Pulse Modulation.pptx

Satellite communications

1. Pulse Modulation Harish I Sathiyan R Srivenkatachalapathy M Sugan G

2. Agenda Pulse Amplitude Modulation(PAM) Pulse Code Modulation(PCM) Pulse Width Modulation(PWM) Pulse Position Modulation(PPM) 9/5/2023 PULSE MODULATON 2

3. Introduction This type of modulation is based on the “sampling principle” which states that  a continuous message waveform that has a spectrum of finite width could be recovered from a set of discrete instantaneous samples whose rate is higher than twice the highest signal frequency . 9/5/2023 PULSE MODULATION 3

4. Pulse Amplitude Modulation Pulse Amplitude Modulation (PAM)  is an analog modulating scheme in which the amplitude of the pulse carrier varies proportional to the instantaneous amplitude of the message signal. The width and position of the pulse remains unchanged. The pulse amplitude modulated signal, will follow the amplitude of the original signal, as the signal traces out the path of the whole wave. In natural PAM, a signal sampled at the Nyquist rate is reconstructed, by passing it through an efficient  Low Pass Frequency (LPF)  with exact cutoff frequency. 9 /5 /2023 PULSE MODULATION 4

5. Generation of PAM 9/5/2023 PULSE MODULATION 5 Block Diagram x(t) c(t) y(t) s(t) Pulse Shaping Network

6. Detection of PAM 9/5/2023 PULSE MODULATION 6 Reconstruction lowpass filter PAM Signal Reconstructed PAM Signal Block Diagram The PAM signal is passed through a lowpass reconstruction filter. The filter reconstructs the analog signal from PAM pulses.

7. Transmission bandwidth of PAM signal 9/5/2023 PULSE MODULATION 7 In a pulse amplitude modulated (PAM) signal the pulse duration ‘τ’ is considered to be very small in comparison to time period (i.e., sampling period) T s  between any two samples i.e. << --------------(1) Now, if the maximum frequency in the modulating signal x(t) is f m , then according to sampling theorem, the sampling frequency f s  must be equal to or higher than the Nyquist rate, i.e. 1/ ≤1/   But according to equation (1), we have, << Therefore, << ≤1/ Now, if the ‘ON’ and ‘OFF’ time of the pulse amplitude modulated (PAM) pulse is same then maximum frequency of the PAM pulse will be equal to, f max = 1/( + ) = 1/2  Therefore, the bandwidth required for the transmission of a PAM signal would be equal to the maximum frequency f max  given by the above equation. Thus, we have Transmission bandwidth, B T B T Since << 1/ , B>>  

8. Advantages PAM can be easily Generated. PAM forms the basis for many other pulse modulation techniques such as PCM. Disadvantages Bandwidth needed for transmission is very large when compared to maximum frequency. Interference noise is maximum as the amplitude keeps varying. Peak power required by the transmitter keeps varying. Applications PAM is used for transmitting signals over a short distance baseband channels and simple communication. It is used in analog-to-digital converters for computer interfacing. 9/5/2023 PULSE MODULATION 8

9. Pulse Code Modulation 9 /5 /2023 PULSE MODULATION 9

10. PCM Transmitter 9/5/2023 PULSE MODULATION 10

11. S ignal Representation of PCM 9/5/2023 PULSE MODULATON 11

12. PCM Receiver 9/5/2023 PULSE MODULATION 12

13. Advantages Very high noise immunity. It is possible to store the PCM signal due to its digital nature. It is possible to use various coding techniques so that only the desired person can decode the received signal. Integration with other form of digited data is possible. Disadvantages The encoding, decoding and quantizing circuitry of PCM is complex. PCM requires a large bandwidth as compared to the other systems. Applications In telephony (with the advent of fibre optic cables). In the space communication, space craft transmits signals to earth. Here the transmitted power is very low (10 to 15W) and the distances are huge (a few million km). Still due to the high noise immunity, only PCM systems can be used in such applications . 9/5/2023 PULSE MODULATION 13

14. Pulse Time Modulation In Pulse time modulation (PTM), amplitude of the carrier is kept constant and the Position or width of the carrier varies with the amplitude of the modulating signal at the time of sampling. There are two types of Pulse Time Modulation(PTM): 1)Pulse Width Modulation(PWM) 2)Pulse Position Modulation(PPM) 9 /5 /2023 PULSE MODULATION 14

15. PULSE WIDTH MODULATION PWM Pulse width modulation reduces the average power delivered by an electrical signal by converting the signal into discrete parts. In the PWM technique, the signal’s energy is distributed through a series of pulses rather than a continuously varying (analogue) signal. The width of the pulse is directly proportional to amplitude of the modulating signal at the sampling instant. The amplitude and position of the pulse remains unchanged. Types of PWM: Trail Edge Modulation –  In this technique, the signal’s lead edge is fixed, and the trailing edge is modulated. Lead Edge Modulation – In this technique, the signal’s lead edge is modulated, and the trailing edge is kept fixed   . Pulse Center Two Edge Modulation –  In this technique, the pulse center is fixed and both edges of the pulse are modulated. 2/2/2023 PULSE MODULATION 15

16. Generation of PWM 9/5/2023 PULSE MODULATION 16 A sawtooth generator generates a sawtooth signal of frequency f s , and this sawtooth signal in this case is used as a sampling signal. It is applied to the inverting terminal of a comparator. The modulating signal x (t) is applied to the non-inverting terminal of the same comparator. The comparator output will remain high as long as the instantaneous amplitude of x (t) is higher than that of the ramp signal.

