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Circuit Design: Pulse Width Demodulation

 

A burst power when used other than the continuous power can save the total power supplied to an inertial load while achieving the same performance from the device. The performance can be varied by varying the width of the pulses. This is the technique called Pulse Width Modulation (PWM) which is in use since a long time for controlling motor speed and other similar inertial machineries.

The PWM technique is use in devices like DC motors, Loudspeakers, Class -D Amplifiers, SMPS etc. They are also used in communication field as-well. The modulation techniques like AM, FM are widely used RF communication whereas the PWM is modulation technique is mostly used in Optical Fiber Communication (OFC).

The PWM in a communication link greatly saves the transmitter power. The immunity of the PWM transmission against the inter-symbol interference is another advantage. This article discusses the technique of demodulating a PWM wave.

DESCRIPTION:

There are different methods for extracting the message signal from a PWM wave. One method which is very simple is discussed here. It is very easy to convert the PWM wave to a PPM wave, and for the PPM wave the demodulation circuitry is extremely simple. The idea is hence to convert the PWM wave to the PPM wave and then demodulate it. The following block diagram represents the implementation of a PWM de-modulator.

{C  1)  PWM GENERATION

The PWM required for this project is generated using the conventional method of comparing the message signal with a ramp waveform. The message signal used here is a pure sine waveform generated using the Wien Bridge Oscillator (WBO). The 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 is used to produce the PWM wave. The block diagram of the PWM generation circuit is given below:

Block Diagram of Pulse Width Modulation (PWM) Generation Circuit

Fig. 1: Block Diagram of Pulse Width Modulation (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.

Block Diagram of Pulse Width Modulation (PWM) Generation Circuit

Fig. 2: PWM Waveform on CRO Screen

The circuit diagram is given below:

Block Diagram of Pulse Width Modulation (PWM) Generation Circuit

Fig. 3: Circuit Diagram of Pulse Width Modulation (PWM)

The image of PWM circuit wired in the breadboard is given below:

Block Diagram of Pulse Width Modulation (PWM) Generation Circuit

Fig. 4: PWM Circuit on BreadBoard

      2) PPM generator

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 mono-stable time period depends on an RC charger here also which charges from 0V to 5V. The threshold pin (pin number 6) and the discharge pin (pin number 7) of the 555 are shorted together. The shorted point is then connected across the charging capacitor, so that whenever the voltage across the capacitor just reaches the threshold voltage, the discharging of the capacitor occurs. The discharge pin remains low till the time the 555 is triggered again preventing the capacitor from charging again.

The circuit diagram of the 555 wired as a mono-stable is given below:

Block Diagram of Pulse Width Modulation (PWM) Generation Circuit

Fig. 5: Circuit Diagram of Mono-Stable Multi-Vibrator Designed using 555 Timer IC

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 as shown in the above diagram.

The above circuit helps in the negative edge triggering of the 555 mono-stable. The circuit is triggered to generate a very short pulse of constant time period each and every time a falling edge is received from the PWM generator circuit.

Block Diagram of Pulse Width Modulation (PWM) Generation Circuit

Fig. 6: PPM Generator Circuit on Breadboard

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

Block Diagram of Pulse Width Modulation (PWM) Generation Circuit

Fig. 7: PPM Modulator Circuit on Breadboard

3)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 pulse width demodulation circuit with the first order low pass filter used for the demodulation is given in the following figure.

Block Diagram of Pulse Width Modulation (PWM) Generation Circuit

Fig. 8: Circuit Diagram of Pulse Width Demodulation Circuit with 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.

Block Diagram of Pulse Width Modulation (PWM) Generation Circuit

Fig. 9: Pulse Width Demodulation Waveforms on CRO Screen

Pulse Width Demodulation Circuit On Breadbroad

Fig. 10: Pulse Width Demodulation Circuit On Breadboard