# Analog Communications Questions and Answers – Spectrum of AM SC Waves

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This set of Analog Communications Questions and Answers for Aptitude test focuses on “Spectrum of AM SC Waves”.

1. What is the modulation index of an AM signal if the peak voltage goes from Emax to Emin?
a) $$\frac{E_{min}}{E_{max}}$$
b) $$\frac{E_{max}}{E_{min}}$$
c) $$\frac{E_{max} – E_{min}}{E_{max} + E_{min}}$$
d) $$\frac{E_{max} + E_{min}}{E_{max} – E_{min}}$$

Explanation: Modulation index tells us the amount by which the carrier wave is varied with respect to the message signal. It is defined as $$\frac{E_{max} – E_{min}}{E_{max} + E_{min}}$$ where Emax and Emin are respectively the maximum and minimum voltage of the modulated wave.

2. Which frequencies is produced if Va sin⁡(ωa t) amplitude modulates the carrier Vc sin⁡(ωc t)?
a) (ωc + ωa), (ωc – ωa) and ωc
b) (ωc) and (ωa)
c) (ωc + ωa) and (2ωc + 2ωa)
d) (2ωc) and (2ωa)

Explanation: If we modulate the carrier signal having frequency ωc with respect to the message wave having frequency ωa, then the resultant modulated wave has frequency components (ωc + ωa), (ωc – ωa) and ωc.

3. What is the total sideband power if there is 100% modulation?
a) half the carrier wave
b) twice the carrier wave
c) equal to the carrier wave
d) one-fourth of the carrier wave

Explanation: The sidebands are identical to each other and contain same information. Thus, for 100% modulation, value of modulation index should be equal to 1. If modulation index, m=1 then total sideband power is equal to the half of carrier wave.

4. If AM radio station increases its modulation index then there is an increase in receiving RF signal.
a) True
b) False

Explanation: Modulation index tells us the amount by which the carrier wave is varied with respect to the message signal. Increase in it also increase the ability of increase reception of RF signals.

5. How an AM radio station increase its signal-to-noise ratio?
a) by increasing its modulation index
b) by decreasing its modulation index
c) by maintaining a constant modulation index
d) by increasing the frequency

Explanation: Signal to noise ratio is the ratio of signal power to the noise power. It is the measure of signal present with respect to the surrounding noise. It is directly proportional to the modulation index and thus it increases with modulation index.

6. What is the main problem in using quadrature amplitude modulation?
a) high bandwidth is required
b) high power is required
c) incompability with ordinary AM radio waves
d) bandwidth required is less

Explanation: Quadrature Amplitude Modulation (QAM) is widely used for combining two amplitude modulated waves into a single channel. It is both an analog and digital modulation technique. It has many advantages over other modulations such as PSK but its disadvantage is its incompability with ordinary radio waves.

7. A single sideband modulation system requires a more complex demodulator circuit.
a) True
b) False

Explanation: 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. For its demodulation it requires a complex demodulator circuit

8. To demodulate a USB SSB signal, the receiver must ________
a) be set to USB mode
b) reinsert the carrier
c) be set to USB mode and reinsert the carrier
d) have low-fidelity

Explanation: Single side band suppresses the carrier and uses only one of the two sidebands. The two sidebands carry exactly the same information and thus removing a sideband does not have any effect on the signal. To demodulate upper sideband of signal it is necessary that the receiver must be set to USB mode and have the ability to reinsert the carrier.

9. The __________ of an AM signal resembles the shape of baseband signal.
a) upperband
b) lowerband
c) efficiency
d) envelope

Explanation: Envelope is basically a smooth curve that outlines the extremes of any baseband signal in the amplitude modulated wave. So basically it is message or baseband signal that determines the envelope.