In AM modulation technique, the amplitude of a carrier is varied according to the amplitude of the modulating signal. The advantage is the simplicity in the transmitter and receiver design but the noise performance is very poor. The FM modulation keeps the amplitude constant and varies the frequency according to the amplitude of the modulating signal. Since the amplitude is constant the noise performance is very high, but with a disadvantage of using large bandwidth for transmission. The PWM is a technique which can save the total transmitter power, but the power consumption varies momentarily. 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. This article discusses the technique of generating a PPM wave corresponding to a modulating sine wave.
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 technique is to generate a very small pulse of constant width at the end of the duty time of each and every PWM pulses. The PPM modulation of an input signal can be achieved using the following circuit blocks:
1) Variable frequency sine wave generator
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.
The circuit diagram of the variable frequency sine wave oscillator is shown in the following:
The frequency of the above circuit can be varied by simply varying the potentiometer R2 and the amplitude of the wave form can be adjusted by varying the potentiometer R. The snapshot of the waveform formed at the CRO screen using the WBO circuit is shown in the following image:
2) Ramp generator
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 image of the circuit wired in the bread board is shown in the following figure:
The frequency of the ramp wave depends on the charging period of the RC circuit. The charging period depends on the RC constant which is the product of the values of the Resistance and the Capacitance.
The image captured from the CRO screen displaying a ramp waveform is shown below:
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 comparator circuit is shown in the following figure;
The image of the circuit wired on the breadboard is shown below:
The following is the image captured from the CRO screen displaying the PWM waveform: The circuit diagram is given below: The 1M ohm resistor is used to adjust the amplitude of the sine wave signal generated by the WBO. The amplitude of the sine wave should be adjusted in such a way that it matches with the amplitude of the ramp signal generated.