Designing a high-fidelity photonics telecommunications system
PRACTICAL ACTIVITY 20 Audio transmission with a
light beam
C
A
B
E D
5.3 QUESTIONS
The following questions refer to the laser telecommunication system developed in Section 5.3.
1 Which modulation method was used for the laser telecommunication system that transmitted the audio signal?
A intensity modulation B frequency modulation C wavelength modulation D sound modulation
2 With the laser telecommunications system used to transmit an audio signal, why is an LD used in preference to an LED?
A LEDs emit a wider range of wavelengths than LDs.
B Unlike an LED, an LD’s beam spot does not spread out much even over long distances.
C Unlike an LED, an LD’s light output can be easily modulated.
D All of the above.
3 With the laser telecommunication system used to transmit an audio signal, why is it preferable to use a phototransistor rather than an LDR?
A The phototransistor generates a larger output voltage for the modulation.
B The LDR has a poorer high-frequency response.
C The phototransistor uses ‘transistor action’ to amplify the signal illuminating its base.
D All of the above.
4 What is the most significant limitation associated with using an opaque object to interrupt the laser beam to create modulation of the audio signal?
A Difficulty with permanently gluing object to speaker membrane.
B Loss of high-frequency response of speaker when object is attached.
C Movement of speaker membrane insufficient to interrupt all the laser beam
D Opaque object heats up when irradiated by the laser beam.
5 With the laser pointer used in the laser
telecommunication system, the positive terminal of the external battery or power supply:
A should be connected only to the central spring of the drive circuit
B can be connected to either the central spring or outer barrel casing
should be connected only to the outer barrel casing
D should be connected to neither the central spring nor the outer barrel casing.
6 The output of the laser pointer used in the laser telecommunication system varies as a function of the power supply voltage. Which one of the following is true?
A The output varies linearly between 0 and 2.9 V.
B The output varies linearly between 0 and 4.5 V.
C The output varies linearly between 4.5 and 12 V.
D The output varies linearly between 2.9 and 4.5 V.
7 If the power supply to the laser diode pointer was increased to 12 V, which one of the following would occur first?
A The laser diode would have too much current flowing through it and the diode would be permanently damaged.
B The laser diode would draw too much current from the power supply and the supply would be
permanently damaged.
C The laser diode would emit too much light and the receiver speaker would be permanently damaged.
D The laser diode would emit too much light and the optical detector would overheat.
8 The laser diode pointer should be mounted into an insulated supporting frame so that:
A the laser diode’s outer case is not accidentally shorted to earth
B the experimenter only handles the frame and not the laser diode, thus avoiding electrocution
C the supporting frame does not overheat D all of the above.
9 The power supply for a laser diode should never exceed its normal operating value by more than:
A 500%
B 0%
C 50%
D 100%
10 The 0.1 µF capacitor in the optical phototransistor detector circuit:
A reduces the ‘transistor action’ of the detector B increases the ‘transistor action’ of the detector C filters out any DC contribution of the laser beam D none of the above.
CHAPTER REVIEW
1 Photodiodes, as used in fibre optic telecommunication systems, are normally:
A forward biased to generate a voltage that is directly proportional to the light intensity
B reverse biased to generate a current that is directly proportional to the light intensity
C forward biased to generate a current that is directly proportional to the light intensity
D unbiased to generate a voltage that is directly proportional to the light intensity.
2 Which one of the following is generally true for the response times of photodetectors?
A LDRs are faster than phototransistors which are faster than photodiodes.
B Phototransistors are faster than LDRs which are faster than photodiodes.
C Photodiodes are faster than LDRs which are faster than phototransistors.
D Photodiodes are faster than phototransistors which are faster than LDRs.
3 Which one of the following is suitable for a high-speed fibre-optics telecommunication system?
A A receiver which uses an LDR.
B A receiver which uses a reverse-biased photodiode.
C A transmitter which uses a reverse-biased LD.
D A transmitter which uses a forward-biased
phototransistor.
4 An LED draws 100 mA when 2 V is applied across its terminals.
Under these conditions, the LED produces an optical power of 2 mW.
a How much electrical power is dissipated in the LED?
b What is the LED’s conversion efficiency from electrical to optical power?
5 A photodiode has a circular active area with a radius of 2 mm.
Determine the light intensity (W m–2) at the detector, if the optical power detected is 0.1 mW.
6 The output of the circuit shown is Vout=3 V. If the LDR has the same characteristics as that shown in Figure 5.13, determine the intensity of light illuminating the detector.
7
If Vout=6 V, determine the light intensity illuminating the photodiode, assuming it has the same I–V characteristics as those of Figure 5.16.
8
The circuit shown has an LDR detector (V1) and a photodiode detector (V2). The LDR and the photodiode are both illuminated with light of the same intensity (Φ).
The LDR and the photodiode have the same characteristics as those shown in Figure 5.13 and Figure 5.16, respectively.
a If the intensity of the light increases (Φfinal> Φinitial), determine what happens to V1 and V2(increase, decrease, no change). Give reasons for your answer.
b If the light intensity (Φ) is 3 W m–2, calculate V1. c The dual detection system is
to be calibrated so that both outputs are the same for a light intensity of 3 W m–2. Determine the value of R2that ensures that V1=V2for Φ =3 W m–2.
d Is there any other particular light intensity where V1=V2? Give reasons for your answer.
9
a Determine the resistance (Rx) of the LDR at which the transistor just turns off.
Assume the transistor just turns off when Vbe≈0.65 V, and that the transistor’s base current (Ib) is much smaller than the current flowing in the Rx/R2voltage divider.
b If RLDRincreases to 10Rx, what
EXAM - STYLE QUESTIONS