6

3.1.6 Practical 2: Diode Detector 3-11

3.1.7 Practical 3: Suppressed Carrier ASK 3-13

3.1.8 Practical 4 : Demodulation of SC ASK 3-17

3.2 Assignment: Frequency Shift Keying 3-21




3.2.4 Theory 3-22

3.2.5 Practical 1: Operation of VCO 3-25

3.2.6 Practical 2: Production of FSK 3-30

3.2.7 Practical 3: PLL Demodulation of FSK 3-33

3.3 Phase Shift Keying Assignment 3-37




3.3.4 Theory 3-38

3.3.5 Practical 1: PSK Modulation 3-39

3.3.6 Practical 2: Demodulation


53-002S TOC-5

3.5.4 Theory 3-62

3.5.5 Practical 1: Generation of QPSK 3-65

3.5.6 Practical 2: Demodulation of QPSK 3-70

3.6 Differential Phase Shift Keying Assignment 3-76




3.6.4 Theory 3-77

3.6.5 Practical 1: Generation of DPSK 3-79

3.6.6 Practical 2: DPSK Demodulation 3-83

3.7 Differential Quadrature Phase Shift Keying Assignment 3-86




3.7.4 Theory 3-87

3.7.5 Practical 1: Generation of DQPSK 3-88

3.7.6 Practical 2: DQPSK Demodulation 3-93

4 Assignments using the Digital Data Formatting Workboard 4-1

4.1 Sampling Assignment 4-1




4.1.4 Theory 4-2

4.1.5 Practical 1: Pulse Amplitude Modulation 4-3

4.1.6 Practical 2: Sample and Hold 4-7

4.1.7 Practical 3: Aliasing 4-10

4.2 A to D Conversion Assignment 4-13




TOC-6 53-002S


4.2.4 Theory 4-14

4.2.5 Practical 1: Quantisation 4-14

4.2.6 Practical 2: Binary Coding 4-17

4.2.7 Practical 3: Companding 4-20

4.3 Data Formatting Assignment 4-24




4.3.4 Theory 4-25

4.3.5 Practical 1: NRZ 4-25

4.3.6 Practical 2: RZ 4-29

4.3.7 Practical 3: Bi-phase (Manchester) 4-32

4.3.8 Practical 4: Bipolar RZ 4-35

4.3.9 Practical 5: AMI (Ternary) 4-38

4.4 Synchronisation Assignment 4-41




4.4.4 Theory 4-42

4.4.5 Practical: Word Synchronisation 4-42

Chapter 1 DIGITAL COMMUNICATIONS - STUDENT’S WORKBOOK Introduction

53-002S 1-1

1 Introduction

This manual provides computer-based assignments which make use of Digital Workboards, the Discovery II environment and Digital Telecommunications 53-922 software package to provide an understanding of the fundamental principles on which complex analogue communication systems are based.

Assignments are divided into practicals whose objectives are clearly defined. Every practical is designed to contain as much circuit investigation, measurement and observation as possible. Explanatory text, diagrams and instrumentation are fully integrated.

Details of hardware and software installation are given in manual 53-001-3 together with Discovery II environment and product operating instructions.

Chapter 1 DIGITAL COMMUNICATIONS - STUDENT’S WORKBOOK Introduction

1-2 53-002S

Notes

Chapter 2 DIGITAL COMMUNICATIONS - Assignments using the STUDENT’S WORKBOOK PCM and Link Analysis Workboard

53-002S 2-1

2 Assignments using the PCM and Link Analysis Workboard

2.1 Familiarisation with the PCM & Link Analysis Board Assignment

2.1.1 Objectives

On completion of this assignment you will be:

• Familiar with the PCM and Link Analysis workboard,

• Able to describe the functions of the controls down the right-hand side of the workboard,

• Conversant with the effects of synchronism and non-synchronism as applied to a digital link,

• Able to use these controls to ensure that the digital link is synchronised.

2.1.2 Practicals

Practical exercises are provided as follows:

• Practical: Picture transmission.

2.1.3 Workboard Required

PCM and Link Analysis Workboard 53-170 which comprises the following blocks:

• Signal Generators

• Time Division Multiplex Circuit

• Noise Generator

• Analogue to Digital Converter

• NRZ, RZ, BPRZ, SP, AMI Links

• Fibre Optics Link (NRZ)

• ASK, FSK, PSK, QPSK, DPSK, DQPSK Links

• Synchronisation Circuits

• Digital to Analogue Converter

• De-multiplex Circuit

• Filter


2-2 53-002S

2.1.4 Theory

2.1.4.1 Synchronisation of a Digital Link

A communications link comprises a transmitter, a receiver and some communications medium between them. This medium could be wire, optical fibre, atmosphere, etc.

With this Digital Communications Trainer, the information that is carried on the communications link is in digital form, i.e. a stream of digital 1s and 0s.

For correct operation, a communications link carrying digital information must be synchronised. This means that the receiver must have precisely the same clock rates as the transmitter and that the decisions as to whether a received bit is a 1 or a 0 is made precisely at the correct time.

If this synchronisation between received and transmitted data is not maintained on a digital link, then the received information will not be decoded correctly, and the received signal will be incorrect.

As the PCM & Link Analysis workboard is a digital link, synchronism must be maintained to get correct results from the assignments. The board contains a number of controls that must be adjusted correctly to get that synchronism and a display to indicate when synchronism is achieved, or lost.

The object of this assignment is to familiarise you with the controls and the display and the technique required to achieve synchronism.

2.1.5 Practical : Picture Transmission

Down the right-hand side of the PCM & Link Analysis workboard you will see nine potentiometer controls. In most of the Assignments these controls need to be set for the correct operation of the link.

The functions of these controls need to be understood to allow you to appreciate how to set them up.

The top two controls, labelled DC channel 0 control (1) and DC channel 1 control (2), provide variable DC levels that are used in some Assignments, for example to provide an adjustable level as the input for an A/D converter.

In most Assignments they are not used.

The Carrier level control (3) adjusts the amplitude of the carrier when links using data modulating a carrier are investigated. For instance, when ASK, FSK, etc. are used.


53-002S 2-3

The PCM level control (4) adjusts the amplitude of the pulse coded waveform used either on baseband PCM links, or as the modulation waveform on ASK, FSK and PSK links.

The PCM bandwidth control (5) adjusts the cut-off frequency of a filter which may be used to show the effects of limiting the bandwidth on a PCM link.

With the bandwidth control at maximum the shape of the pulses is square, however adjusting this control effectively lowers the bandwidth of the link and causes rounding of the pulses, etc.

The Noise level control (6) is used extensively in the Assignments which investigate the link performance with noise added.

