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Optical Communications Multiple Choice Questions on “Receiver Sensitivities”.
1. ASK with heterodyne detection can achieve same SNR limit as ASK with asynchronous detection.
a) True
b) False
Answer: a
Explanation: The coherent optical systems’ receiver sensitivities depend on the SNR limit and the 3dB improvement in the receiver bandwidth. The ASK with heterodyne and asynchronous detection allows for the same receiver SNR limit.
2. In a ______________ receiver, the analyses of signal and noise phenomena are more complicated than in the IM-DD case.
a) Homodyne
b) Heterodyne
c) Circular
d) Heterogeneous
Answer: b
Explanation: The optical output appearance shows the analysis of signal and noise phenomenon. In a heterodyne receiver, the analyses of signal and noise phenomena are more complicated than in the IM-DD case because the optical detector output appears as an IF signal and not as a baseband signal.
3. FSK modulation is attributed to the use of __________ frequencies unlike ASK modulation.
a) One
b) Three
c) Two
d) Four
Answer: c
Explanation: FSK heterodyne detection has receiver sensitivity of 3dB more than tha ASK. Hence, it is attributed to the use of two frequencies unlike ASK where only one frequency is used.
4. BER of FSK modulation scheme is ________ as/to the ASK modulation scheme.
a) Twice
b) Thrice
c) Unequal
d) Same
Answer: d
Explanation: FSK modulation uses two frequencies. The similar BER can be obtained with the two modulation schemes when the average power is transmitted.
5. ___________ can also be used in place of multilayered filters in the dual filter direct detection FSK receiver.
a) Bragg gratings
b) Ceramic gratings
c) Aluminum arsenide
d) Bragg diodes
Answer: a
Explanation: A dual optical filter uses two multilayered dielectric filters. Bragg gratings are an excellent alternative to the dual dielectric filters as they perform the same function as dual dielectric multilayer filters.
6. FSK synchronous detection is _________ more sensitive than asynchronous heterodyne detection.
a) 0.24 dB
b) 0.45 dB
c) 0.9 dB
d) 0.12 dB
Answer: b
Explanation: During FSK detection, the probability of error is increased with respect to direct detection as a result of amplified noise from the orthogonal polarization. Thus, the synchronous detection is more sensitive than the asynchronous heterodyne detection.
7. The asynchronous heterodyne detection is _________ more sensitive than the dual filter direct detection FSK receiver.
a) 0.9 dB
b) 0.23 dB
c) 0.43 dB
d) 0.40 dB
Answer: d
Explanation: The orthogonal polarization leads to the amplified noise structures and behavior in the FSK receiver. This leads to the loss in the sensitivity and accuracy of the FSK receiver.
8. The use of __________ was undertaken to separate the polarization in an analog to digital conversion.
a) FSK
b) DSP
c) ASK
d) DP-FSK
Answer: b
Explanation: DSP is abbreviated as digital signal processing. Digital signal processing involves the cutting, trimming and manipulating of signals in the process of analog to digital conversion. The signals are converted in the form of numerous pulses.
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