20-07-2013, 02:54 PM
SETTING UP OF FIBER OPTIC “DIGITAL LINK”
AIM:
The objective of this experiment is to study a 950-nm Fiber optic Digital Link.
THEORY:
Transmitter: -LED and digital DC coupled transmitters are one of the most popular varieties due to their
ease of fabrication. We have used a standard TTL gate to drive a NPN transistor, which modulates the
LED SFH 450V source.
Receiver: - SFH-551V is a digital opt detector. It delivers a digital output, which can be processed
directly with little additional external circuitry. The integrated circuit inside the SFH 551V opth detector
comprises the photo diode device, a Trans impedance amplifier, a comparator and a level shifter. The
photo diode converts the detector light in to the photo current. With the aid of an integrated lens the light
emanating from the plastic fiber is almost entirely focused on the surface of the diode. At the next page
the Tran impedance amplifier converts the photo current into a voltage. In the comparator the voltage is
compared to a reference voltage. In over to ensure good synchronism between the reference and the
transimpedance output voltage, the former is derived from a second circuit of a similar kind, which
incorporates a blind photo diode. The comparator derives a level shifter with an open collector output
stage. Here a catch diode prevents the saturation of the output transistor, thus limiting the output voltage to
the supply voltage.
PROCEDURE
1. Slightly unscrew the cap of IR LED SFH 450 V (950nm). Do not remove the cap from the connector. Once
the cap is loosened, insert the fiber in to the cap.
2. Now short the jumpers JP15, JP17, JP18, JP20&JP21 as shown in the block diagram.
3. Connect the power chord to the kit, switch on the power supply.
4. Feed the TTL signal of about 1 KHz from the function generator to connector labeled as EXT-TTL using
connecting cables provided with the kit.
5. Connect the other end of the Fiber to detector SFH 551V (digital detector) very carefully.
6. Observe the received signal on CRO at TP 22. The transmitted signal and the received signal are same
except for a slight delay in the received signal due to transmission through fiber.
7. Vary the frequency of the received signal & observe the output response. Determine the max. Bit rate that
can be transmitted on the digital link.
8. Slightly unscrew the cap of IR LED SFH756V (660nm) and insert the fiber optic cable.
9. Short jumpers JP16, JP17, JP18, JP20 and JP21 as in the diagram
10. Feed the TTL signal of about 1 KHz from the function generator to connector labeled as EXT-TTL using
connecting cables provided with the kit.
11. Connect the other end of the Fiber to detector SFH 551V (digital detector) very carefully.
12. Observe the received signal on CRO at TP 22. The transmitted signal and the received signal are same
except for a slight delay in the received signal due to transmission through fiber.
13. Vary the frequency of the received signal & observe the output response. Determine the max. Bit rate that
can be transmitted on the digital link.