A Seminar Report

On

PULSE TIME MODULATION TECHNIQUES

Submitted in partial fulfillment for the award of the Degree of

Bachelor of Technology in Electronics and Communication Engineering

by

Jithin R. J. (Roll No. EC04B081)

Department of Electronics Engineering

NATIONAL INSTITUTE OF TECHNOLOGY CALICUT NIT Campus P.O., Calicut - 673601, India

2008

CERTIFICATE

This is to certify that the seminar report entitled “PULSE TIME

MODULATION TECHNIQUES” is a bona fide record of the seminar presented by

Jithin R. J.(Roll No.EC04B081) during the seventh semester in partial fulfillment of

the requirements for the award of Degree of Bachelor of Technology in Electronics &

Communication Engineering from National Institute of Technology Calicut for the

year 2008.

Mr. Dhanaraj K. J. Dr. Lillikutty Jacob

(Seminar Coordinator) Professor & Head

Sr. Lecturer Electronics Engg. Dept.

Electronics Engg. Dept.

Place: Calicut Date: 01.02.2008

AACCKKNNOOWWLLEEDDGGEEMMEENNTT

First and foremost, I wish to express my sincere gratitude to my seminar co-

coordinator, Mr. Dhanaraj K. J., Senior Lecturer, Electronics and Communication

Department for his continuous guidance and support throughout the course. We

would also like to thank Dr. Lillikutty Jacob, Head of Department, for allowing me

to go ahead with my report. I would also like to thank all my friends for their whole-

hearted support and help in accumulating details for my report.

Jithin R. J.

ABSTRACT

The choice of modulation scheme is vital in obtaining high-performance

bandwidth-efficient fibre optic communication system. In this context, PTM

Techniques represent the best choice as compared to purely analogue or digital

methods. PTM can be defined as the general class of pulse-code modulation in which

the time of occurrence of some characteristic of the pulsed carrier is varied with

respect to some characteristic of the modulating signal. The characteristic can be their

position or frequency or width or any other easily distinguishable property, varied

according to the amplitude of the input signal. They also have an additional advantage

of not requiring a decoder in the receiver end. The PTM family is studied and their

potential for use in high-speed fibre systems intended for transmitting analog data is

examined.

CONTENTS

Chapter No TITLE Page no.

1. INTRODUCTION 1

2. PULSE WIDTH MODULATION 3

2.1 Modulation 3

2.2 Spectra of PWM 4

2.3 Demodulation 4

3. PULSE POSITION MODULATION 5

3.1 Modulation 5

3.2 Spectra of PPM 6

3.3 Demodulation 6

4. PULSE INTERVAL MODULATION 7

4.1 Modulation 7

4.2 PIM spectra 8

4.3 Demodulation 8

5. PULSE INTERVAL AND WIDTH MODULATION 9

5.1 Modulation 9

5.2 Spectra of PIWM 9

5.3 Demodulation 10

6. PULSE FREQUENCY MODULATION 11

6.1 Modulation 11

6.2 Spectra of PIWM 11

6.3 Demodulation 12

7. SQUARE WAVE FREQUENCY MODULATION 13

7.1 Modulation 13

7.2 Spectra of PIWM 13

7.3 Demodulation 14

8. APPLICATION 15

9. CONCLUSION 16

REFERENCES 17

LIST OF FIGURES

Figure No Title Page No.

2.1.1 PWM uniform sample generation 2

2.1.2 PWM output signal 3

2.2.1 Spectra of PWM 3

2.3.1 PWM demodulator for uniform sampling 3

3.1.1 PPM generation 4

3.1.2 PPM output 4

3.2.1 PPM spectra 4

4.1.1 PIM output signal 5

4.1.2 PIM modulator 5

4.2.1 PIM spectra 6

4.3.1 PIM demodulator 6

5.1.1 PIWM modulation 7

5.1.2 PIWM output 7

5.2.1 PIWM spectrum 7

5.3.1 PIWM to PIM converter 7

6.1.1 PFM generation 8

6.1.2 PFM signal 8

6.2.1 PFM spectra 8

6.3.1 PFM demodulation 9

7.1.1 SWFM signal 9

7.2.1 Spectra of SWFM 9

7.3.1 SWFM to DPFM 10

7.3.2 Spectra of DPFM 10

LIST OF TABLES Table No Title Page No.

1.1 The PTM Family 1

CHAPTER 1

INTRODUCTION

Optical fibre networks are being used to provide broadband

telecommunication services that utilize multiplexes of video, data and voice channels.

Therefore the choice of modulation technique used is of prime importance.

An analog Modulation scheme in which modulation is done in a continuous

manner, is both simple and band-width efficient, but often cannot deliver the required

signal-to-ratio. Besides, they suffer from the nonlinearity of the optical channel

causing crosstalk and intermodulation. On the other hand, Digital modulation

schemes such as Pulse Code Modulation (PCM) have been shown immune to channel

nonlinearity but they are expensive due to the requirement of coding circuitry.

