Physics Class 12 NCERT Solutions: Chapter 15 Communication System Part 2 (For CBSE, ICSE, IAS, NET, NRA 2022)
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Q: 6. A modulating signal is a square wave, as shown in Figure.
The carrier wave is given by volts.
(i) Sketch the amplitude modulated waveform
(ii) What is the modulation index?
Answer:
It can be observed from the given modulating signal that the amplitude of the modulating signal,
It is given that the carrier wave
∴ Amplitude of the carrier wave,
Time period of the modulating signal
The angular frequency of the modulating signal is calculated as:
The angular frequency of the carrier signal is calculated as:
From equations (i) and (ii) , we get:
The amplitude modulated waveform of the modulating signal is shown in the following figure.
(ii) Modulation index,
Q: 7. For an amplitude modulated wave, the maximum amplitude is found to be while the minimum amplitude is found to be . Determine the modulation index . What would be the value of if the minimum amplitude is zero volt?
Answer:
Maximum amplitude,
Minimum amplitude,
Modulation index , is given by the relation:
If
Then
Q: 8. Due to economic reasons, only the upper sideband of an AM wave is transmitted, but at the receiving station, there is a facility for generating the carrier. Show that if a device is available which can multiply two signals, then it is possible to recover the modulating signal at the receiver station.
Answer:
Let and be the respective frequencies of the carrier and signal waves.
Signal received at the receiving station,
Instantaneous Voltage of the carrier wave,
At the receiving station, the low-pass filter allows only high frequency signals to pass through it. Obstructs the low frequency signal . Thus, at the receiving station, one can record the modulating signal , which is the signal frequency.