The PWM is a technique which is used to drive the inertial loads since a very long time.The simple example of an inertial load is a motor. Apply the power to a motor for a very short period of time and then turn off the power: it can be observed that the motor is still running even after the power has been cut off from it. This is due to the inertia of the motor and the significance of this factor is that the continuous power is not required for that kind of devices to operate. A burst power can save the total power supplied to the load while achieving the same performance from the device as it runs on continuous power.
The PWM technique is use in devices like DC motors, Loudspeakers, Class -D Amplifiers, SMPS
etc. They are also used in communication field as-well. The modulation
techniques like AM, FM are widely used RF communication whereas the PWM
is modulation technique is mostly used in Optical Fiber Communication
(OFC).
As
in the case of the inertial loads mentioned previously, the PWM in a
communication link greatly saves the transmitter power. The immunity of
the PWM transmission against the inter-symbol interference is another
advantage. This article discusses the technique of generating a PWM wave
corresponding to a modulating sine wave.
DESCRIPTION:
The Pulse Width Modulation
is a technique in which the ON time or OFF time of a pulse is varied
according to the amplitude of the modulating signal, keeping t
he
(ON time + OFF time) time of the pulse as constant. The (ON time + OFF
time) of a pulse is called ‘Period’ of the pulse, and the ratio of the
ON time or OFF time with the Period is called the ‘Duty Cycle’. Hence
the PWM is a kind of modulation which keeps the Period of pulses
constant but varying their duty cycle according to the amplitude of the
modulating signal.
The
conventional method of generating a PWM modulated wave is to compare
the message signal with a ramp waveform using a comparator. The block
diagram required for the generation of a simple PWM is shown in the
following:
1) Variable frequency sine wave generator
A sine wave generator circuit
is used in this project which is based on the Wien Bridge Oscillator
(WBO) circuit. The Wien Bridge oscillator circuit can produce distortion
less sinusoidal sweep at its output. The circuit is designed in such a
way that both the amplitude and frequency of the oscillator can be
adjusted using potentiometers.
The circuit diagram of the variable frequency sine wave oscillator is shown in the following:
The
frequency of the above circuit can be varied by simply varying the
potentiometer R2 and the amplitude of the wave form can be adjusted by
varying the potentiometer R. The frequency of the sine wave generated by
the above circuit depends on the components R1, R2, C1 and C2 and the
equation for the frequency is given below:
For
the ease of adjusting the amplitude of the wave to obtain proper
sinusoidal sweep, a coarse and fine adjustment has been implemented
using potentiometers. A low value (1K) potentiometer is connected in
series with the high value (100K) potentiometer so that the coarse
adjustment can be done with the high value resistor and the fine
adjustment with the low value resistor.
The snapshot of the waveform formed at the CRO screen using the WBO circuit is shown in the following image:
2) Ramp generator
The Ramp generator used in this circuit is designed with an op-amp
and an RC charging circuit. The RC charging circuit is connected to the
output of the op-amp and the voltage across the capacitor is connected
to one of the input of the op-amp. To another input of the op-amp the
variable pin of a potential divider is connected to which divides the
voltage from the output of the op-amp.
The
op-amp here acts as a simple comparator and the potential divider is
used to set the threshold of comparison. As the capacitor charges
through the output potential of the op-amp (either 5V or -5V), the
voltage across the capacitor increases. At some point the voltage across
the capacitor becomes greater than which has been set using the
potentiometer and as a result the output voltage of the comparator
changes. It forces the capacitor to discharge immediately. The capacitor
charges slowly through a resistor, but it discharges immediately
through a diode which conducts only when the current flows in the
discharging direction. Due to this slow charging and very fast
discharging the voltage across the capacitor appears like a ramp
waveform. The image of the circuit wired in the bread board is shown in
the following figure:
The
frequency of the ramp wave depends on the charging period of the RC
circuit. The charging period depends on the RC constant which is the
product of the values of the Resistance and the Capacitence.
Time period of the ramp,
Tramp = R * C
The image captured from the CRO screen displaying a ramp waveform is shown below:
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