Pulse position modulation
Table of Contents
Pulse Position Modulation (PPM) is a modulation method that enables variation in the position of the pulses in accordance with the amplitude of the sampled modulating signal. This is a different kind of PTM in which the only variable is the pulses’ position and not their amplitude or width.
Simply put, the message signal’s sampled value and pulse displacement are directly proportional. Understanding Pulse Width Modulation (PWM or PDM) is required to comprehend how the PPM signal is generated. The first type of PTM is PWM, which is covered in the earlier content.
Basics of Pulse Position Modulation
Pulse position modulation is used to transmit data while varying the position of the pulses. The fundamental concept behind the creation of a PPM waveform is that the pulse shifts in accordance with the reference as the message signal’s amplitude increases. The transmission power varies as a result of the pulses’ variable width, as we already discussed in PWM. With PPM, this is not the case because the pulses’ width is constant while only their position changes. As a result, transmission power does not change.
Now, the question arises how the position of the pulses show variation?
A PPM signal is produced in reference to a PWM signal, as we have already discussed. As a result, the PWM signal’s trailing edge serves as the starting point of the PPM signal’s pulses.
Block diagram for generation of PPM signal
As we already discussed, a PWM signal can be used to produce a PPM signal with ease. As a result, we’ve presummated that a PWM signal has already been produced at the comparator’s output, and we now need to produce a PPM signal.
The figure below shows the block diagram for generating a PPM signal:
Here, we’ve created a thorough block diagram showing how a PAM signal is first created and then further processed at the comparator to produce a PWM signal. The monostable multivibrator receives the comparator’s output as input. It is triggered by the negative edge. As a result, the output of the monostable goes high with the trailing edge of the PWM signal.
This is why a pulse of PPM signal begins with the trailing edge of the PWM signal.
In the case of PPM, it should be noted that the multivibrator’s RC components determine how long the output will be high. This is the cause of the PPM signal producing a pulse with a constant width. The trailing edge of the PWM signal shifts with the modulating signal, and the PPM pulses exhibit positional shifts as a result.
The figure below shows the waveform representation of the PPM signal:
Here, the modulating signal is displayed in the first image, and the carrier signal is displayed in the second. The following image displays a PWM signal that is used as a guide for creating the PPM signal in the previous image. The dotted line in the above figure makes it clear that the PWM pulse’s end and the beginning of the PPM pulse coincide at this point.
Detection (Demodulation) of PPM signal
The figure below shows the block diagram for the detection of a PPM signal at the receiver:
The demodulation circuit, as shown in the above figure, is made up of a pulse generator, SR flip-flop, reference pulse generator, and PWM demodulator. The noise during transmission distorts the PPM signal sent from the modulation circuit. The demodulator circuit receives this distorted PPM signal. A pulsed waveform is produced by the pulse generator used in the circuit. This waveform, which has a fixed duration, is fed to the SR flip-flop’s reset pin (R).
When a transmitted PPM signal is applied to the reference pulse generator, it produces a reference pulse with a fixed period. The flip-flop is set using this reference pulse. At the flip-flop’s output, the set and reset signals produce a PWM signal. The original message signal is then produced using additional processing on this PWM signal.
Advantages of Pulse Position Modulation
- Because the pulses’ amplitude and width are constant. As a result, the transmission power also doesn’t change and stays constant.
- PPM exhibits better noise immunity than PAM, much like PWM does. This is true because the position of the pulses rather than their amplitude contains the information content.
- When compared to PAM and PWM, noise interference is much less severe.
- It is simple to restore a PPM signal from distorted PPM.
- Transmitter and receiver must be synchronized for the signal to be properly detected at the receiver.
- The bandwidth requirement is large.
The method is employed in radio control, an optical communication system, and military applications.