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EXPERIMENT NO: 1

Aim: Setting up a Fiber Optic Analog Link.

Objective:The objective of this experiment is to study a 950 mm Fiber Optic Analog Link. In this

experiment we will study relationship between input signal and received signal.

Theory:Fiber Optics Link can be used for transmission of digital as well as analog signals. Basically

a fiber optic link contains three main elements, a transmitter, a fiber optic cable and a

receiver. The transmitter module takes the input signal in electrical form & then transforms

into optical energy containing the same information. The optical fiber is the medium which

carries this energy to receiver. At the receiver, light is converted back into electrical form

with the same pattern as originally fed into transmitter.

Transmitter:Fiber optic transmitters are typically composed of a buffer, driver & optical source. The

buffer electronics provides both an electrical connection & isolation between the transmitter

& the electrical system supplying the data. The driver electronics provides electrical power to

the optical source in a fashion that duplicate the pattern of data being fed to the transmitter.

Finally the optical source converts the electric current to light energy with the same pattern.

The LED SFH450V supplied with kit1 operates outside visible light spectrum. Its optical

output is centred at near infrared wavelength of 950nm. The emission spectrum is broad, so a

faint red glow can usually be seen when the LED is on in a dark room. The LED SFH450V

used in the kit1 is coupled to the transistor driver in common emitter mode. The driver is

preceded by the amplifier buffer. The amplifier in this case is LM741 operational amplifier

configured as voltage follower in the absence of input signal. In the absence of input signal

half of the supply voltage appears at the base of transistor. This biases the transistor near

midpoint within the active region for linear amplification. Thus LED emits constant intensity

of light at this time. When the signal is applied to the amplifier it overrides the DC level at

the base of the transistor which causes the Q point of the transistor to oscillate about

midpoint. So the intensity of the LED varies about its previous constant value. This variation

in the intensity has linear with the input electrical signal. Optical signal is then carries over by

the optical fiber.

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Receiver:The function of the receiver is to convert the optical energy into electrical form which is then

conditioned to reproduce the transmitted electrical signal in its electrical form. The detector

SFH250V used in the kit2 has a diode type output. The parameters usually considered in the

case of detector are its responsivity at peak wavelength & response time. SFH250V hasresponsivity of about 4microA per 10microW of incident optical energy at 950nm and it has

rise & fall time of .01microsec.

PIN photodiode is normally reverse biased. When optical signal falls on the diode, reverse

current start to flow, thus diode acts as closed switch and in the absence of light intensity, it

acts as an open switch. Since PIN diode usually has low responsivity, a trans impedance

amplifier is used to convert this reverse current into voltage. This voltage is then amplified

with the help of another amplifier circuit. This voltage is the duplication of the transmitted

electrical signal.

Equipments:Kit1 and kit2

20 MHz dual channel oscilloscope

1 MHz function generator

1 meter fiber cable

Block Diagram:

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Procedure:1. Slightly unscrew the cap of IR LED SFH450V from kit1. Do not remove the cap from

the connector. Once the cap is loosened, insert the fiber into the cap and assure that

the fiber is properly fixed. Now tighten the cap by screwing it back.

2. Connect the power supply cables with proper polarity to kit1 and kit2. Whileconnecting this, ensure that the power supply is OFF.

3. Connect the signal generator between the AMP I/P and GND posts in kit1 to feed theanalog signal to the pre-amplifier.

4. Keep the signal generator in sine wave mode and select the freq 1kHz with amplitudeof 2Vp-p. (Max level is 4Vp-p).

5. Switch on power supply and signal generator.6. Check the output signal of the pre-amplifier at the post AMP O/P in kit1. It should be

same as that of the input signal.

7. Now rotate the optical power control pot P1 located below power supply connector inkit1 in anticlockwise direction. This ensures minimum current flow through LED.

8. Short the following posts in kit1 with the links provided.a) +9V and +9V. This ensures supply to the transmitter.

b) AMP O/P and Transmitter I/P.9. Connect the other end of the fiber to detector SFH250V in kit2 very carefully as per

the instruction in step1.

10.Ensure that the jumper located just above IC U1 in kit2 is shorted to pins 2 and 3.Shorting of the jumper allows connection if PIN diode to trans impedance amplifier

stage.

11.Observe the output signal from the detector at DETECTOR O/P post on CRO byadjusting optical power control pot in kit1 and you should get reproduction of original

transmitted signal.

12.To measure the analog bandwidth of the link, keep the same connections and vary thefrequency of the input signal from 100Hz onwards. Measure the amplitude of the

received signal for each freq reading.

Conclusion:Thus from the above experiment we have studied about the 950 mm Fiber

Optic Analog Link. We have also studied about the relationship between input signal and

received signal. Fiber Optics Link can be used for transmission of digital as well as analog

signals.

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EXPERIMENT NO: 2

Aim:Study of Pulse Amplitude Modulation

Objective:The objective of this experiment is to obtain Amplitude Modulation of the analog signal,

transmit it over a fiber optic cable and demodulate the same at the receiver end to get back

the original signal.