17. Detection of PWM 9/5/2023 PULSE MODULATION 17 The PWM signal received at the input of the detection circuit is contaminated with noise. This signal is applied to pulse generator circuit which regenerates the PWM signal. Thus, some of the noise is removed and the pulses are squared up. The regenerated pulses are applied to a reference pulse generator. It produces a train of constant amplitude, constant width pulses. These pulses are synchronized to the leading edges of the regenerated PWM pulses but delayed by a fixed interval. The regenerated PWM pulses are also applied to a ramp generator. At the output of it, we get a constant slope ramp for the duration of the pulse. The height of the ramp is thus proportional to the width of the PWM pulses. At the end of the pulse, a sample and hold amplifier retains the final ramp voltage until it is reset at the end of the pulse. The constant amplitude pulses at the output of reference pulse gtenerator are then added to the ramp signal. The output of the adder is then clipped off at a threshold level to generate a PAM signal at the output of the clipper. A low pass filter is used to recover the original modulating signal back from the PAM signal.

18. Advantages Unlike PAM, noise is less, since in PWM, amplitude is held constant. Signal and noise separation is very easy. PWM communication does not require synchronization between transmitter and receiver. Disadvantages In PWM, pulses are varying in width and therefore their power contents are variable. This requires that the transmitter must be able to handle the power contents of the pulse having maximum pulse width. Large bandwidth is required for the PWM communication as compared to PAM. Applications PWM is used for asynchronous transmission over noisy channel. PWM is used for generating PPM. Motor control. 9/5/2023 PULSE MODULATION 18

19. PULSE POSITION MODULATION PPM In PPM, the amplitude and width of the pulses is kept constant but the position of each pulse is varied in accordance with the amplitudes of the sampled values of the modulating signal. The position of the pulses is changed with respect to the position of reference pulses. 9/5/2023 PULSE MODULATION 19

20. GENERATION OF PPM The sawtooth generator generates the sawtooth signal of frequency f, (i.e. period T). The modulating signal x(t) is applied to the noninverting input of the comparator. The output of the comparator is high only when instantaneous value of (1) is higher than that of sawtooth waveform. The trailing edge of output of comparator depends on the amplitude of signal x(t) When sawtooth waveform voltage is greater than voltage of x(t) at that Instant, the output of comparator remains zero. The trailing edge of the output of comparator (PWM) is modulated by the signal x(t)If the sawtooth waveform is reversed, then trailing edge will be fixed and leading edge will be modulated. The Pulse Duration Modulation (PDM) or PWM signal is nothing but output of the comparator. The amplitude of this PDM or PWM signal will be positive saturation of the comparator, which is shown as 'A' in the waveforms. The amplitude is same for all pulses To generate Pulse Position Modulation (PPM), PDM signal is used as the trigger input to one monostable multivibrator. The monostable output remains zero untill it is triggered. The monostable is triggered on the falling (trailing) edge of PDM. The width of the pulse can be determined by monostable. The pulse is this delayed from sampling time KT, depending on the amplitude of signal x(1) at KT, 9/5/2023 PULSE MODULATON 20

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22. Detection of PPM The detection of the pulse position modulation block diagram is shown below. In the following block diagram, we can observe that it includes a pulse generator, SR FF, reference pulse generator & a PWM demodulator. The PPM signal which is transmitted from the modulation circuit will get distorted with the noise throughout transmission. So this distorted signal will reach the demodulator circuit. The pulse generator used in this circuit will produce a pulsed waveform with a fixed duration. This waveform is given to the SR FF’s reset pin. The reference pulse generator produces a reference pulse with a fixed period once a transmitted PPM signal is given to it. So this reference pulse is utilized to set the SR FF. At the output of the FF, these set & reset signals will generate a PWM signal. Further, this signal is processed to give the original message signal. 9/5/2023 PULSE MODULATION 22

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24. Advantages Like PWM ,in PPM amplitude is held constant thus less noise interference. Like PWM, signal and noise separation is very easy. Due to constant pulse widths and amplitudes, transmission power for each pulse is same. Disadvantages Synchronization between transmitter and receiver is required Large bandwidth is required as compared to PAM 9/5/2023 PULSE MODULATION 24

25. Thank you 9/5/2023 PULSE MODULTAION 25