Demystifying Pulse Width Modulation(PWM).

In my very first article,I thought to write about one of the topics that has been a little mystery to me yet so important and exciting,the Pulse Width Modulation,or popularly referred to as?PWM.

The easiest definition I could find and have used to date since I first met these three letters but with so much significance is ;A technique of obtaining analog values from digital ones,sounds easy right?

Well it is.It just comes with so much confusion but its applications sound much better than just the definition.Let's dive right into it.

Working Principle

PWM is a way of getting analog outputs by quickly varying the Pulse width.Pulse width is the duration of on time taken for a signal to peak and fall,and not to be confused with?frequency?which simply means number of cycles in a second(Hertz).

Another terms often used is the Duty cycle which is the percentage of on/off time i.e 50% duty cycle implies the signal is on for half and off for the other half.

Digital signals usually represent an on/off state or high/low or 1/0 value.Some applications are not favored by these 2 extreme states since we could use the values along the way for complete control,be it in the speed of motor or as simple as controlling the brightness of a bulb.

So basically PWM is a type of signal that is generated by tweaking duty cycles to kind of imitate analog signals ,since analog signals basically represents changes over time.

Generation of PWM

There are several methods to generate PWM signals, depending on the specific application and available resources. Here are some common techniques:

1. Hardware-Based PWM Generation:

• Timer/Counter Modules: Many microcontrollers and digital signal processors (DSPs) have built-in hardware modules specifically designed for generating PWM signals. These modules often include dedicated timers and comparators, making them efficient and reliable for generating precise PWM signals.
• Analog Comparators: Analog comparators can be used to compare a varying input voltage with a reference voltage. By toggling the output based on the comparison result, a PWM signal can be generated.

2. Software-Based PWM Generation:

• General-Purpose Input/Output (GPIO): In software-based approaches, a microcontroller's GPIO pins can be used to generate PWM signals. By toggling the pin state at specific time intervals, the duty cycle of the PWM signal can be controlled. However, the accuracy and resolution of software-based PWM may be limited compared to hardware-based methods.
• Pulse-Generation Libraries: Many programming languages and development platforms provide libraries or functions specifically designed for generating PWM signals. These libraries utilize the underlying hardware resources of the microcontroller or development board to generate accurate PWM signals.

3 .Digital-to-Analog Converter (DAC) PWM:

• In some cases, a digital-to-analog converter (DAC) can be used to generate a PWM-like signal. By varying the output voltage of the DAC in a step-wise manner, an approximate PWM signal can be obtained. However, this method may not be as precise as dedicated PWM generation techniques.

4 .Programmable Logic Devices (PLDs) and Field-Programmable Gate Arrays (FPGAs):

• PLDs and FPGAs offer high flexibility in generating PWM signals. These devices allow for the implementation of custom PWM generation algorithms using hardware description languages (HDLs) like VHDL or Verilog. This approach is particularly useful for complex and specialized PWM applications.

It's worth noting that the selection of a PWM generation method depends on factors such as the complexity of the application, available hardware resources, desired accuracy, and the capabilities of the microcontroller or development platform being used.

We'll cover the first and second methods for now.

1.Hardware-Based PWM Generation:

The message signal and the carrier waveform is fed to a modulator which generates a PAM signal. This pulse amplitude modulated signal is fed to the non-inverting terminal of the comparator.

A ramp signal generated by the sawtooth generator is fed to the inverting terminal of the comparator.

These two signals are added and compared with the reference voltage of the comparator circuit. The level of the comparator is so adjusted to have the intersection of the reference with the slope of the waveform.

Check out the signals produced by each component below,courtesy of?electronics coach

2 .Software-Based PWM Generation:

Anyone who's played around with MCU boards like Arduino have probably come across PWM pins.

With all the Hardware doing the background work,it becomes easy to just set a desired analog value between 0 and 255 (0 representing 0 volts and 255 being the highest voltage signal like 5V etc.)

Learn more about Arduino?here.

Arduino has 6 dedicated pins for PWM and can be used by the function;

analogWrite(pin, value)        

Perhaps the basic application is the dimming of an LED when you need a value in between total on /off state.

I used 3 LEDs and wrote the code below which controls the brightness using pin 3,

int led=3;
int max=255;
int min=0;
int i;
void setup(){
pinMode(led,OUTPUT);
}
void loop(){
for(i=min;i<=max;i++){
  analogWrite(led,i);
  delay(10);
}
for(i=max;i>=min;i--){
  analogWrite(led,i);
  delay(10);
}}        

For demo check?here;

Another typical application is the motor control,but that's for another day.

Advantages of PWM

1. It is more immune to channel induced noise.
2. Reconstruction of PWM signal to remove noise from the distorted PWM signal is easy.
3. The transmission and reception does not need to be synchronized.

Disadvantages of Pulse Width Modulation

1. Due to changing width of the pulses, variation in transmission power is also noticed.

That's all the basics,still not simple but this article definitely goes a long way to demystify PWM.Check my blog for related content.