Pulse Position Modulation (PPM)

Pulse Position Modulation is another kind of important modulation technique used in electronics communication system apart from amplitude and phase modulation. In this article, we will discuss what is pulse position modulation, we will understand its block-diagram and how do we detect pulse position modulation. Later we will understand the working of pulse position modulation through its circuit diagram. We will discuss the advantages, disadvantages and applications of this modulation technique. In the end, we will conclude the article.

What is Pulse Position Modulation ?

Pulse position modulation is a modulation technique in which the position of pulse varies according to instantaneous value of amplitude of sampled modulating signal. If we try to summarize it,

Displacement of pulse is directly proportional to amplitude value of sampled message signal.

The basic picture in this modulation is that with increase in amplitude of signal, the position of pulse shifts according to some reference. It is important to note that the width of pulses remains constant only there position changes. Hence transmitted power remains same.,

Pulse Position Modulation Block Diagram

Let us see the basic block diagram used for generation of Pulse Position Modulation:

Block Diagram of Pulse Position Modulation

In this block diagram, Firstly a PAM( pulse amplitude modulated) signal is produced which is processed at comparator to generate a PWM (pulse width modulated signal) . After processing, the output of comparator is fed as an input to monostable vibrator.

Being negative edge-triggered, the output of vibrator goes high with trailing edge of PWM signal. The trailing edge of PWM shifts with modulating signal creating PPM pulses.

Detection of Pulse Position Modulation

Here is the block diagram for detection of PPM wave.

Pulse Position Modulation Detector

It consist of Pulse generator, RS flip flop and PWM demodulator. The modulated PPM signal transmitted from modulation circuit consists of a lot of distortions mainly because of noise.

The pulse generator performs the task of generating a pulsed waveform. This received waveform is fed to reset-pin of SR flip flop. The reference pulse generator generates, reference pulse of a fixed period when transmitted PPM signal is applied to it , we use this pulse to set the flip-flop. The set and Reset pin produce a PWM signal at the output of the flip-flop. This gives us the original message signal.

Working of Pulse Position Modulation

In PPM information is simply transmitted by the position of the pulse. In order to transmit data, it sends electrical, optical, or electromagnetic pulses to another device. This gives us a constraint because transmission is only possible if both transmitting and receiving devices are synchronized, i.e. having the same clock.

In order to reduce the bandwidth consumption, we can use a different sub-type of PPM which is differential pulse position modulation. In this method data is encoded based on the difference between broadcast time. The device which receives the data carefully measures the difference in arrival time to perform decoding of data.

Pulse Position Modulation Waveform

Pulse Position Modulation Circuit

This is the circuit used for performing Pulse Position Modulation , the circuit diagram is given below and we will study how it worked:

Pulse Position Modulation Circuit

The circuit diagram of PPM involves IC 555 timer as a major component. All other components include resistors R₁ and R₂, Capacitors like C₁ & C₂, and diode D₁.

We can define IC 555 as a monolithic IC consisting of a package of 8 pins. Triangle waves, square waves, etc. are used in many applications depending on how the IC is being used. It is used as an unstable multivibrator and a bistable multivibrator to create different waveforms. Therefore, PPM production is also considered as one of the applications of IC 555.

Let’s dive deeper into generation of PPM signal using the PPM circuit of the above 555 IC. The 555 timer operates in monostable mode to generate PWM pulses and PPM pulses. It is a monostable type multivibrator. Multivibrators can be described as electronic devices that generally do not have one or two stable states. Depending on the steady state multivibrators can be divided into three types: astable, bistable and monostable.

The input PWM pulse is fed to pin 2 of the class 555 IC trigger input through the differential network consisting of diode D₁, resistor R and capacitor C₁. Now, since the input is taken at pin 2, the output will be taken at pin 3 of the 555 timer IC. The output will remain high for a period of time determined by resistors R₂ and C₂, keeping the width and amplitude of each pulse constant and thereby we will get a PPM signal at the output.

This is how 555 timer integrated is used to generate PPM signal.

Advantages of Pulse Position Modulation

These are some advantages of Pulse Position Modulation

• Due to constant width of pulse , the transmission power remains constant and displays no variations.
• PPM shows better noise immunity as compared to an amplitude modulated signal. This is because information in PPM is stored in position rather than amplitude.
• The recovery process of PPM signal from its distorted form is easy and simple.
• The PPM system has very low system requirements so it is frequently used in lightweight appliances with simple accessories.
• The all-over power required for such system is low as compared to PAM making it a power-efficient modulation technique.

Disadvantages of Pulse Position Modulation

These are some disadvantages of Pulse Position Modulation

• The major limitation of this technique is that it requires large bandwidth as compared to other methods increasing the bandwidth consumption.
• In order to perform detection of this signal at receiver, it is necessary that transmitter and receiver are synchronized. This is not possible every time.
• It is really sensitive to phenomena like interference which can disturb a transmission by changing the difference in arrival times of every signal.

Applications of Pulse Position Modulation

PPM is used in a variety of appliances and here are some of its applications.

• This modulation technique finds its application in air traffic control systems, in radio control and in military applications.
• Since PPM is used in non-coherent detection when we don’t require a receiver having a PLL(Phase lock loop ) for tracking carrier phase.
• Since this modulation technique helps in compression of data, it is often used for storage purpose.
• We can employ PPM in optimal communication. This is done to minimize the dispersion effect and efficiently use the laser power.
• PPM is employed in remote control systems, in which a stream of pulses is used to encode the information in bits.

Conclusion

So now if someone asks you what is PPM? You can easily answer that question : In addition to this we have seen various circuits involved in modulation and demodulation of PPM signals. As we talked about the application of PPM technique in real-life, we realized PPM is a signification technique and has vast multitude if applications. There is still a scope of improvement in performing PPM which can help to work on limitations of PPM like lowering the bandwidth consumption and working on synchronization between transmitter and receiver. These techniques need time and technology for development and scientists are working on it.

FAQs on Pulse Position Modulation

1. Name some other modulation techniques.

When information is stored in amplitude then we get PAM(amplitude modulation) and when it is stored in width then we get PWM(width modulation).

2. Which IC is used for generating PPM?

The 555 timer IC is used for PPM generation. It is an integrated circuit used in a variety of timer, delay, pulse generation, and oscillator applications.

3. What is a mono-stable multivibrator used for in PPM ?

A mono-stable vibrator is used in PPM for converting Pulse Width modulated signal to Pulse position modulated signal.