Normally, when noise is added to the link (usually using an on-screen button) the amplitude of the signal is initially reduced to allow noise to be added on top of it. The Carrier level (3) or PCM level (4) controls often have to be adjusted to compensate for this. Instructions to do this are given in the relevant Practicals.

2.1.5.1 Controls and Synchronisation

The Bit Clock control (7) adjusts the characteristics of the receiver's bit clock recovery circuit so that the bit clock at the receive end of the link is running in synchronism with the received bitstream.

If this synchronism is not maintained the received data will be sampled at the wrong times and errors will result. More information into the importance of maintaining bit clock synchronism is given in the Assignments using this workboard and the underlying principles are demonstrated with the 53-150, Digital Data Formatting, workboard.

The Data 1 (8) and Data 0 (9) controls set the switching thresholds (or decision levels) of the receiver to allow it to determine whether a received bit is a 1, or a 0.

In most of the Assignments on this workboard unipolar formatted data is used (more information on data formats is given in the Assignments associated with the 53-150, Digital Data Formatting, workboard).

With unipolar formatted data only one decision (threshold) level is required and the Data 0 control (9) is used. This control has to be set to ensure that the decision as to whether each bit received is a 1, or a 0 is made correctly.

This correct setting is normally indicated by looking at the received data waveform with the oscilloscope. With the control set incorrectly the received data waveform will be either constantly at 1, or at 0. The control must be adjusted so that the data waveform switches between the two logic levels, as required.


2-4 53-002S

The LED display may be used to determine the correct setting instead of the oscilloscope. The technique for this is given in the section on the display that follows.

A procedure for achieving synchronism and setting the correct threshold level has to be performed whenever a link is set up.

This is true of any commercial link and not just the links associated with this workboard.

Sometimes the optimum setting of these controls can be very sensitive, especially when the link has noise associated with it. Maintaining synchronism under noisy conditions is a significant problem with digital links.

Controls (7) and (9) are somewhat interdependent when trying to find an optimum setting and care is needed in their adjustment.

This is made even more difficult when a bipolar data format is used because bipolar formats require two threshold (decision) levels: one for 0 and the other for 1. Both controls (8) and 9 are used, together with the Bit Clock control (7) to achieve the correct reception of data.

Guiding instructions are given in the Assignments on the typical settings for these controls, however each workboard and each link configuration may require slightly different settings which must be found by adjustment.

A display is provided at the top right-hand corner of the workboard to facilitate this setting up.

Any difficulties you may have in setting the controls reflect the problems encountered on typical commercial links.

2.1.5.2 Information on LED Display

At the top right-hand corner of the 53-170 workboard you will see a 7x5 matrix LED display. In a number of Assignments this display is of significance and is a useful aid to setting up and synchronising the digital link.

After switching on the equipment, the first display that should be seen is of alternating vertical and horizontal bars travelling across the matrix. This indicates that the workboard is correctly powered up and waiting for a command from the computer. This is the normal display that should be seen first. If a computer is not connected, or if the communications between computer and workboard is not correct, this display will stay permanently alternating.

If the communications are correct and a Practical is selected, the alternating display will cease and the bottom right-hand LED will be seen to be blinking (there may also be other LEDs lit as well).


53-002S 2-5

For most Assignments, when performing a Practical, the top line of LEDs gives the state of the link being used. The top left-hand LED blinks whenever a synchronisation byte (sequence of bits) is sent. The next LED to the right blinks whenever the synchronisation byte is detected at the receive end of the link. The next right LED lights if the receiver misses a sync byte but is still in synchronisation from the last sync byte. The link will probably still be functioning. The fourth LED lights if synchronisation has been lost completely.

Whenever the link is set up correctly and is fully synchronised the first two LEDs will be blinking, as well as the bottom right-hand one.

Many of the Practicals instruct you to adjust the Data 1 and Data 0 controls controls 8 and 9 and the Bit Clock control 7 for synchronism. Usually it is fairly easy to see from the oscilloscope display when synchronisation has been achieved, however the LED matrix display gives a more sensitive and reliable indication. The general procedure is to [Adjust the controls until the top left-hand two LEDs blink]

Note: the second row of LEDs is used in some Practicals. The significance of the LEDs is the same as for the top row, except bit synchronisation is being used

2.1.5.3 Procedure

The data source is a digitised picture. This is transmitted through a PCM link. The effect of gaining, or loosing synchronisation can be seen on the received picture. The 7 x 5 matrix LED display on the workboard is used to indicate when synchronism has been attained.

Set the Carrier Level (3), PCM Level (4) and PCM Bandwidth controls (5) to maximum, the Noise Level (6) control to minimum and all other controls to their mid positions.


2-6 53-002S

Make sure you have read the background to this Practical and understand the significance of the LEDs in the top left-hand corner of the matrix display.

Observe the received picture and the LED matrix display.

It is unlikely that you have synchronisation.

Adjust the Data 0 control (9) to get a received picture and then the Bit Clock control (7) to get the best resolution.

These controls are somewhat interdependent. Adjustment for synchronisation can be rather sensitive. You will probably find that your first attempts take a long time to achieve synchronism. It will become easier with practice!

It is best if you use both hands simultaneously – one to adjust the Data 0 control (9), the other to adjust the Bit Clock control (7). Keep looking at the LED matrix display and adjust until you have the two top left-hand LEDs flashing.

When you have achieved this, you should see that the picture is received without any break-up.


53-002S 2-7

2.2 Introduction to Digital Communications Assignment

2.2.1 Objectives

On completion of this assignment you will be familiar with:

• The concept of information,

• How information may be represented in binary form,

• Binary signals in the time and frequency domains,

• The spectrum of a pulse train,

• Bit and baud rates.

2.2.2 Practicals


• Practical 1: Information

• Practical 2: t & f Domain

• Practical 3: Pulse Train Spectrum





• Noise Generator








• Filter


2-8 53-002S

2.2.4 Theory

2.2.4.1 Information

A communications system is used for communicating information, which originates from an information source.

Information is that which is conveyed.

The meaning, or lack of meaning, of the information is not important when dealing with communications systems, only the quantity of information and its integrity are important.

No information is transmitted by a continuous symbol; information requires change.

The simplest information system is binary, where the information is contained in the choice of one state out of two possible states, normally given the symbols 1 and 0.

These are binary digits (often shortened to the word bits). One bit is the smallest division of binary information.

The bit is a division of information and the choice of 1 or 0 constitutes the information.

2.2.4.2 Information Transfer and Signalling Rate

The Information Transfer Rate is expressed in bits per second.

If 10 bits are transmitted in 2ms, the information transfer rate (often called the bit rate) is 5000 bit/s. This is normally written as 5 kbps, or 5 kb/s.

The Signalling Rate is expressed in Baud. This is the rate at which transmission changes occur. The unit (baud) is named after J.E.M. Baudot and the signalling rate is often called the baud rate.

