Pulse Position Modulation : Block Diagram, Circuit, Working, Generation with PWM & Its Applications

Pulse modulation (PM) is one type of modulation where the signal is transmitted in the pulse form. In this type of modulation, continuous signals are sampled at normal intervals, so this modulation technique is used to transmit analog information. Pulse modulation is classified into two types analog modulation and digital modulation. Analog modulation is classified into three types PAM, PWM, and PPM whereas digital modulation is classified into pulse code and delta modulation. So this article discusses an overview of one of the types of pulse modulation namely – pulse position modulation theory or PPM.

What is Pulse Position Modulation?

Pulse position modulation is one type of analog modulation which allows variation within the position of the pulses based on the sampled modulating signal’s amplitude is called PPM or Pulse Position Modulation. In this type of modulation, the amplitude & width of the pulses are kept stable & the position of the pulses only varied.

The PPM technique allows computers to transmit data by simply measuring the time taken to reach each data packet to the computer. So is frequently used within optical communication where there is small multi-pathway interference. This modulation totally transmits digital signals & cannot be utilized by analog systems. It transmits simple data which is not efficient while transferring files.

To know more on the difference between the PPM, PWM and PAM click here

Pulse Position Modulation Block Diagram

The pulse position modulation block diagram is shown below which generates a PPM signal. We know that a pulse position modulation signal is easily generated by using a PWM signal. So, here at the o/p of the comparator, we have assumed that a PWM signal is generated already & now we have to produce a PPM signal.

In the above block diagram, a PAM signal is generated from the modulator once, and further, it is processed at the comparator to produce a PWM signal. After that, the output of the comparator is given to a monostable multivibrator which is negative edge triggered. Thus, with the trailing edge of the PWM signal, the output of the monostable goes high.

Thus, a pulse of the PPM signal starts by the trailing edge of the PWM signal. Here, it is to be noted that the high output duration mainly depends on the multivibrator’s RC components. So this is the main reason why a stable width pulse is attained in the case of the PPM signal.

The PWM signal’s trailing edge shifts through the modulating signal, so with this shift, the pulses of PPM will show shifts within its position. The PPM signal’s waveform representation is shown below.

In the above waveform of pulse position modulation, the first waveform is the message signal, the second signal is a carrier signal and the third signal is the PWM signal. This signal is considered a reference for the PPM signal generation as shown in the last diagram. In the above waveforms, we can notice that the ending point of the PWM pulse as well as the starting point of the PPM pulse is coinciding, which is shown with the dotted line.

Detection of Pulse Position Modulation

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.

How Does Pulse Position Modulation Work?

Pulse position modulation (PPM) simply works by transmitting electrical, optical, or electromagnetic pulses to a computer/another device to communicate simple data. So it needs both the devices to be coordinated to a similar clock so that it decodes the data based on once the pulses were broadcasted. Alternately, one more form of PPM called differential pulse position modulation permits all signals to be encoded depending on the dissimilarity between broadcast times. This means that a receiving device has to monitor only the dissimilarity in arrival times to decode a transmission.

Pulse Position Modulation Circuit

Generally in PPM, the amplitude & width of the pulses are kept stable whereas the arrangement of every pulse with reference to reference pulse position is modified based on the modulating signal’s instant sampled value. The circuit diagram of pulse position modulation with a 555 timer is shown below.

This circuit can be built with different electronic components like 555 timer IC, resistors R1 and R2, Capacitors like C2 & C3, and diode D1. Give the connections as per the circuit given below.

Basically, the 555 IC is a monolithic IC that is available in an 8-pin DIP package. It is used in many applications used as an astable multivibrator and bistable multivibrator to generate the triangular wave, square wave, etc. So, the generation of PPM is also considered one of the applications of 555 IC.

Let’s see how the PPM signal is generated using the above PPM circuit with 555 IC. For a generation of PWM pulses and PPM pulses, the 555 timer operates in monostable mode. Monostable mode is one of the modes of multivibrators. Multivibrators are generally electronic circuits which are having no one or two stable states. Based on the stable states, there are three types astable, bistable and monostable multivibrators.

The input PWM pulse is applied to the pin2 of 555 IC-like triggered input through a differentiator network formed by diode D1, resistor R, and capacitor C1. Now based on the received input at pin2, the output will be obtained at pin3 of 555 timer IC. The output will remain high for the duration of the time period decided by resistors R2 and C2 so that the width and amplitude of each pulse remain constant and we will get a PPM signal at the output.

In this way, the 555 timer IC is used for generating a PPM signal.

Advantages

The advantages of pulse position modulation include the following.

• PPM has the most power efficiency as compared to other modulations.
• This modulation has less stable amplitude noise interference.
• This modulation separates the signal easily from a noisy signal.
• It needs less power as compared to PAM.
• Separation of signal & noise is extremely simple
• It has constant transmitted power output.
• This technique is simple to divide the signal from a noisy signal.
• It needs extremely less power as compared to PAM & PDM because of amplitude & short duration pulse.
• Easy noise removal & separation is extremely easy in this type of modulation.
• Power utilization is also extremely low as compared to other modulations because of stable pulse amplitude & width.
• PPM communicates only simple commands from a Tx to an Rx, so it is frequently used in lightweight applications because of its low system necessities.

Disadvantages

The disadvantages of pulse position modulation include the following.

• PPM is very complex.
• It needs more bandwidth for transmission as compared to PAM.
• It is extremely sensitive to multi-pathway interference like echoing that can disturb a transmission by changing the difference in arrival times of every signal.
• Synchronization is necessary between transmitter & receiver which is not feasible each time & we require a dedicated channel for it.
• Special devices are required for this kind of modulation.

Applications

The applications of pulse position modulation include the following.

• The PPM is mainly used in telecommunication systems & air traffic control systems.
• This modulation is used in radio control, an optical communication system & military applications.
• This technique is used in planes, remote-controlled cars, trains, etc.
• PPM is used in noncoherent detection wherever a receiver does not require any Phase lock loop or PLL to track the carrier’s phase.
• It is used in RF (radio frequency) communication.
• It is also utilized in high-frequency, contactless smart cards, radio frequency ID tags, etc.

Thus, this is all about an overview of pulse position modulation – working and its applications. Here is a question for you, what is PWM?