Amrita Sai Institute of Science & Technology
Approved by AICTE, Permanently Affiliated to JNTU, KAKINADA
Accredited by NAAC with ‘A’ Grade
Paritala, Kanchikacherla (MD), Krishna (DT). AP, India – 521 180
DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING
EXPT.No:10 ASIST
CHARACTERISTICS OF LED
Aim: The aim of the experiment is to study the relationship between the LED dc forward
current and the Light Emitting Diode (LED) optical power output and determines the
linearity of the device at 660nm as well as 850nm.
Equipment Required:
S.No Equipment Quantity
1. LINK A KIT 1
2. POWER SUPPLY UNIT 1
3. Fiber optic cable 1
4. Digital Multi meters 2
Circuit Diagram:
DMM 1 TX-Unit
RX-Unit
Vo LED1
OF
LED2 Cable FO PT
SET PO
PO
DMM 2
Theory: LEDs and laser diodes are the commonly used sources in optical communication
systems, whether the system transmits digital or analogue signals. In the case of analogue
transmission, direct intensity modulation of the optical source is possible, provided the
optical output from the source can be varied linearly as a function of modulating electrical
signal amplitude. LEDs have a linear optical output with relation to the forward current over
a certain region of operation. It may be mentioned that in many low-cost, short-haul and
small bandwidth applications, LEDs at 660nm, 850nm and 1300nm are popular. While direct
intensity modulation is simple to realize, FM modulating the base-band signal achieves
higher performance prior to the intensity modulation.
The relationship between an LED optical output Po and the LED forward current I F is given by
Po=K.IF (over a limited range), where K is a constant
Procedure:
1. Connect one end of Cable 1 to the LED1 port of FO analog transmitter and the other
end to the FO pin (power meter) port of FO analog receiver.
2. Set DMM1 to the 200mV range and connect the green wires marked Po on the Rx unit
to it. The power meter is ready for use. Po = (Reading)/10dBm.
3. Set DMM2 to the 200mV range and connect it between the Vo1 and ground in the TX
unit IF1=Vol (mV)/100(mA).
4. Plug the AC mains for both units. Adjust the SETPO knob on the TX unit to the
extreme anti clockwise position to reduce IF to 0mA. The reading on the power meter
should be out of range.
5. Slowly turn the SETPO knob in clockwise to increase I F1. The power meter should read
30.0dB approximately. From here change in suitable steps and note the power
mater readings, Po. Record up to the extreme clockwise position.
6. Repeat the complete experiment for Po LED2 and tabulate the readings for
Vo2&Po.IF2 = Vo2 (mV)/50(mA). Apply the correction of 2.2dB discussed.
7. Record of Observations:
Amrita Sai Institute of Science & Technology
Approved by AICTE, Permanently Affiliated to JNTU, KAKINADA
Accredited by NAAC with ‘A’ Grade
Paritala, Kanchikacherla (MD), Krishna (DT). AP, India – 521 180
DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING
FOR 660nm:
Sno Vol(mV) IF1=Vo1/100(mA) Po(dBm)
1
2
3
4
5
6
7
8
9
10
FOR 850nm:
Sno Vo2(mV) IF2=Vo2/100(mA) Po(dBm)
1
2
3
4
5
6
Modal graph:
Po (dbm)
If (ma)
Precautions:
1. The Tx-Unit and Rx-Unit must be handled with care.
2. Fiber optic cable connection must be firm.
Result: Thus the characteristics of Light Emitting Diode’s (LED’s) are studied .
