Pulse-width modulation

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

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Telecommunications

In telecommunications, the width of the pulses correspond with specific data values encoded at one end and decoded at the other.

Pulses of various lengths (the information itself) will be sent at regular intervals (the carrier frequency of the modulation). 

          _      _      _      _      _      _      _      _     
         | |    | |    | |    | |    | |    | |    | |    | |    
Clock    | |    | |    | |    | |    | |    | |    | |    | |    
       __| |____| |____| |____| |____| |____| |____| |____| |____

                 _      __     ____          ____   _
Data            | |    |  |   |    |        |    | | |
                | |    |  |   |    |        |    | | |
       _________| |____|  |___|    |________|    |_| |___________

Data       0     1       2      4      0      4     1      0

The inclusion of a clock signal is not necessary, as the leading edge of the data signal can be used as the clock if a small offset is added to the data value in order to avoid the lack of a pulse for zero values.

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Types

Three types of pulse-width modulation (PWM) are possible.

  1. The pulse center may be fixed 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.
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Spectrum

The resulting spectra (of the three cases) are similar, and each contains a dc component, a base sideband containing the modulating signal and phase modulated carriers at each harmonic of the frequency of the pulse. The amplitudes of the harmonic groups are restricted by a <math>\sin x / x</math> envelope (sinc function) and extend to infinity.

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Power delivery

Image:Pwm.png PWM can be used to reduce the total amount of power delivered to a load without losses normally incurred when a power source is limited by resistive means. This is because the average power delivered is proportional to the modulation duty cycle. With a sufficiently high modulation rate, passive Electronic filters can be used to smooth the pulse train and recover an average analogue waveform.

High frequency PWM power control systems are easily realisable with semiconductor switches. The discrete on/off states of the modulation are used to control the state of the switch(es) which corespondingly control the voltage across or current through the load. The major advantage of this system is the switches are either off and not conducting any current, or on and have (ideally) no voltage drop across them. The product of the current and the voltage at any given time defines the power dissipated by the switch, thus (ideally) no power is dissipated by the switch. Reallistically, semiconductor switches such as MOSFETs or BJTs are non-ideal switches, but high efficiency controllers can still be built.

Examples of PWM use are DC motor speed control, Class D audio amplfiers and light dimmers common in homes. In the light dimmer case the electricity being modulated is AC. Simple adjustment to the brightness of the light can be implemented by setting at what voltage in the AC cycle the dimmer begins to conduct electricity to the light bulb (using a triac). Because the duty cycle of the modulation is the same as the AC frequency of the line (50 Hz or 60 Hz in most countries) and PWM dimmers are often used with incandescent lamps, the human eye sees only the average intensity (see flicker fusion). Their intensity when dimmed this way is the same as the intensity they would have given the same average power via constant current (as from a resistive dimmer). LEDs, on the other hand, do flicker instantaneously and would appear steady only because of the flicker fusion effect.

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Voltage regulation

(main article: switched-mode power supply)

PWM is also used in efficient voltage regulators. By switching voltage to the load with the appropriate duty cycle, the output will approximate a voltage at the desired level. The switching noise is usually filtered with an inductor and a capacitor.

One method measures the output voltage. When it is lower than the desired voltage, it turns on the switch. When the output voltage is above the desired voltage, it turns off the switch.

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Audio effects and amplification

PWM is sometimes used in sound synthesis, in particular subtractive synthesis, as it gives a nice effect similar to chorus or slightly detuned oscillators played together. (In fact, PWM is equivalent to the difference of two sawtooth waves. [1]) The ratio between the high and low level is typically modulated with a low frequency oscillator, or LFO.

A new class of audio amplifiers based on the PWM principle is becoming popular. Called "Class-D amplifiers", these amplifiers produce a PWM equivalent of the analogue input signal which is fed to the loudspeaker via a suitable filter network to block the carrier and recover the original audio. These amplifiers are characterised by very good efficiency figures (≥ 90%) and compact size/light weight for large power outputs.

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See also

es:Modulación por anchura de pulsos fr:Modulation de largeur d'impulsion it:Modulazione di larghezza di impulso pl:Pulse Width Modulation pt:Modulação por largura de pulso ru:Широтно-импульсная модуляция ta:துடிப்பு நீள பண்பேற்றம்

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