In a binary system, the bit rate and the baud rate are the same.

In a system of greater than two choices per symbol the baud rate (signalling rate) will be different from the information transfer rate (bit rate). For example, in a system with four choices per symbol, 2 bits are required to specify each choice, thus the bit rate will be twice the baud rate.

2.2.5 Practical 1 : Information

This practical introduces the concept of information and how it may be represented and conveyed in binary digital form.

A communications system is used for communicating information, which originates from an information source.


53-002S 2-9

Information is that which is conveyed

The meaning, or lack of meaning, of the information is not important when dealing with communications systems, only the quantity of information and its integrity are important.

No information is transmitted by a continuous symbol.

Information requires change

The simplest information system is binary, where the information is contained in the choice of one state out of two possible states, normally given the symbols 1 and 0.

These are binary digits (often shortened to ‘bits’). One bit is the smallest division of binary information.

The bit is a division of information and the choice of 1 or 0 constitutes the information.

2.2.5.1 Procedure

Set the PCM level control (4) and the PCM bandwidth control (5) to maximum. Set all the other controls to their mid positions. Consider the information sources shown below. Monitor their signals using the oscilloscope. Click on the Change Rate button to change between sources.

Answer the questions provided. You will need to make some observations in order to answer them. When you have answered the questions, proceed to the next practical.


2-10 53-002S

2.2.5.2 Questions

1. If the low state of a signal represents 0 and the high state 1, what form of information system does this represent?

2. What information is being carried by the signal with rate 1?

3. Does the signal with rate 2 carry any information?

4. Does the signal with rate 3 carry any information?


53-002S 2-11

2.2.6 Practical 2: t & f Domain

In this practical you will investigate the rate of information transfer and the relationship between this and the frequency spectrum required.

Theoretically, a square waveform has a frequency spectrum that contains the fundamental frequency and all its odd harmonics up to infinity.

The magnitude of the harmonic components decreases as the order goes up; eg, the component at the third harmonic is larger than that at the fifth, and so on.

In practice, the number of harmonics that are considered to have a significant amplitude is limited to about nine, however, to convey a really sharp-edged square waveform requires a very wide bandwidth.

2.2.6.1 Information Transfer Rate

The number of binary digits (bits) that is transmitted in unit time (normally one second) is known as the Information Transfer Rate.

This is normally expressed in bits, or kilobits, per second.

Another term often used for this is the Bit Rate.

2.2.6.2 Signalling Rate

The Signalling Rate specifies how fast the signal states change in a communications channel.

This is normally expressed in Bauds. One baud is a rate of one change per second.

Another term often used for this is the Baud Rate.


2-12 53-002S

2.2.6.3 Procedure

The information sources below are the same as in the previous practical. Monitor their signals using the oscilloscope and spectrum analyser (use the buttons to change instruments).

Pay particular attention to the bandwidths of the three signals when they are displayed on the spectrum analyser.

Answer the questions for this practical.

You will need to return to the practical and make some measurements in order to answer them. Then proceed to the next practical.


53-002S 2-13

2.2.6.4 Questions

1. What frequency components are present for the signal of rate 1?

2. Measure the period of the waveform of rate 2. What is the bit rate of this signal and in what units is it measured?

3. What is the bit rate of the waveform of rate 3?

4. How does this relate to the frequency components of the waveform as seen on the spectrum analyser?

5. Look at the spectrum on the large spectrum analyser. At about 160kHz, how do the magnitudes of the components of the signals at rates 2 and 3 compare?


2-14 53-002S

6. Does the bandwidth required by the signals depend on the bit rate?

7. Click on Rate 2 and turn the PCM bandwidth control (5) to minimum. Notice the effect on the waveshape as seen on the oscilloscope. Change to the spectrum analyser and adjust the PCM bandwidth. Observe the changes in the spectrum. Does a waveform with fast edges require a wider bandwidth than one with rounded edges, or vice versa ?

8. Explain your answer to question 7.


53-002S 2-15

2.2.7 Practical 3: Pulse Train Spectrum

In this practical you will investigate the spectrum of, and the bandwidth required by, a pulse train.

In digital communication the transmission of a regular square waveform is not particularly common. It is far more usual to have a train of pulses representing a string of binary data.

The previous practical shows that a square waveform has many frequency components and requires a significant bandwidth to convey it.

A pulse train has a spectrum with many components also.

The frequencies of these components and the bandwidth required to convey the signal depend on the pattern of the bits in the pulse train.

The pattern that requires the greatest bandwidth is that where alternate bits change state.

This is effectively a square waveform.

The fundamental frequency of this waveform will depend on the transmission format of the data, e.g. whether it is in RZ (return-to-zero) or NRZ (non-return-to-zero) format.

Formats for digital transmission are covered in detail on the Digital Data Formatting workboard, 53-150.

Because the RZ format has twice the number of transitions compared with the NRZ format for a pattern of alternate 1's and 0's, the bandwidth required will be twice as much.

A practical communications channel has to be capable of conveying any pattern of data transmitted, thus the worst-case of a square waveform is normally used to determine the bandwidth required.

The absolute minimum bandwidth of a channel is theoretically the fundamental frequency of the square waveform.

This would only pass the fundamental frequency component of the waveform, from which the square data waveform could be recovered.

In practice, the bandwidth used is greater than this absolute minimum.


2-16 53-002S

2.2.7.1 Procedure

Consider an information source, as shown below. Monitor its signal using the oscilloscope.

Ensure the PCM bandwidth control (5) is at maximum and alter the transmitted bit pattern to the patterns given below using the button block provided. A red light indicates a bit is in the 1 state.

Bit

7 6 5 4 3 2 1 0

1 1 1 1 1 1 1 1

1 0 1 0 1 0 1 0

1 1 0 0 1 1 0 0

Observe their signals on the oscilloscope and the spectrum analyser and answer the questions.


53-002S 2-17

2.2.7.2 Questions

1. Do the frequency components of the spectra vary as the bit patterns are changed?

2. How are the frequency components of the spectra related to the patterns 2 and 3 set up?

3. Set up the patterns 10000000 and 011111111. Do the spectra for these patterns differ?

Can you explain why this is?

4. Which of the bit-patterns investigated occupies the greatest bandwidth?


2-18 53-002S

2.3 PCM Encoding Assignment

2.3.1 Objectives

On completion of this assignment you will be:

• Digital codes, including Morse code, 5-bit Baudot, ASCII and BCD,

• Asynchronous transmission and the need for start and stop bits,

• Coding efficiency,

• Synchronous transmission, synchronisation words and bits.

