Communication Systems
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A diode detector is used to detect an amplitude modulated wave of 60% modulation by using a condenser of capacity 250 pico farad in parallel with a load resistance 100 kilo ohm. Find the maximum modulated frequency which could be detected by it.
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10.62 MHz
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10.62 kHz
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5.31 mHz
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5.31 kHz
B.
10.62 kHz
The frequency is given as
μ =0.6
R = 100 k = 100 x1000 Ω
c = 250 pico farad = 250 x 10-12 F
So,
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A radar has a power of 1 Kw and is operating at a frequency of 10 GHz. It is located on a mountain top of height 500 m. The maximum distance upto which it can detect object located on the surface of the earth (Radius of earth = 6.4 x 106 m) is
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80 km
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16 km
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40km
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64 km
A.
80 km
Maximum distance on earth where object can be detected is d, then
(h + R)2 = d2 + R2
⇒ d2 = h2 + 2Rh
Since, h<<R,
⇒d2 =2hR
A signal of 5 kHz frequency is amplitude modulated on a carrier wave of frequency 2 MHz. The frequencies of the resultant signal is/are:
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2005 kHz, and 1995 kHz
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2005 kHz, 2000 kHz and 1995 kHz
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2000 kHz and 1995 kHz
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2 MHz only
C.
2000 kHz and 1995 kHz
Frequency associated with AM are
fc - fm, f, fc + fm
according to the question
fc = 2 MHz = 2000 kHz
Thus, frequency of the resultant signal is are carrier frequency fc = 2000 kHz, LSB frequency fc-fm = 2000 kHz-5kHz = 1995 kHz and USB frequency fc+fm = 2005 kHz
A sound absorber attenuates the sound level by 20 dB. The intensity decreases by a factor of
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1000
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10000
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10
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100
D.
100
A train is moving on a straight track with speed 20 ms-1 .It is blowing its whistle at the frequency of 1000 Hz. The percentage change in the frequency heard by a person standing near the track as the train passes him is (speed of sound = 320 ms-1 ) close to :
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6%
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12%
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18%
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24%
B.
12%
Apparent frequency heard by the person before crossing the train.
Similarly, apparent frequency heard, after crossing the trains
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