Pulse Width Modulation_741

Theory


Pulse Width Modulation

Pulse Width Modulation (PWM) encodes a signal into periodic Pulses of equal amplitude but varying width. The width of a pulse at a given point in time is proportional to the amplitude of the message signal at the time. For example, a large value of the message signal corresponds to a wide pulse, and a small value of the message yields a narrow pulse. The width of the pulse can be described in terms of its duty cycle, which is defined as:

d = tw / td × 100 %
Where,
tw = width of pulse
td = period of the pulse
Here, td is constant and tw varies.
In Pulse Width Modulation there is a linear relationship between duty cycle (d) and amplitude of message signal (Vin). This relationship is as following:
d ∝ Vin
⇒ d = Md . Vin

Where, Md is the modulation index.

Fig.1 Schematic diagram of Pulse Width Modulation Circuit.

To implement the PWM, the message signal is compared with sawtooth carrier. When the message signal is greater than the carrier, the comparator output becomes high and vice versa; the highs and lows can be represented by + 1 or – 1 respectively. The comparator output is the pulse width modulated signal.


Fig.2 Shows Modulating Signal Waveform.



Fig.3 Shows Carrier Waveform.



Fig.4 Shows Pulse Width Modulated Output.

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.

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.