2.3.2 Practicals


• Practical 1: Codes Simulation

• Practical 2: Asynchronous Link

• Practical 3: Synchronous Link





• Noise Generator








• Filter


53-002S 2-19

2.3.4 Theory

2.3.4.1 Codes

Digital communications codes predate the harnessing of electricity. The lighting of beacons all over the south coast of England in the 16th century to warn of the coming of the Spanish Armada was an example of a binary system of digital communications.

The first well-known electrical form of digital communications was probably that invented by Samuel Morse for the transmission of information across the telegraph network of the United States in the 19th century.

The Morse code is also an example of a binary digital code, but one which uses combinations of long and short characters (dashes and dots) to distinguish the letters.

There are Morse code combinations for the letters and numbers and for the normal punctuation symbols used.

These combinations are not of equal length, but vary from one short character (e) to five long characters (0), or six characters (in the case of most of the punctuation symbols).

More modern codes have equal length characters (bits) and have the same number of characters in each combination (word). Examples of this are the 5-bit code of J.M.E. Baudot, which became the CCITT-2 code used on teleprinter, and the ASCII code, which is very commonly used in computing.

Another commonly used code is the binary coded decimal, BCD, code and its extended version EBCDIC.

ASCII is a 7-bit code, with an eighth often added for parity reasons which will be investigated at a later stage, and EBCDIC is an 8-bit code.

2.3.4.1.1 Asynchronous Transmission

Asynchronous transmission is used when the data to be transmitted comes from a source that produces it at random intervals: for example the communication between a VDU terminal and the server.

With this form of transmission the receiver has to be able to resynchronise at the start of each new character received.

To allow the receiving equipment to determine the start and end of each character the code for each character is preceded by a bit to indicate its start and succeeded by one, or more, bits to indicate the finish.


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These are called START and STOP bits.

Probably the most used form of code for asynchronous transmission is the ASCII code.

This code has 7 data bits, often an added parity bit (see Assignment 6 on Error Detection), one start bit and usually two stop bits.

These extra bits are often added to the data automatically by an integrated circuit called a UART (a Universal Asynchronous Receiver/ Transmitter).

Because extra bits are added for synchronisation the ratio of data bits to the total number of bits is lower and thus coding efficiency is reduced.

2.3.4.1.2 Synchronous Transmission

An application which transmits data at a more regular rate, or at a high speed, may use a synchronous mode of transmission. With this form of transmission, once the receiver is synchronised to the transmitter data clock it will remain in synchronism for a significant period of time.

Start and stop bits are not necessary with synchronous transmission.

Obviously synchronisation must be established before the start of data transmission but once this is achieved it is maintained by locking the receiver clock to the transmitted data bit timing.

This is known as bit clock recovery.

2.3.5 Practical 1 : Codes Simulation

There are many codes used in digital and data communications. Examples of these are:

• Morse Code

• CCITT-2 code

• ASCII code

• BCD and EBCDIC codes

The Morse Code was an early form of binary digital code developed for telegraphic applications. It has two lengths of symbol and variable length characters. It was extensively used for manual information transmission but was also automatically machine generated.

The 5-bit Baudot code, which became the CCITT-2 code, was developed for automatic teleprinter message transmission. It has control characters in addition to the letters, numerals and punctuation characters. It was specifically developed for automatic transmission and reception.


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The ASCII code (American Standard Code for Information Interchange) is a 7-bit code. Again, it has control characters in addition to the letters, numerals and punctuation characters. It is a commonly used code for data transmission and computer communication.

The Binary Coded Decimal code (BCD) is used in computers and instrumentation and control systems, especially where numerical values and calculations are required. It is used extensively when analogue values are to be represented in digital form (e.g. in analogue to digital conversion).

The Extended Binary Coded Decimal Interchange Code (EBCDIC) is used as a communications code.

Go to the Codes Simulation Practical to investigate various codes.

2.3.5.1 Procedure

Consider the coding of the letter shown below. Look at the signals using the display provided. Change the letter using the arrows and click on the code boxes, or use the Code Buttons to change between codes. Observe the different codes used. The Zoom In and Zoom Out buttons can be used to expand and contract the display.

Answer the questions in the relevant section. You will need to return to the simulation and make some changes in order to answer them.


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2.3.5.2 Questions

1. Are the code patterns for the Morse letters of the same length?

2. Are the code patterns for the ASCII letters of the same length?

3. Why do you think that the Morse code is not particularly suitable for automatic, high speed data transmission?

4. To what unit associated with data transmission did J.M.E. Baudot give his name?


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2.3.6 Practical 2: Asynchronous Link

In this practical you will investigate asynchronous transmission links, the requirement for start and stop bits in addition to the data and the loss in coding efficiency that results.

In an asynchronous transmission of data, if there is no means of indicating where one character or word stops and the next one starts the resultant received data could well be undecipherable.

Consider binary data being transmitted:

.....1100101010011001001110110111010100111010100110.....

and suppose each data word is known to be ASCII of seven bits long. Without any indication where the start of each word is, the data may well be decoded in error.

For instance, it could be:

.... 1100101 0100110 0100111 0110111 0101001 1101010 0110...

.... S 2 r v J + .....

or it could be:

.....11 0010101 0011001 0011101 1011101 0100111 0101001 10.....

.. T L \ ] r J .....

There is no way of knowing which, if either, is correct.

To be sure that the data is decoded correctly extra bits to designate where the words start and stop must be added.

Not unreasonably, these are known as Start and Stop bits!

The state of the start and stop bits is always different. This ensures that there is always a transition between successive characters, irrespective of the bit pattern of the characters themselves.

The first transition after an idle period is used to determine the start of a new character.

The next 'n' bits will be known to be data, then comes the stop bit(s). The system will now be in the state waiting for the next start bit to occur.


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2.3.6.1 Coding Efficiency

This is a measure of the quantity of information that is transmitted as a ratio of the total number of bits.

With the 7-bit ASCII code all the combinations of bit patterns are used for either alphanumeric characters, control characters or other symbols. The efficiency of this code is thus 100%.

An eighth bit is often added for parity checking. This carries no information (other than allowing an error to be detected) and thus degrades the efficiency.

The efficiency will be 7/8 x 100 % = 87.5%

If a start and two stop bits are added, as is common with the ASCII format for asynchronous systems, a further reduction in efficiency results.

The efficiency will now be 7/11 x 100 % = 63.6%

This means that only two thirds of the time will be used to transmit information.

In fact the control characters associated with the ASCII code are not truly carrying information. This will further reduce the efficiency.

In applications where high speed data transfer is not required, for example: communication between a keyboard and a PC, this low efficiency is not significant.

However, in an application such as the transmission of large amounts of data at high speed via a satellite link, an efficiency of only just over 50% would be totally unacceptable.

It is for applications such as this that synchronous transmission is more suitable.


