Amplitude Modulation MCQ || Amplitude Modulation Questions and Answers

Answer.3. Modulation index

Explanation

The modulation index (or modulation depth) of a modulation scheme describes by how much the modulated variable of the carrier signal varies around its unmodulated level.

For undistorted modulation to occur, the voltage of modulating signal V_m must be less than the carrier voltage Vc. Therefore, the relationship between the amplitude of the modulating signal and the amplitude of the carrier signal is important. This relationship, known as the modulation index m, is the ratio m = Vm⁄Vc.

Answer.4. 16.7 to 19.7 kHz 

Explanation

Given:

f_c = 20 kHz

Speech band = 300 to 3300 Hz

The range of LSB is given as:

(-fc + fm₁) and (fc – fm₁) = -19.7 kHz  and 19.7 kHz

(-fc + fm₂) and (fc – fm₂) = -16.7 kHz  and 16.7 kHz

Note:

Negative frequency does not exist practically so:

The range of LSB is: 16.7 kHz  to 19.7 kHz 

Answer.3. SSB

Explanation

Total power = Carrier power + Sideband power

In SSB, we have one sideband only hence the power is less than DSB and VSB.

In VSB, we have one sideband and some part of the other sideband and hence power is more than SSB and less than DSB.

Answer.3. 83.33

Explanation

The modulation index m = P_m/P_c

PM is the peak value of the modulating signal

PC is the peak value of the carrier signal.

=100 × 7.5/9 = 83.33.

Answer.1. 7.4%

Explanation

The total transmitted power for an AM system is given by:

${P_t} = {P_c}\left( {1 + \frac{{{μ^2}}}{2}} \right)$

Pc = Carrier Power

μ = Modulation Index

Total sideband power is calculated as

Ps = Pc.µ²/2

% of sideband power is given as:

$\begin{array}{l} \frac{{{P_c}\frac{{{\mu ^2}}}{2}}}{{{P_c}\left( {1 + \frac{{{\mu ^2}}}{2}} \right)}}\\ \\ \frac{{{\mu ^2}}}{{{\mu ^2} + 2}} \end{array}$

putting μ = 0.4, we get

$\frac{{{{0.4}^2}}}{{{{0.4}^2}\: + \:2}} \times 100 = 0.074 \times 100$

= 7.4 %

Answer.4. SSB-SC

Explanation

The modulation technique that takes the lowest bandwidth is SSB-SC

Single Side Band (SSB SC):

1) SSB SC is a form of amplitude modulation where only a single sideband is transmitted and 1 sideband and carrier are suppressed.

2) The advantage of SSB is bandwidth and power-saving over both conventional AM and DSB SC, i.e. DSB SC has more power consumption than SSB SC.

3) The Bandwidth requirement is only fm, therefore It needs half the bandwidth as the DSB-SC transmission.

Answer.2. Modulating signal voltage > Carrier voltage

Explanation

• Amplitude distortion is the unintended modification of a signal, typically while undergoing amplification.
• Overmodulation results in spurious emissions by the modulated carrier, and distortion of the recovered modulating signal. This means that the envelope of the output waveform is distorted.

Over modulation occur when Modulating signal voltage > Carrier voltage

• Over-modulation is a condition in which the modulating signal voltage is much greater than the carrier voltage. The received signal will produce an output waveform in the shape of the envelope, whose negative peaks have been clipped off.
• Intermodulation distortion is a result of nonlinearities in the system such that one frequency component tends to modulate another frequency component—e.g., a high audio frequency modulating a low audio frequency.

Answer.4. DSB /SC signal

Explanation

In electronics, ring modulation is a signal processing function, an implementation of frequency mixing, in which two signals are combined to yield an output signal. The ring modulator and balanced modulator are generally used for the modulation of the DSB SC signal.

Answer.3. Modulating signal voltage = Carrier voltage

Explanation

The condition for the greatest output power at the transmitter without distortion is when the Modulating signal voltage is equal to Carrier voltage.

When the modulation index is 1 or the percentage of modulation is 100, modulating signal voltage is equal to the carrier voltage. This results in the greatest output power at the transmitter and the greatest output voltage at the receiver, with no distortion.

Answer.4. SSB

Explanation

Single Side Band (SSB SC):

1) SSB SC is a form of amplitude modulation where only a single sideband is transmitted and 1 sideband and carrier are suppressed.

2) The advantage of SSB is bandwidth and power-saving over both conventional AM and DSB SC, i.e. DSB SC has more power consumption than SSB SC.

3) The Bandwidth requirement is only fm, therefore It needs half the bandwidth than the DSB-SC transmission.

Answer.4. 1.2

Explanation

When the voltage of the modulating signal exceeds the voltage of the carrier signal over-modulating occurs.

From the given open i.e 12/10 = 1.2 which is greater than 1 and hence would cause over-modulation.

Answer.1. 2.35 V

Explanation

Modulation signal voltage is given by

M = (V_max − V_min)/2

Given

Max voltage = 5.9 V

Min Voltage = 1.2 V

M = (5.9 − 1.2)/2

M = 2.35 V