Objective: Using the concept developed in earlier experiments to design a simple set up for
transmission of analog signal.
To understand the design aspects and performance testing involved.
Aim: Design, build and test a simple fiber optical link for transmission of analog signal.
Apparatus and specification: Transmitter & receiver, power supply, 20 MHz oscilloscope, fiber cable.
Optical fiber used: MMSI
Optical source used: LED/Laser
Optical transmitter TIL38 : PN gallium arsenide infrared emitting diode.
Features:
1. Designed to emit near infrared region.
2. Output spectrally compatible with silicon sensors.
3. High power output.
4. High power efficiency.
5. Plastic package with two leads for ease of handling
Optical receiver [TIL 81]
[N-P-N Si photo transistor]
Features:
1. Designed for automatic or hand insertion in sockets or PC boards.
2. Recommended for industrial application requiring low cost discrete photo transistor.
3. Spectrally & mechanically matched with TIL 38 IR emitter.
Theory: Fig: Basic elements of an analog link and the major noise contributors
Above figure shows the basic elements of analog link. The transmitter contains either an LED or a laser diode optical source. In analog applications, one first sets a bias point on the source approximately at the midpoint of the linear output region. The analog signal can then be sent using one of several modulation techniques. The simplest form for optical fiber links is direct intensity modulation, wherein the optical output from the source is modulated simply by varying the current around the bias point in proportion to the message signal level. Thus, the information signal is transmitted directly in the baseband.
A somewhat more complex but more efficient method is to translate the baseband signal onto electrical subcarrier prior to intensity modulation of the source. This is done using standard amplitude modulation, frequency modulation or phase modulation techniques. No matter which method is implemented, one must pay careful attention to signal impairments in the optical source. These include harmonic distortions, intermodulation products, relative intensity noise (RIN) in the laser, and laser clipping.
In relation to the fiber-optic element, one must take into account the frequency dependent of the amplitude, phase, and group delay in the fiber. Thus, the fiber should have a flat amplitude and group-delay response within the passband required to send the signal free of linear distortion. In addition, since modal-distortion-limited bandwidth is difficult to equalize, it is best to choose a single mode fiber. The fiber attenuation is also important, since the carrier-to-noise performance of the system will change as a function of the received optical power.
Procedure:
Set the switch SW8 to the ANALOG position. Switch the power on. The Power On switch is located at the top right- hand corner.
INPUT-OUTPUT RELATIONSHIP OF LINK
1. Feed a sinusoidal wave of 1KHz, 1VP-P [with zero d.c.] from the function generator to P11. The PIN detector output signal is available at P32 in the optical Rx1 block. Vary the input signal level driving the LED and observe the received signal at the PIN detector. Plot the received signal peak-to-peak amplitude with respect to the input signal peak-to-peak amplitude.
2. Using the 3m fiber instead of the 1m fiber repeat above step. Plot the received signal amplitude at the PIN detector as a function of input signal amplitude.
GAIN Control
1. The PIN detector signal at P32 is amplified, with amplifier gain controlled by the GAIN potentiometer as shown in Fig 1.3. With a 3VP-P input signal at P11, observe P31 as the gain potentiometer is varied.
Bandwidth of the Fiber Link
Measure the bandwidth of the link as follows:
1. Apply a 2VP-P sinusoidal signal [with zero d.c.] at P11 and observe the output at P31. Adjust GAIN such that no clipping takes place. Vary the frequency of the input signal from 100 Hz to 5 MHz and measure the amplitude of the received signal. Plot the received signal amplitude as a function of frequency [using a logarithmic scale for frequency]. Note the frequency range for which the response is flat.
Conclusion:
transmission of analog signal.
To understand the design aspects and performance testing involved.
Aim: Design, build and test a simple fiber optical link for transmission of analog signal.
Apparatus and specification: Transmitter & receiver, power supply, 20 MHz oscilloscope, fiber cable.
Optical fiber used: MMSI
Optical source used: LED/Laser
Optical transmitter TIL38 : PN gallium arsenide infrared emitting diode.
Features:
1. Designed to emit near infrared region.
2. Output spectrally compatible with silicon sensors.
3. High power output.
4. High power efficiency.
5. Plastic package with two leads for ease of handling
Optical receiver [TIL 81]
[N-P-N Si photo transistor]
Features:
1. Designed for automatic or hand insertion in sockets or PC boards.
2. Recommended for industrial application requiring low cost discrete photo transistor.
3. Spectrally & mechanically matched with TIL 38 IR emitter.
Theory: Fig: Basic elements of an analog link and the major noise contributors
Above figure shows the basic elements of analog link. The transmitter contains either an LED or a laser diode optical source. In analog applications, one first sets a bias point on the source approximately at the midpoint of the linear output region. The analog signal can then be sent using one of several modulation techniques. The simplest form for optical fiber links is direct intensity modulation, wherein the optical output from the source is modulated simply by varying the current around the bias point in proportion to the message signal level. Thus, the information signal is transmitted directly in the baseband.
A somewhat more complex but more efficient method is to translate the baseband signal onto electrical subcarrier prior to intensity modulation of the source. This is done using standard amplitude modulation, frequency modulation or phase modulation techniques. No matter which method is implemented, one must pay careful attention to signal impairments in the optical source. These include harmonic distortions, intermodulation products, relative intensity noise (RIN) in the laser, and laser clipping.
In relation to the fiber-optic element, one must take into account the frequency dependent of the amplitude, phase, and group delay in the fiber. Thus, the fiber should have a flat amplitude and group-delay response within the passband required to send the signal free of linear distortion. In addition, since modal-distortion-limited bandwidth is difficult to equalize, it is best to choose a single mode fiber. The fiber attenuation is also important, since the carrier-to-noise performance of the system will change as a function of the received optical power.
Procedure:
Set the switch SW8 to the ANALOG position. Switch the power on. The Power On switch is located at the top right- hand corner.
INPUT-OUTPUT RELATIONSHIP OF LINK
1. Feed a sinusoidal wave of 1KHz, 1VP-P [with zero d.c.] from the function generator to P11. The PIN detector output signal is available at P32 in the optical Rx1 block. Vary the input signal level driving the LED and observe the received signal at the PIN detector. Plot the received signal peak-to-peak amplitude with respect to the input signal peak-to-peak amplitude.
2. Using the 3m fiber instead of the 1m fiber repeat above step. Plot the received signal amplitude at the PIN detector as a function of input signal amplitude.
GAIN Control
1. The PIN detector signal at P32 is amplified, with amplifier gain controlled by the GAIN potentiometer as shown in Fig 1.3. With a 3VP-P input signal at P11, observe P31 as the gain potentiometer is varied.
Bandwidth of the Fiber Link
Measure the bandwidth of the link as follows:
1. Apply a 2VP-P sinusoidal signal [with zero d.c.] at P11 and observe the output at P31. Adjust GAIN such that no clipping takes place. Vary the frequency of the input signal from 100 Hz to 5 MHz and measure the amplitude of the received signal. Plot the received signal amplitude as a function of frequency [using a logarithmic scale for frequency]. Note the frequency range for which the response is flat.
Conclusion:
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