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2.3.6.2 Procedure

On the workboard, set controls PCM level (4) and PCM bandwidth (5) to maximum and all others to their mid-positions. Drag the probe to monitor point 8 and adjust Data 0 (9) control on the workboard to give the best received square wave, as seen on the oscilloscope. The letter below is ASCII coded. Click on the arrows to alter the letter transmitted and observe the letter received.

Answer the questions provided. You will need to make some observations in order to answer them.


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2.3.6.3 Questions

1. Make sure that the start/stop bits are removed. Does the received letter correspond with the transmitted one? Try it for a number of different letters.

2. Why do you think this is?

3. How does the pattern of red lights received compare with the pattern transmitted? Can you see any similarity between them?

4. The red lights and the display showing the transmitted waveform use the normal convention of least significant bit on the right. The word is transmitted down the link LSB first. Why does the oscilloscope show the waveform with the LSB on the left?

5. Add the start/stop bits. Observe these on the transmitted waveform display and on the oscilloscope. Does the received letter correspond with the transmitted one now? Try it for a number of different letters.

6. What is the main disadvantage of having start and stop bits added to the data bits?


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2.3.7 Practical 3: Synchronous Link

In this practical you will investigate synchronous transmission links, the requirements for synchronisation and clock recovery and the improvement in efficiency that synchronous transmission has over asynchronous.

As has been seen, asynchronous transmission is normally used where the rate at which characters is generated is indeterminate, slow, and when there may be considerable idle time between characters.

For high speed transmission of considerable blocks of information, for example: computer-to-computer communications, asynchronous transmission, with its large proportion of start and stop bits, etc, is too inefficient.

Synchronous transmission methods are normally used.

With synchronous transmission, the digital information is broken into blocks, known as frames. Each frame is then transmitted as a single bit stream, with no breaks in-between characters.

Information to allow the system to synchronise, so that the received data can be decoded, is obviously still required. However, as the frame length may be several hundred bits, this synchronising information is not transmitted as frequently as for the asynchronous case. The efficiency is thus considerably increased and the throughput of information is speeded up.

Just like the asynchronous character has start and stop bits, the frame in synchronous transmission may have a start-of-frame character(s) and end-of-frame character(s) and other synchronisation bits or characters.

Two forms of synchronisation are required for the receiver to be able to decode the information correctly:

• bit synchronisation

• frame synchronisation

The receiver must be in bit synchronisation with the data so that the data bits are read at the correct time (ideally in the middle of the bit-period).

The receiver must also be in frame synchronism with the data so that the frames are decoded correctly.

Synchronisation must be maintained at all times with a synchronous link for proper decoding of data.


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There are many synchronising protocols that may be used to accomplish this but the general procedure is for a sync (short for synchronisation) word to be sent before the start of data transmission. This character is recognised by the receiver and is used to synchronise its clock (known as bit clock recovery) and also to indicate that the data is following.

In this Practical, you can set the sync word. The effectiveness of synchronisation will depend on the pattern of bits that you choose.

Ideally, the sync word bit pattern should be unique and not be like any data word pattern.

If this is so, synchronism will be maintained for all data word combinations.

If the sync pattern is similar to a data word pattern the system can mistake the sync word timing.

For example:

Consider the ASCII coded data AAAAAAA.. being sent with a sync word 01010101 between the second and third A's.

The ASCII code for A is 01000001.

This gives a bit stream of:

A A sync A

01000001 01000001 01010101 01000001

(of course, in practice there are no gaps between words; they are just shown here to make the explanation clearer).

The system is looking for the sync word pattern of 01010101 to give synchronisation.

The first time this pattern occurs is just before the end of the second A and the system will synchronise on this.

This will then give:

A A sync P P....

01000001 010000 01010101 01010000 01.......

and a series of P's will be received.

This is obviously an error and is undesirable.


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This effect is called aliasing.

The sync word does not have to be the same length as the data words and, in general, the longer the sync word is, the less likely aliasing is to occur. However, the coding efficiency is lowered with long sync words.

An alternative method of sending a sync word is by using one bit of each data word and sending the sync pattern sequentially on that bit.

This may be referred to as sync on one bit.

With this form of synchronisation the sync information and the data are effectively less likely to be confused one with the other by the system and thus aliasing seldom occurs. However, efficiency is low as one bit per data word is used for sync purposes.

2.3.7.1 Procedure

Consider an information source, as shown below. Monitor its signal using the display and the oscilloscope. Set the PCM bandwidth (5) control to maximum, Noise level (6) control to minimum and Data 0 (9) control for data at monitor point 8, all others to their mid positions.



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2.3.7.2 Questions

1. Ensure that the transmitted letter is A. Set the sync word to be 00011010 Is the received letter the same as the transmitted one?

2. Why do you think that the display of the received word is flashing between two letters?

3. Are these received letters related to the required transmitted letter and the sync word?

4. Reset the transmitted letter to A and set up the sync word 01010101. Are the received letters now related to the required transmitted letter and the sync word?

5. Why do you think this is so?


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6. Change the transmitted letter. Is the received data now the same as transmitted?

Experiment with different sync words and notice that some give good performance and some poor with respect to aliasing.

7. Change to Sync on One Bit. The synchronising word is transmitted using the right-hand bit. Set up the sync word to be 00011010. Does the right-hand bit in the received pattern change?

8. Set the transmitted letter to A. How many different letter patterns are received, and why?

9. Change the sync word pattern. Is there any noticeable difference in the received patterns?

10. Change the sync word pattern to all zeros. How many different received patterns are now visible? Try this again with all ones for the sync word.


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2.4 Introduction to Noise Assignment

2.4.1 Objectives


• The concept of noise,

• Noise in a digital communications system,

• Signal-to-Noise Ratio.

2.4.2 Practicals


• Practical 1: Noise

• Practical 2: Noise on a Link

• Practical 3: Signal to Noise Ratio





• Noise Generator








• Filter


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2.4.4 Theory

2.4.4.1 Noise

A definition of noise in electronic systems may be unwanted electrical signals that occur in electrical or electronic devices, circuits or systems resulting in spurious signals occurring at the output.

Noise can come from many sources. It may be man-made, or natural.

An example of man-made noise is the unwanted signals that are radiated from a car ignition system, or from a faulty insulator on a overhead electricity power distribution system.

An example of natural noise is that which emanates from the sun as a result of the violent reactions and radiations that occur there.

Noise occurs randomly. It may be classified into two types:

• white noise, and

• impulse noise

White noise has a wide, flat frequency spectrum. Impulse noise is due to a single, or a series of momentary disturbances, and may have specific frequency components with very different magnitudes.

For the examples given, ignition noise is one of impulse noise, whereas that from the sun is white noise.

In a communications system, the total effect of noise is a function of the frequency used, the location, the type of antenna and receiver used, the weather conditions, etc, etc.

2.4.4.1.1 Effects of Noise

Noise in a digital communications system has a very different effect than in an analogue one.

In a digital system only fixed signal levels are allowed. When noise is present in the system the receiver must decide which of the allowed levels the noisy signal represents.

If the decision is correct the noise has no effect on the received signal.

If the decision is made incorrectly the results can be catastrophic.


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The theory of this decision is complex and requires statistical analysis methods to predict whether it will be made correctly, or not.

It is not possible to predict the instantaneous amplitude of a noise voltage at any given instant. But it is possible to predict the probability that the amplitude of the noise waveform will exceed a given value. To do this the shape of the amplitude distribution function must be known. This function is the relationship between the probability density function and the amplitude of the noise voltage.

It can be shown mathematically that for white noise this function has a Gaussian distribution (basically bell-shaped), and that the probability of making an error in the decision of the state of a noisy signal can be expressed as

Pe = 0.5(1 - k [A / (2 (2s)0.5) ] )

where Pe is the probability of a decision error,

A is the peak signal pulse voltage,

s is the rms noise voltage.

k is a constant, known as the error function.

This shows that the error probability is proportional to the ratio of A to s

This ratio is known as the signal to noise ratio and is usually quoted in decibels (dB).

The probability of errors occurring increases as the signal to noise ratio decreases.

If practical values are put into the expression for Pe it is found that when the signal to noise (S/N) ratio is greater than about 18dB, the probability of errors occurring drops very rapidly with increasing S/N ratio, whereas with a S/N below about 14dB the probability changes much more slowly with S/N ratio.

Typical values may be Pe = 10-2 for 14dB S/N

Pe = 10-4 for 17.5dB S/N

Pe = 10-8 for 21dB S/N

An error probability of 1 in 105 (Pe = 10-5) is considered acceptable for a practical data transmission system.


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2.4.4.1.2 Signal to Noise Ratio

The signal to noise ratio, which is normally quoted in dB, is generally a ratio of voltages (though it can be expressed as a power ratio). It is given by the expression:

S/N = 20log10(Vs/Vn)

where Vs is the signal voltage and Vn is the noise voltage.

2.4.5 Practical 1: Noise

This practical introduces the concept of noise. You will see how noise is random and how the magnitude of noise is shown in both the time and frequency domain.

With any electrical or electronic system, including telecommunications systems, there will always be signals present that cause unwanted, spurious signals to be present at the output.

These signals are generally referred to as Noise.

Noise can come from many sources. It may be man-made, or natural. For example: noise radiated from a television timebase, or from a sparking contact breaker is man-made, whereas noise radiated from the Sun, or due to ionisation in the atmosphere is natural in origin.

Noise occurs randomly. It may be classified into two types:

• White noise

• and Impulse noise.

White noise has a wide, flat frequency spectrum. Impulse noise is due to a single, or a series of momentary disturbances, and may have specific frequency components with very different magnitudes.

For the examples given, the sparking contactor is one of impulse noise, whereas that from the Sun is white noise.

The noise source in the workboard you are using gives an output approximating to white noise.

If white noise is observed on an oscilloscope its voltage at any instant is not predictable. It is totally random.

Also it has a fairly flat frequency spectrum and the amplitudes of the components are unpredictable and random too.


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2.4.5.1 Procedure

Consider the noise source shown below. Set the Carrier level control (3) to minimum and all other controls to their mid positions. Monitor the source output using the oscilloscope and the spectrum analyser.

Answer the Questions associated with this Practical. You will need to return to the practical and make some measurements in order to answer them.


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2.4.5.2 Questions

1. Observe the output of the noise source on the oscilloscope. Ensure that the Carrier level (3) is at minimum and adjust the Noise level control (6) to mid position. Is the amplitude of the noise constant with respect to time?

Use the large oscilloscope to estimate the average amplitude of noise voltage.

Adjust the Noise level control (6) and verify that the amplitude of the noise changes.

2. Observe the output of the noise source on the spectrum analyser. What is the form of the spectrum of the noise?

3. To what type of noise does this source approximate?


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2.4.6 Practical 2: Noise on a Link

In this practical you will investigate the effects of introducing noise onto a digital communications link. You will see that there is a definite threshold below which the noise has little or no effect, but above which the effects are catastrophic.

To illustrate the typical effects of noise on a digital communications link a triangle-wave input signal is used in this practical. This is, of course, an analogue waveform. It must be converted into a digital bitstream before it can be transmitted down the link.

This is done by an analogue-to-digital (A/D) converter.

The digitised waveform is fed to the transmitter (Tx) for transmission down the link.

For this practical the link is a synchronous baseband PCM link, though similar effects would be found irrespective of the type of link used. In later Assignments the effects will be investigated in more detail.

The received data is decoded in the receiver (Rx) and converted back to an analogue waveform using a digital-to-analogue converter (DAC).

The received and converted waveform can then be compared with the original triangle waveform. Without any noise injected into the system the two waveforms should be very similar, showing that the link is operating correctly.

Noise can then be injected onto the link and its effects seen by investigating the waveforms.

When a bit transmitted down a digital link is received by the receiver the first operation that must be carried out is to determine whether that bit is a 1, or a 0.

If there is no noise present on the link the decision as to the state of the bit will normally be made without error. It is just as if there is a short length of wire connecting Tx and Rx.

The circuits in the receiver will have logic gates whose characteristics are such that there is a range of input voltage over which it is deemed to be 'logic 0' and another range for 'logic 1'. These ranges will be reasonably wide in practice.

If a small amount of noise is present on the link the decision is still made without error as the noise is not enough to go outside these ranges and spuriously switch the input logic gate.

If more noise is present there will come a point when the instantaneous amplitude of the noise is high enough to switch the state of the gate in error. This will cause a logic 1 to be registered instead of a logic 0, or vice-versa. This is, of course, an error.


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If this error occurs in a data bit, the result will be a wrong data word received. With the triangle wave that is being used for this practical this is noticeable as a glitch, or spike on the waveform.

If the error occurs in a sync word the link may lose synchronism briefly until the next sync word is recognised, when the link will resynchronise. The result may be a period where the output waveform is not correct.

If the magnitude of the noise is sufficient, these spurious switchings become so frequent that data and synchronism are permanently lost and the decoded data is totally corrupted.

2.4.6.1 Procedure

In this practical a triangle wave input signal is digitised and transmitted down a pcm link. Use the oscilloscope to monitor the signals down the link. Initially, set the Noise level control (6) to minimum, the PCM bandwidth control (5) to maximum, Data 0 (9) and Bit clock (7) controls for synchronism and all other controls to their mid positions.

The trace initially shown on the oscilloscope at monitor point 21 is the digital representation of the triangle wave. Move the probe to monitor points 20 and 27 to see the input and output signals.