PTM represents an alternative method to both of the aforesaid schemes and it

forms an intermediate between the two. The modulation scheme is simple, requiring

no coding, and the pulse form of the scheme makes the PTM immune to

nonlinearities of the channel.

Different types of PTM.

Type Variable

PPM Position

PWM Width(Duration)

PIM Interval(space)

PIWM Interval and Width

PFM Frequency

SWFM Frequency

Table 1.1 The PTM family.

All PTM techniques produce modulation spectra that share a common set of

features. Each modulation gives rise to spectra with diminishing set of side tones

centered around the carrier frequency and its harmonics separated in frequency by an

amount equal to the signal frequency. The number and the strength of side tones is a

characteristic unique to each PTM technique. In addition, a baseband component is

also present depending on the type of sampling employed.

Either uniform or natural sampling can be employed for PTM. In natural

sampling, the sampling instants may be varying but for uniform sampling, the input

signal is routed via a sample and hold circuit which produces flat-topped amplitude

modulated pulses and hence the PTM modulator is operating upon uniformly sampled

and stored samples.

In the demodulator, the particular PTM is converted into form having a

baseband component and filtering out the carrier and side tones using low pass filter.

When uniform sampling is employed, a sample and hold circuit is included to recreate

the amplitude modulated waveform, followed again by a low pass filter.

CHAPTER 2

PULSE WIDTH MODULATION

2.1 Modulation

In PWM, the width of the pulse carrier is changed according to the sampled

value of the modulated signal. Basically the samples are compared with the pulse

carrier. For the naturally sampled PWM, this comparison is performed directly at the

comparator, wherein for the uniformly sampled PWM, the input signal is routed first

through a sample and hold circuit so that samples are obtained at uniform intervals

rather than depending on signal amplitude.[1]

Fig 2.1.1 PWM uniform sample generation

Fig 2.1.2 PWM output signal

2.2 Spectra of PWM

Fig 2.2.1 Spectra of PWM

The PWM spectra consist of a baseband component having the frequency

equivalent to input signal frequency. Also there are frequency components around

the carrier frequency and its harmonics. The sidetones are separated by ωm, the input

frequency. The even harmonics are created as strength of modulation increases. When

uniform sampling is employed, the baseband signal contains diminishing harmonics.

2.3 Demodulation

Fig 2.3.1 PWM demodulator for uniform sampling.

For the naturally sampled PWM, threshold detection and low pass filter is

sufficient where as in uniform sampling, conversion to PAM is necessary with the

complexity of sample and hold circuit.

CHAPTER 3

PULSE POSITION MODULATION

3.1 Modulation

PPM can be considered as the differentiated version of PWM. That is, after

modulating the pulse carrier, we remove the unwanted portion of PWM. The position

of a narrow pulse is varied within a time interval depending on the signal frequency.

The naturally sampled PPM generator consists of a comparator detecting the

equivalence between the input signal and the linear ramp, followed by a monostable

or other pulse generating circuit.

Fig 3.1.1 PPM generation

. Fig 3.1.2 PPM output

3.2 Spectra of PPM

Fig 3.2.1 PPM spectra

Spectral components are generated at sampling frequency and its harmonics, along

with the diminishing group of sidetones separated by input frequency. If the pulse

duration is increased, the spectra become similar to PWM (as expected). The spectra

also contain a baseband component, composed of a differentiated version of

modulating signal.

3.3 Demodulation

The demodulation in its simplest form consists of integrating, threshold

detection and lowpass filtering to obtain the baseband component.

CHAPTER 4

PULSE INTERVAL MODULATION

4.1 Modulation

As the name indicates, the interval between is determined by the input signal

amplitude.

Fig 4.1.1 PIM output signal

The duration of each sampling interval is determined by the input signal

amplitude. So it is called anisochronous modulation.

. Fig 4.1.2 PIM modulator

The modulator consists of a sample and hold circuit whose feedback (i.e., the

hold signal) is the PIM output. The ramp is reset whenever the comparator detects the

equivalence between DC shifted input signal and ramp. The input signal is DC shifted

in order to accommodate the full dynamic range of the ramp signal.

4.2 PIM Spectra

Fig 4.2.1 PIM spectra.

Similar to previous PTM methods, the PIM spectra consists of diminishing set

of sidetones centred on carrier frequency and its harmonics.

4.3 Demodulation

Fig 4.3.1 PIM Demodulator

Although it is enough to low pass filter the PIM to recover the baseband

component, the PIM pulses are used to reset and initiate a ramp signal, whose

maximum values constitute the sampled points on the reconstructed modulating

waveform. Finally, a lowpass filter is used. If uniform sampling is used a sample and

hold circuit is used prior to the filter.

CHAPTER 5

PULSE INTERVAL AND WIDTH MODULATION

5.1 Modulation

PIWM is derived directly from its counterpart, PIM, by passing PIM through a

bistable so that both mark and space convey information. Both uniform and natural

sampling can be employed and the set up is similar to PIM modulator. As with the

PIM, the ramp is reset at a point in time determined by the input signal and not by a

predetermined interval controlled by the choice of sampling frequency.

