Electronic Devices and Circuits

Amplitde Modulation

Question 1
Marks : +2 | -2
Pass Ratio : 25%
A modulating signal is amplified by a 80% efficiency amplifier before being combined with a 20 kW carrier to generate an AM signal. The required DC input power to the amplifier, for the system to operate at 100% modulation, would be
5 kW
8.46 kW
12.5 kW
6.25 kW
Explanation:
Pi = Pt -Pc = 30 – 20 = 10 kW. DC input = 10/0.8 or 12.5 kW.
Question 2
Marks : +2 | -2
Pass Ratio : 50%
A carrier is simultaneously modulated by two sine waves with modulation indices of 0.4 and 0.3. The resultant modulation index will be
1.0
0.7
0.5
0.35
Explanation:
α2 = 0.32 + 0.42 = 0.52 or α = 0.5.
Question 3
Marks : +2 | -2
Pass Ratio : 25%
Total sideband power is
4 W
8 W
16 W
2 W
Explanation:
Pt – Pc = 204 – 200 = 4W.
Question 4
Marks : +2 | -2
Pass Ratio : 25%
In a DSB-SC system with 100% modulation, the power saving is
100%
55%
75%
100%
Explanation:
This is so because the power is suppressed by two thirds of the total. hence the power saving is 66%.
Question 5
Marks : +2 | -2
Pass Ratio : 25%
1-Q.3) An AM signal is represented by x(t) = (20 + 4sin(500Ï€t)) cos(2Ï€t x 105)V.
Explanation:
Question 6
Marks : +2 | -2
Pass Ratio : 25%
The modulation index is
20
4
0.2
10
Explanation:
20 + 4sin(500Ï€t) = 20(1 + 0.2sin(500Ï€t)), hence the modulation index is 0.2.
Question 7
Marks : +2 | -2
Pass Ratio : 25%
The aerial current of an AM transmitter is 18 A when unmodulated but increases to 20 A when modulated.The modulation index is
0.68
0.73
0.89
0.95
Explanation:
400/326 = 1 + (α2)/2, therefore α is 0.68.
Question 8
Marks : +2 | -2
Pass Ratio : 25%
Total signal power is
208 W
204 W
408 W
416 W
Explanation:
Question 9
Marks : +2 | -2
Pass Ratio : 50%
A 2 MHz carrier is amplitude modulated by a 500 Hz modulating signal to a depth of 70%. If the unmodulated carrier power is 2 kW, the power of the modulated signal is
2.23 kW
2.36 Kw
1.18 kW
1.26 kW
Explanation:
Pt = Pc (1 + 0.49/2).
Question 10
Marks : +2 | -2
Pass Ratio : 25%
An AM broadcast station operates at its maximum allowed total output of 50 kW with 80% modulation. The power in the intelligence part is
12.12 kW
31.12 kW
6.42 kW
none of the mentioned
Explanation:
Pi = Pt – Pc = 50 – 37.88 kW or 12.12 kW.