Easy Pulse Width Modulation Circuit

Once you know how pulse width modulation is created then it is easy to create a circuit from that knowledge.

You will need three things:
  1. A sine wave source
  2. A triangle wave source (555 Timer)
  3. A differential amplifier (Op Amp)
I will explain this circuit in the context of continuous transmission e.g. a land-line phone.

If you examine the diagram below you will start to understand quickly how PWM works.

 

Picture: http://pcbheaven.com/wikipages/PWM_Modulation/

As the waves cross over the differential amplifier will saturate high or low. 
As the sine wave cuts through the top of the triangle wave the pulse generated is short because the peak of the triangle wave is very thin where as at the bottom of the triangle wave is wider and so the pulse generated will also be larger as a direct consequence. 
(I realise the picture is actually a sawtooth wave but there is no difference in the concept).

I'm sure you have grasped the concept by now, if not then the circuit diagram should be enlightening.


























As you can see I have use the rising and falling of the RC in the 555 astable to create my triangle wave.
The frequency of the 555 can be changed by changing the RC connected to the chip.

I have also decoupled the triangle wave with a capacitor so it will swing around 0v just like the sine wave generator does. You will also want to do this to prevent any current escaping the RC pair but op amps have enough input impedance to do that for you.
The capacitor will also add capacitance to the RC so the total capacitance in the RC of the 555 will be 101uF, I have made the capacitance small enough so the effect on the timing element will be negligible.

As I said I will explain the circuit in terms of a continuous transmission signal.
You will find that the sine wave is represented by a signal generator, but the input sine wave could be anything as long as it's voltage is kept below the triangle wave voltage, otherwise it will not work properly.

Also if you have an increasingly complex waveform it will be better represented on the receiving end if you choose a higher frequency for the triangle wave, as it will give better resolution.

As a rule of thumb try to keep the frequency of the triangle wave at least ten times higher than the maximum sine wave frequency so the sine wave will be decently resolved when demodulated.

The depth of modulation is controlled by the ratio of input voltages, the depth of modulation will also affect the amplitude of the demodulated wave.