Theory:Pulse Amplitude Modulation is a technique of communication in which the high freq square

wave is modulated by low freq signal. The modulating signal is sampled by the pulses. The

PAM signal is nothing but the high freq square wave in which the amplitude of each pulse is

equal to that of the information signal at the respective sampling instant.

The block diagram clearly indicates the pulse amplitude modulation and demodulation

concept.




1 meter fiber cable

Block Diagram:

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Procedure:1. Connect the power supply cables with proper polarity to kit1 and kit2. While

connecting this, ensure that the power supply is OFF.

2. Connect the signal generator between the PAM I/P and GND posts in kit1 to feed theanalog signal to the pre-amplifier.

3. Keep the signal generator in sine wave mode and select the freq 1kHz with amplitudeof 1Vp-p.

4. Switch on power supply and signal generator.5. Check that the clock circuit is properly working by connecting the oscilloscope probe

at CLK O/P post. You will find the square wave output with freq 32 MHz.

6. Now observe the output waveform at post PAM O/P post.7. Slightly unscrew the cap of IR LED SFH450V from kit1. Do not remove the cap from

the connector. Once the cap is loosened, insert the fiber into the cap and assure that

the fiber is properly fixed. Now tighten the cap by screwing it back. Similarly connect

the other end of the fiber to detector SFH250V in kit2.

8. Connect output of PAM circuit at PAM O/P post to the AMP I/P post with theshorting links provided in kit1.

9. Short the following posts in kit1 with the links provided.a) +9V and +9V. This ensures supply to the transmitter.

b) AMP O/P and Transmitter I/P.10.Observe the output signal from the detector at DETECTOR O/P post on CRO by

adjusting optical power control pot in kit1 and you should get reproduction of original

transmitted signal.

Conclusion:Thus from above experiment we have studied about how to obtain the

Pulse Amplitude Modulation of the analog signal, transmit it over a fiber optic cable and

demodulate the same at the receiver end to get back the original signal.

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EXPERIMENT NO: 3

Aim: Study of simple Time Division Multiplexing

Objective:The objective of this experiment is to study of Time Division Multiplexing for 4 analog

channels using 950nm fiber optic analog link.

Theory:This technique unable simultaneous transmission of number of signals along the same

communication channel. The basic concept behind this is that the signals from different

sources are multiplexed by allowing only a specific time slot for each channel. At the receiver

side, the multiplexed data is distributed to the corresponding channels. The block diagram

shows different parts used in TDM circuit. The circuit consist of three parts.

1. Clock pulse generator2. Four stage Up/Down binary counter3. Analog multiplexer

The circuit action is follows:

The timer 555 is used to generate clock signal of 32 kHz. This is given as clock input to IC

74LS191 which is operational in UP counting mode. All parallel load inputs are tied to

ground so that at power on it, it is in the reset condition. Hence outputs Q0, Q1, Q2, Q3 are

all low. The counter increments from the first clock pulse at the input. Analog inputs are

applied to the analog switch 4016. A two as to four decoder 74LS139 decodes the counteroutput to generate enable signals for analog switches (4016). The outputs of all four analog

switches are short and we observe time division multiplexing of four analog signals at this

output.




1 meter fiber cable

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Block Diagram:

Procedure:1. Connect the power supply cable with proper polarity to kit1 and kit2. While

connecting this, ensure that the power supply is off. Now turn on the supply.

2. Connect the CRO probe to CLK O/P post in kit1. You will see square wave. Freq is32kHz.

3. Kit 1 contains four sinusoidal signal generators of different freq as 2kHz, 1kHz,500Hz and 250 Hz. Observe these signals and adjust their amplitude at 2Vp-p.

4. With connecting cables provided along with the kit, apply these signals to the fourchannels at test CH1, CH2, CH3, CH4 and observe the TDM output at TDM O/P

post.

5. Apply signal available at TDM O/P post to buffer input at AMP I/P post.

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6. Connect AMP O/P post to TRANMITTER I/P post in kit1. This connects theamplified output to input of transmitter LED. Also short posts +9V and +9V.

7. Slightly unscrew the cap of IR LED SFH450V from kit1. Do not remove the cap fromthe connector. Once the cap is loosened, insert the fiber into the cap and assure that

the fiber is properly fixed. Now tighten the cap by screwing it back. Similarly connectthe other end of the fiber to detector SFH250V in kit2.

8. With the help of optical power control pot P1 in kit1 adjust the proper amplitude ofthe received signal at DETECTOR O/P post.

9. Connect the detector output from DETECTOR O/P post to the post TDM I/P in kit2.10.Now establish link between CLK O/P post in kit1 and CLK I/P in kit2.11.With the help of connecting wires short test points CHRX1, CHRX2, CHRX3 and

CHRX4to the test points CH1 I/P, CH2 I/P, CH3 I/P and CH4 I/P respectively in kit2.

12.You will observe four different signals at the output channels marked as CH1, CH2,CH3, CH4 of Time Division Multiplexer in kit2.

Conclusion:Thus from this experiment we have studied about the Time Division

Multiplexing for 4 analog channels using 950nm fiber optic analog link.

Documents

EXP1,2,3