Analog Communications online test focuses on “Demodulation of VSB Modulated Wave”.
1. What is the reason of “envelope” in an amplitude modulated signal?
a) noise signal
b) carrier signal
c) nematic signal
d) baseband signal
Clarification: Envelope is basically a smooth curve that outlines the extremes of any baseband signal. So basically it is message or baseband signal that determines the envelope.
2. AM stands for ________
a) Amplitude Modulation
b) Audio Modulation
c) Antenna Modulation
d) Amplified Modulation
Clarification: AM stands for amplitude modulation. Amplitude modulation is the change in the amplitude of carrier wave in proportion to the instantaneous amplitude of the message signal.
3. What is the equation for full-carrier AM?
a) V(t) = (Ec + Em) × (sin ωc t)
b) V(t) = (Ec + Em) × (sin ωm t) + (sin ωc t)
c) V(t) = (Ec × Em) × (sin ωm t) × (sin ωc t)
d) V(t) = (Ec + Em sin ωm t) × (sin ωc t)
Clarification: Amplitude modulation is the change in the amplitude of carrier wave in proportion to the instantaneous amplitude of the message signal. A carrier can be seen as a waveform with frequency higher than the message signal frequency, that is modulated with respect to input signal for the purpose of transmitting information. The equation for full-carrier AM is V(t) = (Ec + Em sinωm t) × (sin ωc t).
4. What is the cause of Overmodulation?
c) both distortion and splatter
d) half reception of signals
Clarification: Overmodulation is the process in which the modulation index is greater than 1 such that the modulating signal voltage exceeds the required voltage to produce 100% modulation. It results out of both distortion and splatter of waveform and causes distortion and overlapping.
5. If AM radio station increases its modulation index then the audio gets louder at the receiver.
Clarification: Modulation index tells us the amount by which the carrier wave is varied with respect to the message signal. If we increase the modulation index then audio signal gets louder.
6. The modulation index can be derived from ________
a) frequency-domain signal
b) time-domain signal
c) both frequency and time domain signal
d) a highly modulated carrier wave
Clarification: Modulation index tells us the amount by which the carrier wave is varied with respect to the message signal. It can be derived for frequency-domain signals as well as for time-domain signals.
7. A single sideband modulation system is more efficient than a plain amplitude modulated system.
Clarification: In Single Sideband transmission, the carrier is suppressed and only either of the two sidebands is transmitted. Thus, it reduces the total power consumption and also reduces the bandwidth required. Whereas, in AM, the carrier being transmitted along with both the sidebands entails more power and larger bandwidth.
8. At peak modulation an SSB transmitter radiate 1000W, what will it radiate with no modulation?
a) 1000 watts
b) 500 watts
c) 250 watts
d) 0 watts
Clarification: Power of a modulated wave is directly proportional to modulation index. Thus, if there is no modulation in any SSB transmitter than, it will not radiate. So it will radiate 0 watts when there is no modulation.
9. Why AM stations has “low-fidelity”?
a) AM is susceptible to noise
b) Commercial AM stations use low power
c) Commercial AM stations have a narrow bandwidth
d) High quantization to noise ratio
Clarification: Fidelity is the ability of receivers to reproduce all modulating signals eually. Low fidelity can be seen as sound recording that contain technical flaws to make sound better compared with the sound that is recorded live. High fidelity refers to the equipment that very accurately produces without any harmonic or resonance. AM stations have low fidelity to have narrow bandwidth.
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