This set of Analog Communications Multiple Choice Questions & Answers (MCQs) focuses on “Power Calculation”.

1. If peak voltage of a carrier wave is 10V, what is the peak voltage of modulating signal if modulation index is 50%?

a) 10V

b) 20V

c) 8V

d) 5V

View Answer

Explanation: From the relation, Modulation Index (µ) = V

_{m}/V

_{c}= 50% = 0.5,

where V

_{m}= Peak voltage of modulating signal,

V

_{c}= Peak voltage of a carrier wave = 10V,

Therefore, V

_{m}= 10 X 0.5 = 5V.

2. Maximum Amplitude of an amplitude modulated 10V and minimum amplitude is 5V. Find its modulation index?

a) 0.65

b) 0.9

c) 0.33

d) 1

View Answer

Explanation: We know, Modulation Index(µ) = (V

_{max}-V

_{min})/(V

_{max}+V

_{min}),

Where, V

_{max}= Maximum Amplitude of an amplitude modulated = 10V

V

_{min}= Minimum amplitude of an amplitude modulated = 5V

Therefore, µ = (10-5)/(10+5) = 0.33.

3. 24 channels, each band limited to 3.4 KHz, are to be time division multiplexed. Find the bandwidth required for 128 quantization level? (Given that sampling frequency is 8 KHz)

a) 2436 KHz

b) 1002 KHz

c) 1536 KHz

d) 1337 KHz

View Answer

Explanation: N = 24, f

_{m}= 3.4 kHz

m = 128,

2

^{n}= m = 128, n = 7

But f

_{s}= 2f

_{m}, where, f

_{s}= sampling frequency

instead at 2f

_{m}2 x 3.4 kHz 6.8 KHz.

B.W. = N(n+1)X f

_{s}= [24(7 + 1)] 8 kHz = 1536 KHz.

4. Sampling frequency of a signal is 6 KHz and is quantized using 7 bit quantizer. Find its bit rate?

a) 48kbPs

b) 64kbPs

c) 16kbPs

d) 8kbPs

View Answer

Explanation: Bit rate refers to the rate at which data is processed or transferred. It is usually measured in seconds, ranging from bps for smaller values to kbps and mbps.

Bit rate is also known as bitrate or data rate.

Bit rate, R

_{b}=

^{1}⁄

_{Tb}where where n = number of bits and f

_{s}= Sampling Frequency

T

_{b}= 1/42, therefore Bit rate = 42 Kbps.

5. Calculate power in each sideband, if power of carrier wave is 96W and there is 40% modulation in amplitude modulated signal?

a) 11.84W

b) 6.84W

c) 3.84W

d) 15.84W

View Answer

6. For 50% modulation, power in each sideband is ________ of that of carrier.

a) 10%

b) 4.32%

c) 5%

d) 6.25%

View Answer

7. For 100% modulation, total power is ________

a) 1.5P_{c}

b) 2P_{c}

c) 3.75P_{c}

d) 1.25P_{c}

View Answer

Explanation: Total power, P

_{t}= P

_{c}(1 +

^{µ2}⁄

_{2}), where P

_{c}= Carrier Power

where µ = 1 (for 100% modulation),

so P

_{t}= P

_{c}(1 +(1

^{2}/2)). On solving it we get P

_{t}= 1.5P

_{c}.

8. If each element of signal occupies 70ms, what will its speed?

a) 11.23 bauds

b) 14.28 bauds

c) 17.39 bauds

d) 13.33 bauds

View Answer

Explanation: The carrier signal is characterized by the number of signal intervals, or pulses, that are transmitted per second. Each pulse is called a baud. Bps stands for bits per second. Bps is a measure of how many bits can be transmitted during one pulse (one baud).

9. Power of carrier wave is 300W and modulation index is 0.75. Find its total power?

a) 465W

b) 384W

c) 323W

d) 502W

View Answer

Explanation: Total power, P

_{t}= P

_{c}(1 +

^{µ2}⁄

_{2}), where P

_{c}= Carrier Power = 300W

where Modulation Index (µ) = 0.75,

So P

_{t},

10. If a wave is modulated by two waves. One of them has modulation index equal to 0.75 and other has 0.2, the total modulation index will be ________

a) 0.67

b) 0.58

c) 0.77

d) 0.35

View Answer

Explanation: Given that m1 = 0.75 and m2 = 0.2. Total modulation index will be equal to By substituting values we have which is equal to 0.77.

11. Find the power saving for DSB-SC wave with 100% modulation?

a) 66%

b) 86%

c) 50%

d) 33%

View Answer

Explanation: In DSB-SC carrier is suppressed.

So Total Power required in DSBSC Modulation = (u

^{2}XP

_{c})/2 = P

_{c}/2

In normal AM, carrier is not suppressed.

So total power required in AM Modulation = (1+(u

^{2}/2))XP

_{c}= 3P

_{c}/2

Therefore, Power saving = ((P

_{c}/2)/(3P

_{c}/2)) x 100% = 66%.

12. If power transmitted is 45kW, field at a distance of 23km will be ________

a) 0.02

b) 0.75

c) 0.05

d) 0.03

View Answer

Explanation: Field is a region around a charged particle or object within which a force would be exerted on other charged particles or objects.

13. Find the number of pulses, if the number of level is 128 in PCM?

a) 3

b) 6

c) 4

d) 7

View Answer

Explanation: A pulse in signal processing is a rapid, transient change in the amplitude of a signal from a baseline value to a higher or lower value, followed by a rapid return to the baseline value.

Therefore,

2

^{n}= 128, so n = 7.

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