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DIY Circuit Design: Pulse Position Demodulation

 

The Pulse Position Modulation (PPM) is a modulation technique designed to achieve the goals like simple transmitter and receiver circuitry, noise performance, constant bandwidth and the power efficiency and constant transmitter power. In Pulse Position Modulation the amplitude of the pulse is kept constant as in the case of the FM and PWM to avoid noise interference. Unlike the PWM the pulse width is kept constant to achieve constant transmitter power. The modulation is done by varying the position of the pulse from the mean position according to the variations in the amplitude of the modulating signal.

The Pulse Position Modulation (PPM) can be actually easily generated from a PWM waveform which has been modulated according to the input signal waveform. The PPM can be demodulated both synchronously and asynchronously. The synchronous demodulation is complex as it requires synchronization of the receiver with the transmitter. When using the asynchronous demodulation technique the quality will be comparatively less, but with an advantage of very simple circuit for demodulation. This article discusses how to demodulate the PPM waves using the simplest possible method.

DESCRIPTION:

There are different methods for extracting the message signal from a PPM wave synchronously and asynchronously. The simplest asynchronous demodulator uses a low pass filter to filter out the message signal from the modulated wave. The following block diagram represents the implementation of a PWM modulator.

Block Daigram of PWM Modulator

Fig. 1: Block Diagram of PWM Modulator

{C   1)  PPM GENERATION

The PPM required for this project is generated from a PWM wave which is modulated with the message signal. The message signal used here is a pure sine waveform generated using the Wien Bridge Oscillator (WBO). A ramp signal is generated with the help of a RC charging circuit and a comparator IC. Another comparator IC which is fed with ramp signal as one of its input and the message signal as other can produce a PWM wave at its output. This PWM wave is then used to generate the PPM wave using a mono-stable multi-vibrator. The block diagram of the PWM generation circuit is given below:

Block Diagram of PWM Generation Circuit

Fig. 2: Block Diagram of PWM Generation Circuit

A sine wave generator circuit is used in this project which is based on the Wien Bridge Oscillator (WBO) circuit. The Wien Bridge oscillator circuit can produce distortion less sinusoidal sweep at its output. The circuit is designed in such a way that both the amplitude and frequency of the oscillator can be adjusted using potentiometers. Here the sine wave generator is adjusted to produce a waveform of frequency 1 KHz.

The Ramp generator used in this circuit is designed with an op-amp and an RC charging circuit. The RC charging circuit is connected to the output of the op-amp and the voltage across the capacitor is connected to one of the input of the op-amp. To another input of the op-amp the variable pin of a potential divider is connected to which divides the voltage from the output of the op-amp.

The ramp waveform is applied to one of the input of another comparator circuit and the output of the comparator circuit will be a PWM waveform.

PWM Waveform Displayed on CRO Screen

Fig. 3: PWM Waveform Displayed on CRO Screen

The PPM generation is achieved with the help of a mono-stable multi-vibrator designed using a 555 timer IC.  The advantage of using a 555 timer IC is that it reduces the circuit complexity and requires only a few components and single power supply.

The PWM pulses acts as a triggering source for the mono-stable circuit. To enable triggering from the PWM pulses, a triggering circuit needs to be implemented with the mono-stable circuit. The triggering circuit can be easily made from a couple of resistors and a capacitor.

The PPM circuit diagram is shown in the following image:

Circuit Diagram of PPM

Fig. 4: Circuit Diagram of PPM

The complete image of the PPM modulator circuit wired in a breadboard is shown in the following image:

Circuit of PPM modulator on Breadboard

Fig. 5: Circuit of PPM modulator on Breadboard

Low Pass Filter

{C  2) LOW PASS FILTER

A PPM modulated wave can be modulated in synchronous mode and asynchronous mode. In synchronous mode the receiver is provided with the original un-modulated pulses. These pulses are then used to compare the received wave in which the positions of the pulses are varied according to the amplitude of the modulating signal. This synchronous receiver circuitry is very complex as it requires a local generation of pulses having synchronization with the transmitter.

The PPM can be demodulated using the asynchronous method also, in which the there is no synchronization required and all that required is a low pass filter of good quality. In this project the message signal is retrieved from a PPM wave is demodulated using a low pass filter of first order. As the order of the filter increases the quality of the demodulated signal also increases.

The Low-pass filter is designed in such a way that the time constant of the filter matches with the time period of the modulating signal. Since 1KHz sine wave is used as modulating signal, the time constant of the low pass filter using the RC circuit is 1 ms. Assuming the value of the resistor ‘R’ as 1K, then the capacitor selected should be of 1uF so that the time constant will be exactly 1 ms.

The first order low pass filter used for the demodulation is given in the following figure.

Circut Diagram of PPM Demodulation using Low Pass Filter

Fig. 6: Circut Diagram of PPM Demodulation using Low Pass Filter

The demodulated signal might require amplification and further filtering to achieve good quality message signal. Usually the filters used are of third order or more followed by high gain amplifiers.

PPM Demodulated Waveforms Dispalyed on CRO Screen

Fig. 7: PPM Demodulated Waveforms Displayed on CRO Screen

Circuit of Pulse Position Demodulation on Breadboard

Fig. 8: Circuit of Pulse Position Demodulation on Breadboard

Comments

hello, 

 i realy need your help,

 i want to build the ppm modem as you show with the 555, but i need to have as input digitized voice, please any help

thank you

Hello Guys, I'm needing your hemp. I know how to generate a PWM signal with a ramp generator and a 555 IC, and transmit by led pules over fiber optic, but need help to receive that signal and demodulate. I'm tryng remove the ramp singnal, but it inst working. Thanks