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PULSE WIDTH MODULATION AND DEMODULATION
ABSTRACT
This document gives an overview of how pulse width modulation and demodulation can be performed using bipolar junction transistors and operational amplifiers. The simulation for the same was carried out using ngspice. The results obtained have been presented at the end of this document.
INTRODUCTION
Pulse Width Modulation is a modulation process or technique used in most communication systems for encoding the amplitude of a signal right into a pulse width or duration of another signal (carrier signal) for transmission. It is actually used for controlling the amplitude of digital signals in order to control devices and applications requiring power or electricity.
Modulation can be performed using the following figure. The operational amplifier is used as a comparator with the input signal i(t) given to its non-inverting terminal and the triangular signal r(t) given to its inverting terminal.
When the input signal is higher than the triangular signal, we get logic level „1‟.
When input signal is lower than the triangular signal, we get logic level „0‟.
Increase in duty cycle increases the power delivered. This leads to better efficiency of the signal.
Figure 1: Circuit for Modulation
Thus, pulses are formed with the frequencies of both, the input signal and the modulated signal, being the same. The on-off behavior of the signal A simple emitter follower Q1 serves as the PWM's output stage. This transistor is driven into one of two states: fully ON or fully OFF. And therein lies the virtue of the PWM. In either of these states, the
transistor power is low. The power of Q1 can be calculate by
PQ1 = VCE x ILOAD
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Q1 |
Q1 power |
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state |
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Full ON |
PQ1 = VCE (LO) x |
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ILOAD (HI) = LO Power |
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Full OFF |
PQ1 = VCE (HI) x |
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ILOAD (LO) = LO Power |
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On comparing the power dissipation with linear output stage, we observe that the equation PQ2 = VCE x ILOAD gives us a value that is more than PQ1.
Demodulation can be performed by implementing the circuit shown in Figure 2. PWM can be demodulated by converting it to PAM and sending it through low pass filter. Input PWM is applied to a ramp generator and a synchronous pulse generator. The synchronous pulse generator will generate a pulse waveform such that the pulse will end at the beginning of each PWM pulse and the ramp generator will produce a ramp signal whose amplitude is proportional to the width of the PWM signal. On adding these two, we clip the waveform at a particular level, thus giving the PAM signal. This is then passed through the low pass filter to obtain the original input signal.
Figure 2: Circuit for Demodulation
NGSPICE CODES
Subcircuit of op-amp
*op amp*
.subckt subopamp 1 2 6
r1 1 2 1meg
r2 3 4 1k
c1 4 0 1.5u
e1 3 0 2 1 100k
e2 5 0 4 0 1
r3 5 6 100
.ends opamp
Subcircuit of PWM
.subckt subpwm 1 2 3
Rsin 1 0 1K
Rtri 2 0 1MEG
Bpwm 3 0 V = PWL( V(1,2), -10m,0, -1m,0, 1m,5, 10m,5 )
Rpwm 3 0 1MEG
.ends subpwm
Modulation
*PWM*
VIN 1 0 sin(5V 4V 500HZ)
RIN 1 0 1K
VTRI 2 0 pulse(0V 10V 0 49US 49US 1US 100US)
RTRI 2 0 1MEG
ECOMP 3 0 table {V(1,2)} = (-1MV,0V) (1MV,10V)
RCOMP 3 0 1MEG
.model bjt npn
VCC 10 0 DC 10V
Q1 10 3 11 bjt
RL1 11 0 20
.model bjt npn
Q2 10 1 12 bjt
RL2 12 0 20
.control
tran 5US 2000US
run
set color0=white
set color1=black
plot V(1) V(2)
plot V(3)
.endc
.end
Figure 3: Input to Modulator
Figure 4: Modulated Output
Demodulation
pwm demodulation
.include subpwm.cir
x1 1 2 3 subpwm
Vsin 1 0 SIN(0.5 1 200K)
Vtri 2 0 PWL(0 0 2us 1.3 2.1us 1.3 2.2us 0) r=0
r1 3 4 560
.model bjt npn
q1 5 4 0 bjt
q2 8 7 0 bjt
r2 6 5 1k
r3 5 7 100
r4 6 8 10k
r5 8 9 2.7k
r6 9 10 10k
c2 9 0 .1u
c3 10 0 .22u
.include subopamp.cir
x2 11 10 11 subopamp
v2 6 0 dc 12v
r7 0 13 1k
r8 13 14 3k
x3 13 11 14 subopamp
v3 14 15 dc 27
d1 16 15
r9 16 17 1k
v4 17 0 dc 1.4
.tran .1m 100m
.control
run
display
set xbrushwidth=2
plot v(16)
.endc
.end
Figure 5: Demodulated output
ADVANTAGES OF PWM
1. Synchronisation between the transmitter and receiver is not required
2. Less noise interference
3. High Efficiency
4. Low power consumption
5. Less complex circuitry
6. Cost effective
DISADVANTAGES OF PWM
1. Variable power due to the varying width of pulse.
2. Bandwidth requirement is large.
APPLICATIONS
1. In telecommunication systems
2. In power delivering systems to control the amount of power delivered to the load without incurring losses.
3. In robotics to control the speed of robots
4. In audio signals and amplification purposes