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2.4.6.2 Questions

1. Ensure that the noise source is off. Trace the signal through the link with the oscilloscope. The Data 0 (9) and Bit clock (7) controls may have to be adjusted to obtain a received signal. These set receiver threshold and bit clock recovery. Is the received and decoded waveform (monitor point 27) of the same shape as the transmitted one?

2. Switch in the noise source. Look at monitor point 27 with the oscilloscope and slowly increase the Noise level control (6) until the waveform just shows signs of breaking up. Then decrease the noise level very slightly. The controls may have to be re-adjusted to obtain a received signal. Look at point 10 with the oscilloscope. Switch the noise source in and out. Does the signal at point 10 change significantly?

3. Look at point 20 with the oscilloscope. Switch the noise source in and out. Ignoring the change in amplitude of the signal (which is a because the workboard hardware can only deal with a finite maximum amplitude of signal+noise, so the signal amplitude has to be lowered when noise is applied), does the shape of the signal at point 20 change significantly?

4. Can you identify the wanted digital signal at point 20 when the noise is switched on? (Hint: The large oscilloscope gives a better display) Re-connect the oscilloscope to point 27. Increase the Noise level control very slowly and observe the triangle waveform breaking up. Does this break-up occur progressively, or suddenly?

5. What is the reason for this?

6. Now, can you identify the wanted digital signal at point 20 when the noise is switched on?


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2.4.7 Practical 3: Signal to Noise Ratio

In this practical you will investigate the concept of signal to noise ratio as applied to digital communications. A knowledge of the meaning of ratios expressed in decibels (dB) is assumed.

As the name suggests, this is a ratio, normally expressed in decibels (dB), relating the signal voltage to the noise voltage.

It is given by the expression:

S/N = 20log10(Vs/Vn)

where Vs is the signal voltage and Vn is the noise voltage.

The shape of a digital waveform is that of square pulses. The amplitude of the pulses is usually taken to be the peak-to-peak voltage of the pulses. To get the correct ratio for S/N, the noise amplitude must be taken as peak-to-peak, also:

ie, S/N = 20log10(Vs pk-pk/Vn pk-pk)

In this practical you will measure the pulse amplitude and then inject noise and estimate the amplitude of the noise. The S/N can then be calculated.

Because the noise is approximately white noise, its instantaneous amplitude varies randomly. The measure of peak-to-peak amplitude you take will only be an approximate average value. In fact, taking a statistical average of amplitude over a period of time is the most common way to measure noise.


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2.4.7.1 Procedure

Consider an information source with noise added, as shown below. Monitor its signal using the oscilloscope. Turn the Noise level control (6) to minimum, the PCM bandwidth control (5) to maximum and all other controls to their mid positions.



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2.4.7.2 Questions

1. Ensure that the Noise Level control is at minimum.

What is the peak-to-peak amplitude of the square wave?

2. Adjust the Noise Level control to approximately one third scale. Observe the noise on the signal.

What is the average peak-to-peak amplitude of the noise component.

This may be difficult to measure accurately, however, an estimate can be made.

3. What is the approximate signal-to-noise ratio, in dB, for the above settings?


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2.5 Noise on a Digital Link Assignment

2.5.1 Objectives


• Some basic data formats used in digital communications,

• Performance of these formats with respect to noise on a link,

• Some basic keying methods used,

• Performance of these with respect to noise on a link,

• Comparison of the methods,

• Effects of noise on a digitised picture transmitted over a digital link.

2.5.2 Practicals


• Practical 1: Data Link

• Practical 2: Keying

• Practical 3: Picture Transmission





• Noise Generator








• Filter


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2.5.4 Theory

2.5.4.1 Noise on a Link

Ensure that you have read the theory for the ‘Introduction to Noise Assignment’ before you proceed further.

Errors on a digital link are predominantly caused by noise that is added during the transmission of the signals; ie, the signal at the receiver comprises the wanted digital signal plus the noise. The noise may be white noise, impulse noise, quantisation noise, but may all be considered as added together to give a total noise component.

Generally, the noise performance of digital systems is expressed in terms of the probability of an error occurring, rather than in terms of signal-to-noise ratio. This is investigated more quantitatively in the Bit Error Rate assignments.

When binary pulses are being received in a digital communications system there is a possibility that noise will cause an error in the decoding of the signal.

For instance, if the transmitted signal is at the binary 0 level and a noise spike of sufficient amplitude occurs, making the received signal momentarily a binary 1 level then an erroneous 1 will appear at the decoder output of the receiver.

The probability of errors occurring may be calculated using decision theory mathematics.

The most commonly used statistical model for additive noise in a digital communications system is:

ø2 is the noise variance, and may be measured either digitally, or by a power meter with a long time-constant.

This model approximates to a Poisson distribution, with the typical bell-shaped distribution curve, with its peak at n = 0 and standard deviation ø.

It can be shown that there is an optimum decision level, i.e. one that gives a minimum error probability, and that it is given by:

dopt = (ø2/A).ln(P0/P1)

where P0 is the probability of transmitting a 0 and P1 is that of transmitting a 1.

A is the pulse amplitude.


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In a practical system, given a reasonable length of time, the probability of transmitting 0s and 1s will be about equal, giving P0/P1 = 1.

Thus, as ln of 1 = 0, dopt = 0.

In practice, this means that the optimum detecting voltage to determine whether the received bit is a 1, or a 0, is half way between the two expected voltage levels for the two states. Intuitively, this might seem obvious, however, if the received signal is known to contain substantially more 1's than 0's for instance, then the optimum decision level for minimum errors would not be in the middle.

2.5.5 Practical 1 : Data Format

This practical examines the effects of noise on a link as a function of the formatting of the data. Unipolar Return-to-Zero (RZ), Split Phase (biphase) and Bipolar RZ formats are investigated

There are a number of different data formats that are used in digital communications.

Some common ones are:

• unipolar non-return-to-zero (NRZ)

• unipolar return-to-zero (RZ)

• unipolar biphase (split phase, or Manchester)

• bipolar NRZ

• bipolar RZ

• ternary (alternate mark inversion, AMI)

These are dealt with in more detail on the Digital Data Formatting workboard 53-150.

In this practical three of these formats are investigated with respect to their performance with noise.

The three formats are: unipolar RZ, split phase and bipolar RZ.

As explained in the Introduction to Noise assignment, noise has an effect on a digital signal when an error is made in the decision as to the state of any received bit.

The requirements of a receiver in order that it can successfully receive and decode data are:

• the correct decision must be made as to the state of each bit received

• the timing information for clock synchronisation must be derived.


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Of these, the requirement to retrieve timing information is probably the more critical.

If noise causes a decision error in a data bit this will result in the wrong data being received. However, there are methods for detecting and correcting such errors.

