A Seminar Report On PULSE TIME MODULATION TECHNIQUES. Jithin R. J. (Roll No. EC04B081)

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1 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 , India 2008

2 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 Mr. Dhanaraj K. J. (Seminar Coordinator) Sr. Lecturer Electronics Engg. Dept. Dr. Lillikutty Jacob Professor & Head Electronics Engg. Dept. Place: Calicut Date:

3 ACKNOWLEDGEMENT First and foremost, I wish to express my sincere gratitude to my seminar cocoordinator, 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 wholehearted support and help in accumulating details for my report. Jithin R. J.

4 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.

5 CONTENTS Chapter No TITLE Page no. 1. INTRODUCTION 1 2. PULSE WIDTH MODULATION Modulation Spectra of PWM Demodulation 4 3. PULSE POSITION MODULATION Modulation Spectra of PPM Demodulation 6 4. PULSE INTERVAL MODULATION Modulation PIM spectra Demodulation 8 5. PULSE INTERVAL AND WIDTH MODULATION Modulation Spectra of PIWM Demodulation PULSE FREQUENCY MODULATION Modulation Spectra of PIWM Demodulation 12

6 7. SQUARE WAVE FREQUENCY MODULATION Modulation Spectra of PIWM Demodulation APPLICATION CONCLUSION 16 REFERENCES 17

7 LIST OF FIGURES Figure No Title Page No PWM uniform sample generation PWM output signal Spectra of PWM PWM demodulator for uniform sampling PPM generation PPM output PPM spectra PIM output signal PIM modulator PIM spectra PIM demodulator PIWM modulation PIWM output PIWM spectrum PIWM to PIM converter PFM generation PFM signal PFM spectra PFM demodulation SWFM signal Spectra of SWFM SWFM to DPFM Spectra of DPFM 10 LIST OF TABLES Table No Title Page No. 1.1 The PTM Family 1

8 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 PPM PWM PIM PIWM PFM SWFM Table 1.1 The PTM family. Variable Position Width(Duration) Interval(space) Interval and Width Frequency Frequency 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

9 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.

10 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 PWM uniform sample generation Fig PWM output signal

11 2.2 Spectra of PWM Fig 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 ω m, 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 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.

12 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 PPM generation Fig PPM output.

13 3.2 Spectra of PPM Fig 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.

14 CHAPTER 4 PULSE INTERVAL MODULATION 4.1 Modulation As the name indicates, the interval between is determined by the input signal amplitude. Fig PIM output signal The duration of each sampling interval is determined by the input signal amplitude. So it is called anisochronous modulation. Fig 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.

15 4.2 PIM Spectra Fig 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 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.

16 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 PIWM Modulation 5.2 Spectra of PIWM Fig PIWM output Fig PIWM Spectrum

17 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 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.

18 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 PFM generation Fig 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.

19 Fig 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 PFM Demodulation

20 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 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 Spectra of SWFM

21 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 SWFM to DPFM The spectra of DPFM is as shown, Fig 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.

22 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]

23 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.

24 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 [2] [2] _modulation dated 12/01/2008 [3] [3] dated 15/12/2007 [4]

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