Lecture Notes: EEEC6440315 Communication Systems - Pulse Time Modulation

Course: BEng (Hons) in Electrical and Electronic Engineering

1. EEEC6440315 COMMUNICATION SYSTEMS Pulse Time Modulation FACULTY OF ENGINEERING AND COMPUTER TECHNOLOGY BENH (HONS) IN ELECTRICAL AND ELECTRONIC ENGINEERING Ravandran Muttiah BEng (Hons) MSc MIET

2. Pulse Time Modulation Pulse Time Modulation (PTM) is a class of signaling technique that encodes the sample values of an analogue signal onto the time axis of a digital signal . The two main types of pulse time modulation are : (1 ) Pulse Width Modulation (PWM) (2) Pulse Position Modulation (PPM ) In PWM the sample values of the analogue waveform are used to determine the width of the pulse signal. Either instantaneous or natural sampling can be used. In PPM the analogue sample values determine the position of a narrow pulse relative to the clocking time . It is possible to obtain PPM from PWM by using a mono-stable multivibrator circuit. 1

3. 2 Pulse Time Modulation Signaling Figure 3: Corresponding PPM Signal. Figure 2: Corresponding PWM Signal. Figure 1: Analogue Signal.       Sampling Times

4. 3 Instantaneously Sampled PAM Generator Clock Triangle Function Generator Monostable Multivibrator Σ Comparator Sync Triangle Flat-Topped PAM Analogue Input Signal PWM Output PPM Output Reference Level   + + Figure 4: Generation of PWM and PPM using instantaneous sampling. PWM Generation U sing Instantaneous Sampling +  

5. 4 Figure 5: Technique for generating instantaneously sampled PTM signals. PAM Signal Triangle PAM Plus Triangle PWM PPM   Clocking          

6. 5 Monostable Multivibrator Triangle Function Generator Comparator PWM Output PPM Output Σ Reference Level     + Analogue Input Signal + + Figure 6: Generation of PWM and PPM using natural sampling. PWM Signal Generation U sing Natural Sampling

7. 6 Figure 7: Technique for generating naturally sampled PTM signals. Analogue Input Signal Triangle Analogue Plus Triangle PWM PPM            

8. 7 Figure 8: Detection of PWM and PPM signals. Detection Of PWM And PPM Signals PWM or PPM signals may be converted back to the corresponding analogue signal by a receiving systems shown below. Integrator Reset & Start Stop (Pulse) (Pulse ) Sample & Hold Clock Low Pass Filter Differentiator Inverter PWM Input PPM Input From Clock Sync Circuit Clocking Signal To A To B To A To B Analogue Output Truncated Ramp PAM   A B

9. 8 Figure 9: Detection of PWM and PPM signals. Integrator Output (Truncated Ramp) PAM Waveform PWM Input PPM Input                  

10. For PWM detection the PWM signal is used to start and stop the integration of the integrator. After reset integrator starts to integrate during the duration of the pulse and will continue to do so till the pulse goes low . If integrator has a DC voltage connected as input , the output will be a truncated ramp. After the PWM signal goes low, the amplitude of the truncated ramp will be equal to the corresponding PAM sample value. Then it goes to zero with reset of the integrator. 9

11. (1) Erhan A. Ince, Communication Systems, Eastern Mediterranean University, 2003. References 10