These are investigated in more detail in the Error Detection and Bit Error Rate assignments.

If noise causes loss of the timing information the bits may not be sampled at the correct times, thus the decoded data will be totally incorrect. Or the word or frame synchronism may be lost, with a similar result.

Error correction methods will not cope with such catastrophic failure.

2.5.5.1 Procedure

The data source is a triangle waveform which is digitised and may be formatted by clicking on the Change Button. Set the PCM bandwidth control (5) to maximum, the Noise level control (6) to minimum and all other controls to their mid positions. Monitor the signals using the oscilloscope.

The trace initially shown on the oscilloscope at monitor point 21 is the digital representation of the triangle wave.

Move the probe to monitor point 27 to see the transmitted signal. You will have to change to the large oscilloscope and alter the timebase to see the trace more clearly.



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2.5.5.2 Questions

1. Ensure that the noise is switched out. Monitor point 27 and adjust the Data 0 control (9) and Bit clock control (7) to obtain the correct received waveform (Triangle wave). Switch between the three formats and observe the waveform at point 27. Are there any significant differences between the received waveforms for the different formats without noise present?

2. Trace the signal through the link. Switch between the three formats and notice the differences in the digital waveforms. Are the waveforms as you would expect for the three formats?

3. Ensure that the Noise level control (6) is set to minimum. Select the RZ format and add noise to the link. Slowly increase the Noise level control until the received waveform (at monitor point 27) just starts to break up. Note the position of this control. Now switch to the Split Phase format. Is the Split Phase received waveform more, or less, effected by noise than the RZ one? Re-adjust the Noise level control for just break-up of the received waveform and compare the position of this control with that for RZ to answer this question.

4. Repeat the procedure used for Question 2, but with Bipolar RZ format. You may have to readjust controls (8) and (9) to maintain synchronisation. Is this format more, or less, effected by noise than the others?


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2.5.6 Practical 2: Keying

This practical examines the effects of noise on a link as a function of the modulation method used. There are three basic types of modulation that can be applied to a carrier: amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM).

In digital communications modulation is often referred to as keying. This gives rise to:

Amplitude shift keying (ASK), which is amplitude modulation by a digital waveform

Frequency shift keying (FSK), which is frequency modulation by a digital waveform

Phase shift keying (PSK), which is phase modulation by a digital waveform

This Assignment investigates the effects that noise has on signals using these modulation methods.

2.5.6.1 Ampliude Shift Keying

This is an example of amplitude modulation: the amplitude of the signal being dependant on the state of the modulating bit.

The effect of noise is generally an additive one, where the noise voltage is added to the signal voltage.

This means that the noise may substantially change the amplitude of the received signal.

As it is this amplitude which is detected and thus determines the decision as to the state of the received bit, changes in the amplitude due to noise can easily cause errors.

Amplitude modulated systems are particularly susceptible to errors due to noise.

2.5.6.2 Frequency Shift Keying

This is an example of frequency modulation: the frequency of the signal being dependant on the state of the modulating bit.

In a frequency modulated system the amplitude of the signal is relatively unimportant, providing it is sufficient to be received and cause limiting in the receiver. Thus these changes will not cause significant errors.

The noise will have components that cause changes in the timing of the transitions of the carrier and thus effectively changes in the instantaneous frequency of the signal.


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These changes are small compared with the amplitude changes, thus frequency modulated systems are much more immune to noise effects than amplitude modulated systems.

2.5.6.3 Phase Shift Keying

This is an example of phase modulation: the phase of the signal being dependant on the state of the modulating bit.

In a similar way to frequency modulation, in a phase modulated system the amplitude of the signal is relatively unimportant, providing it is sufficient to be received and cause limiting in the receiver. Thus these changes will not cause significant errors.

The timing changes noted for the frequency modulated case will also give changes in the instantaneous phase of the signal.

These changes are small compared with the amplitude changes, thus phase modulated systems are also much more immune to noise effects than amplitude modulated systems.


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2.5.6.4 Procedure

The digitised triangle wave data source is Split Phase formatted and the keying method may be chosen by clicking on the Change box. Set the PCM bandwidth control (5) to maximum, the Noise level control (6) to minimum and all other controls to their mid positions. Monitor the signals using the oscilloscope.

The trace initially shown on the oscilloscope at monitor point 19 is the digital representation of the triangle wave.

Move the probe to monitor point 27 to see the transmitted signal. You will have to change to the large oscilloscope and alter the timebase to see the trace more clearly.



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2.5.6.5 Questions

1. Ensure that the noise is switched out. Adjust the Carrier level control (3) to mid-position. Monitor point 27 and adjust the Data 0 and 1 controls 8 & 9 to obtain the correct received waveform (a triangle wave). Switch between the three keying methods and observe the waveforms at point 27. Are there any differences in the received waveforms without noise present?

2. Ensure that the Noise level control (6) is set to minimum. Select ASK and add noise to the link. Slowly increase the Noise level control until the received waveform just starts to break up. Note the position of this control. Now switch to FSK. Re-adjust the Noise level control for just break-up of the received waveform and compare the position of this control with that for ASK. Is the received waveform for FSK more, or less, effected by noise than that for ASK?

3. Repeat the procedure used for Question 2, but with PSK. Is this form of keying more, or less, effected by noise than the others?


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2.5.7 Practical 3: Picture Transmission

This practical examines the effects of noise on a link carrying data in the form of a digitised picture.

The link is a phase shift keyed, +/-90 degrees, with split phase formatted data.

Initially, without noise added, originating picture data is transmitted down the link and the received picture is built up pixel by pixel and line by line. This can be seen happening on the screen.

When a complete picture has been transmitted and received, the data is repeated automatically. This continues throughout the practical.

In practice, the picture data would not normally be transmitted more than once, unless errors were found and automatic requests for repeat transmission methods were being used. However, in this practical the continuously repeated transmission of data allows changes in the received data and the effects due to the noise to be seen more easily.

When noise is added the effects can be seen. Firstly, as the noise level is slowly increased, some pixel errors become evident but, with only slightly more noise present, the picture completely breaks up due to complete loss of data and synchronisation. The abrupt threshold typical of digital data transmission is readily seen.


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2.5.7.1 Procedure

The data source is a digitised picture of the Feedback building. This is transmitted through a PSK link and noise may be added. The effect of this noise can be seen on the received picture. Set the Carrier level (3) to its mid position, PCM level (4) and PCM bandwidth (5) controls both to maximum, the Noise level control (6) to minimum and all other controls to their mid positions.

Adjust the Data 0 control (9) to get a received picture and then the Bit clock control (7) to get the best resolution. Switch on the noise. Turn up the Noise level control (6) very slowly and

Teknikit Digital Communication - Student's Workbook...

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