12/29/13 SETTING UP A FIBER OPTIC ANALOG LINK

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SETTING UP A FIBER OPTIC ANALOG LINK

OBJECTIVE:

The objective of this experiment is to study an 660 nm/ 950 nm Fiber OpticAnalog Link. In this experiment you will study the relationship between theinput signal & received signal.

EQUIPMENTS:Link-A kit.20 MHz Dual Channel Oscilloscope.

1 Meter Fiber cable.Power supply.

THEORY:Fiber Optic Links can be used for transmission of digital as well as analog

signals. Basically a fiber optic link contains three main elements, a transmitter, anoptical fiber and a receiver. The transmitter module takes the input signal inelectrical form and then transforms it into optical (light) energy containing the sameinformation. The optical fiber is the medium which carries this energy to thereceiver. At the receiver, light is converted back into electrical form with the samepattern as originally fed to the transmitter.

TRANSMITTER:Fiber optic transmitters are typically composed of a buffer, driver and optical

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source. The buffer electronics provides both an electrical connection and isolationbetween the transmitter and the electrical system supplying the data. The driverelectronics provides electrical power to the optical source in a fashion thatduplicates the pattern of data being fed to the transmitter. Finally the optical source(LED) converts the electrical current to light energy with the same pattern. The LEDSFH756V supplied with the kit operates inside the visible light spectrum. It's opticaloutput is centered at near visible wavelength of 660 nm.

The emission spectrum is broad, so a dark red glow can usually be seen when theLED is on. The LED SFH450V supplied with the kit operates outside the visiblelight spectrum. It's optical output is centered at near infrared wavelength of 950 nm.

RECEIVER:

The function of the receiver is to convert the optical energy into electrical formwhich is then conditioned to reproduce the transmitted electrical signal in its originalform. The detector SFH250V used in the kit has a diode type output. Theparameters usually considered in the case of detector are it's responsivity at peakwavelength and response time. SFH250V has responsivity of about 4 mA per 10mW of incident optical energy at 950 nm and it has rise and fall time of 0.01 m sec.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 oflight intensity, it acts as an open switch. Since PIN diode

usually has low responsivity, a trans impedance amplifier is used to convert thisreverse current into voltage. This voltage is then amplified with the help of anotheramplifier circuit. This voltage is the duplication of the transmitted electrical signal.

INTENSITY P3

P1 P2

D

D

R E RLEVEL FREQ

I T

E

OFF ON

V

C

IN OUT TX IN E

FIBRE

ANALOG1-10 HZ

SINE T R

OPTIC RX 1 R OUT10-100 HZ

ANALOG

TX 1/ TX 2100-1KHZ

BUFFER

TRANSMITTERCABLE

TRANSIMPEDANCE

1-10KHZ

RECEIVER

SW 1

S3 AMPLIFIER

O

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SQUARE

TX IN

VI

FUNCTION GENERATOR

BLOCK DIAGRAM FOR SETTING UP FIBRE OPTIC ANALOG LINK

JP 2 JUMPER SETTING DIAGRAM FOR ANALOG LINK

+5V

+12V FOR TX1 (SFH756V)

JP 3

JP 4

+5V

+12V

TX1 TX1

OFF

OFF

ON

ON

JP 5 JP 6

1 2 3 4 1 2 3 4

JP 7

SF1 SF2

TX2

TX2

SWITCH FAULTS

PROCEDURE:

Make the connections and Jumper settings as shown in block diagramConnect the power supply cables with proper polarity to kit. While connectingthis, ensure that the power supply is OFF.Keep all the switch faults in OFF position.Switch on the power supply.Select the frequency range of Function Generator with the help of RangeSelection Switch SW1, frequency can be varied with Pot P2. Adjust thevoltage LEVEL of the Sine Wave with Pot P1 as per following setting.FREQUENCY: 1KHz, LEVEL: 2Vp-p.

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Connect SINE post of the Function Generator section to IN post of Analog BufferSection. Keep Jumpers JP2 & JP4 towards +12V position, JP3 towards sineposition, JP5 towards TX1 position, JP6 towards TX1 position & JP7 shorted.

Keep switch S3 towards TX IN position.Connect OUT post of the Analog Buffer Section to TX IN post of TRANSMITTER.

Slightly unscrew the cap of LED SFH 756V TX1 (660 nm) from kit. Do notremove the cap from the connector. Once the cap is loosened, insert the fiberinto the cap and assure that the fiber is properly fixed. Now tight the cap byscrewing it back. Keep INTENSITY pot P3 at minimum position i.e. fullyanticlockwise.Connect the other end of the fiber to detector SFH 250V (RX 1) in kit verycarefully.Check the output signal of the Analog Buffer at its OUT post in Kit. It should besame as that of the applied input signal.Observe the output signal from the detector at ANALOG OUT post on CRO byadjusting INTENSITY (Optical Power Control) Pot P3 in kit and you should getthe reproduction of the original transmitted signal.

To measure the analog bandwidth of the link, connect the external SignalGenerator with 2Vp-p sine wave to IN post of Analog Buffer Section and vary thefrequency of the input signal from 100 Hz onwards. Measure the amplitude of the

received signal for each frequency reading.

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Plot a graph of gain / Frequency. Measure the frequency range for which theresponse is flat.Keep Jumpers JP5 & JP6 towards TX2 position.Remove Fiber from TX1. Slightly unscrew the cap of LED SFH 450V TX2 (950nm) from kit. Do not remove the cap from the connector. Once the cap isloosened, insert the fiber into the cap and assure that the fiber is properly fixed.Now tight the cap by screwing it backs. Keep INTENSITY pot P3 at minimumposition i.e. fully anti clockwise.Check the output signal of the Analog Buffer at its OUT post in Kit. It should besame as that of the applied input signal.

Observe the output signal from the detector at ANALOG OUT post on CRO byadjusting INTENSITY (Optical Power Control) Pot P3 in kit and you should getthe reproduction of the original transmitted signal.

To measure the analog bandwidth of the link, connect the external SignalGenerator with 2Vp-p sine wave to IN post of Analog Buffer Section and vary thefrequency of the input signal from 100Hz onwards. Measure the amplitude of thereceived signal for each frequency reading.Plot a graph of gain / Frequency. Measure the frequency range for which the

response is flat.

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