Fig 5.1.1 PIWM Modulation

Fig 5.1.2 PIWM output

5.2 Spectra of PIWM

Fig 5.2.1 PIWM Spectrum

In contrast to PIM, the PIWM spectrum doesn’t contain any baseband component.

5.3 Demodulation

Demodulation is carried out by first converting PIWM to PIM and then

employing the PIM demodulation techniques.

. Fig 5.3.1 PIWM to PIM converter

This process is facilitated by the use of a complementary output stage within the

receiver, feeding pairs of logical inverters configured as differentiators followed by

an OR gate to recombine the two pulse streams.

CHAPTER 6

PULSE FREQUENCY MODULATION

6.1 Modulation

In PFM, the instantaneous frequency of the pulse train is varied depending on

the input signal. PFM can be simply performed by using a Voltage Controlled

Multivibrator (VCM) followed by a circuit to produce low duty cycle pulses.

Fig 6.1.1 PFM generation

Fig 6.1.2 PFM signal

6.2 Spectra of PFM

The PFM spectrum consists of a baseband component along with a sidetone

pattern set around carrier frequency and all its harmonics. The sidetone pattern is

slightly asymmetrical with the upper sidetones being stronger than the lower ones.

Fig 6.2.1 PFM spectra.

6.3 Demodulation

Demodulation is usually achieved by threshold detection and some form of

monostable to obtain equal length pulses. This signal is passed through a lowpass

filter to directly recover the baseband component from PFM spectrum.

Fig 6.3.1 PFM Demodulation

CHAPTER 7

SQUARE WAVE FREQUENCY MODULATION

7.1 Modulation

SWFM is a PTM technique closely related to PFM, and is the pulse equivalent

of sine-wave Frequency modulation (FM). Whenever there is zero-crossings of FM,

there is a square wave edge transition resulting in square wave whose frequency is

modulated with respect to input signal. Therefore the SWFM modulator basically

consists of FM generator and a comparator at the end.

Fig 7.1.1 SWFM signal

7.2 Spectra of SWFM

The spectrum of SWFM is similar to the FM except that it is slightly modified at odd

harmonics. The sidetone spread at nth harmonic is n times that at basic frequency.

Fig 7.2.1 Spectra of SWFM

7.3 Demodulation

Although the demodulation schemes similar to FM can be used, a more

convenient method is employed by converting SWFM to Dual-Edged PFM.

Fig 7.3.1 SWFM to DPFM

The spectra of DPFM is as shown,

Fig 7.3.2 DPFM spectrum

DPFM spectrum only has even harmonics and a baseband component (while a

PFM spectrum has all multiples of harmonics). So a lowpass filter can be used to

extract the baseband component.

CHAPTER 8

APPLICATIONS

PWM and PPM are long established techniques in fibre optic transmission.

Because of their fixed timing frame they are easy to multiplex and require a relatively

cheap demultiplexer. PFM has been used for optic fibre transmission of broadcast

quality TV and video signals. SWFM is used for the transmission of HDTV

signals.[2]

Narrow band Radio Frequency channels with low power and low frequency

are affected primarily by flat fading, and PPM is better suited than M-FSK to be used

in these scenarios. One common application with these channel characteristics is the

radio control of model aircraft, boats and cars. PPM is employed in these systems,

with the position of each pulse representing the angular position of an analogue

control on the transmitter, or possible states of a binary switch. The number of pulses

per frame gives the number of controllable channels available. The advantage of

using PPM for this type of application is that the electronics required to decode the

signal are extremely simple, which leads to small, light-weight receiver/decoder units.

PWM is used in efficient voltage regulators. PWM is sometimes used in sound

synthesis, in particular subtractive synthesis, as it gives a sound effect similar to

chorus or slightly detuned oscillators played together. (In fact, PWM is equivalent to

the difference of two saw tooth waves.) The ratio between the high and low level is

typically modulated with a low frequency oscillator, or LFO. A new class of audio

amplifiers called "Class-D amplifiers" based on the PWM principle is becoming

popular.[3]

CHAPTER 9

CONCLUSION

This report gives an insight into the future technology that’s gaining

popularity gradually. Pulse Time Modulation (PTM) is of different types and the

different PTM techniques available have been discussed. Each one of them has its

unique relevance in the industry. A bird’s eye view of Modulation, Spectrum and

Demodulation of each technique has been presented. The circuitry required for the

modulation and the pulse thus obtained has been neatly shown. A brief outline of

practical applications has also been provided.

REFERENCES

[1] B Wilson and Z. Ghassemlooy, "Pulse Time modulation techniques for optical

communication: a review", IEEE Proceedings-J, Vol. 140, No 6, December

1993, pp. 346-347.

[2] [2] http://en.wikipedia.org/wiki/Pulse _modulation dated 12/01/2008

[3] [3] http://www.atis.org/tg2k/_pulse-time_modulation.html dated 15/12/2007

[4]