Table of Contents:
- DIY Circuit Design: Pulse Position Demodulation
- Low Pass Filter
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.
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.
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:
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.
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:
The complete image of the PPM modulator circuit wired in a breadboard is shown in the following image: