Pulse Width Modulation_555

Theory


Pulse Width Modulation

PWM uses a square wave whose duty cycle is modulated resulting in the variation of the average value of the waveform.
If we consider a square waveform f (t) with a low value Ymin, a high value Ymax and duty cycle D. the average value of the waveform is given by


As f (t) is a square wave its value is Ymax for 0 < t < DT and Ymin for DT < t < T, the above expression becomes


PWM of a signal or power source involves the modulation of its duty cycle, to either convey information over a communication channel or control the amount of power sent to a load.


WHY PULSE WIDTH MODULATION ?
One of the advantages of PWM is that the signal remains digital all the way from the processor to the controlled system; no digital-to-analog conversion is necessary. By keeping the signal digital, noise effects are minimized. Noise can only affect a digital signal if it is strong enough to change a logical 1 to a logical 0, or vice versa.
Increased noise immunity is yet another benefit of choosing PWM over analog control, and is the principal reason PWM is sometimes used for communication. Switching from an analog signal to PWM can increase the length of a communications channel dramatically. At the receiving end, a suitable RC (resistor-capacitor) or LC (inductor-capacitor) network can remove the modulating high frequency square wave and return the signal to analog form.

Types
Three types of Pulse width Modulation are possible :
      1) The pulse center may fix in the center of the time window and both edges of the pulse moved to compress or expand the width.
      2) The lead edge can be held at the lead edge of the window and the tail edge modulated.
      3) The tail edge can be fixed and the lead edge modulated.
In telecommunications, the widths of the pulses correspond to specific data values encoded at one end and decoded at the other.






Here the widths of the pulses are varying according to the signal amplitude. As the amplitude going to high value the widths of pulses are going lengthy, and as the amplitude dropping down the widths of pulses are following that and the lengths of pulses are decreasing. At the high value of signal amplitude the Pulse width is high and at low level the pulse width is too short.