fiber optics project presentation

8/3/2019 Fiber Optics Project Presentation

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Pulse-width modulation (PWM), or

pulse-duration modulation (PDM), is a

commonly used technique for controllingpower to inertial electrical devices, made

practical by modern electronic power

switches.



LM555 IC

MC1496 IC

UA741



R4

DC7

Q3

G

N D

1

V C C

8

TR2

TH6

CV5

U1

555

R4

DC7

Q3

G

N D

1

V C C

8

TR2

TH6

CV5

U2

555

C2

100u

C5

100u

C4100u

R21k RV1

5K

C10.01u

R310k

R710k

R42K

R52KC3

0.01u

R1300 R6

300

12V

C60.018u

PWM SIGNAL OUTPUT

Audio Signal Input

pwm modulator circuit diagram



This method consists of a product detector and a low-pass filter.

The PWM and the carrier signals are connected to the inputs of

product detector, and then sequence of pulses having the width

inversely proportional to the width of PWM pulse presents at

output. When the Va signal passes through the low-pass filter, ademodulated output signal is obtained

PWM

input

Carrier input

Demodulated

output



. The differential amplifier (Q5

and Q6) is used to drive the differential

amplifiers Q1Q2 and Q3Q4. The constant-current source generator (Q7 and Q8)

provides the differential amplifier Q5

and Q6 with a constant current. Overall

gain of MC1496 can be controlled by externally connecting a resistor

between pins 2 and 3. For PWM demodulation, the PWM signal should beapplied to pins 1 and 4, and the carrier to pins 8 and 10. The bias current to

pin 5 is commonly provided by connecting a series resistor from this pin to

the power supply. Since the detector has two outputs (pins 6 and 12), one of

the outputs can be used as the detector output and the other used for the use

of automatic gain control (AGC).



The V1 and V2 amplifiers control the input amplitudes of PWM and carrier signals

within the range of 300 mVpp to 1400 mVpp, so that the detector operates in linear

region. The resistor R7 connected between pins 2 and 3 determines the voltage gain

of MC1496. The coupling capacitors C1, C2, C4, C5

and C9 are used to block DC

signals while passing AC signals. The VR1 and VR2 control the gains of V1 and V2,

respectively. The value of VR3 determines the magnitude of PWM signal to the inputof the detector. The U4 operates as a second-order low-pass filter



Connect the PWM OUTPUT of the PWM Modulator circuit into the PWM INPUT of the PWM Demodulator circuit.

Looking at the V 1

OUTPUT using oscilloscope, vary the VR1

of the PWM Demodulator to get a rectangular signal with minimum distortion andmaximum amplitude (from V 1 OUTPUT ).

Connect your oscilloscope probe into DE MO D.(demodulation) OUTPUT .Carefully adjust VR2 and VR3 until getting a proper demodulated signal at DE MO D.OUTPUT . Now see both AUDIO INPUT and DEMOD. OUTPUTsignals together on your oscilloscope . Realize that we have recovered ouroriginal signal ( AU D IO INPUT ).

Using the oscilloscope, observe the signals of PWM INPUT, CA RR I E R

INPUT ,V 1 OUTPUT,V 2 OUTPUT, MC 1496 OUTPUT and DE MO DULATION OUTPUT . Record the results in Table

Remain all conditions but change the input frequency of AU D IO INPUT to500 Hz.

fiber optics project presentation

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