## Analog Communications Questions and Answers Part-5

1. What is Amplitude Modulation?
a) Change in amplitude of carrier according to modulating signal amplitude
b) Change in frequency of carrier according to modulating signal amplitude
c) Change in amplitude of carrier according to modulating signal frequency
d) Change in amplitude of modulating signal according to carrier signal amplitude

Explanation: Amplitude modulation is a modulation process in which amplitude of carrier wave is varied with respect to amplitude of the message signal to be transmitted. Whereas, Frequency modulation is a modulation process in which frequency of carrier wave is varied with respect to amplitude of the message signal to be transmitted.

2. In amplitude modulation frequency and phase of carrier ________
a) varies simultaneously
b) varies alternately
c) initially varies but become same after sometime
d) remains constant

Explanation: In AM, amplitude of carrier signal varies according to instantaneous amplitude of baseband signal. The frequency and phase of carrier signal remains constant.

3. AM spectrum consists of ________
a) Carrier frequency
b) Upper sideband
c) Lower sideband
d) Carrier frequency with both upper and lower sideband

Explanation: Spectrum of AM wave consists of a carrier with upper sideband and lower sideband. If carrier frequency is Wc, then the two sidebands produced by it are (Wc+Wm) and (Wc-Wm), where Wm is the frequency of the message signal. The amplitude of the carrier is A and that of the two sidebands are mA/2, where m is the modulation index.

4. Envelope of AM wave has the same shape as the message of baseband signal.
a) True
b) False

Explanation: Modulation Envelope of an AM wave is said to follow the outline of the message signal when carrier, upper sideband and lower sideband are combined in a single impedance and observed on time versus amplitude plot. Thus, AM wave has same shape as the message signal.

5. When aliasing takes place?
a) Sampling signals more than Nyquist Rate
b) Sampling signals less than Nyquist Rate
c) Sampling signals equal to Nyquist Rate
d) Sampling signals at a rate which is twice of Nyquist Rate

Explanation: Aliasing causes different signals to become indistinguishable during sampling. It happens because sampling rate is less than Nyquist rate. In order to avoid aliasing, signals should be sampled at a rate twice of Nyquist rate.

6. Why the Synchronous detection of AM signals is considered as a disadvantage?
a) Needs additional system for synchronization of carrier
b) Receiver is available at cheap prices
c) Needs less number of system as estimated for generation of carrier

Explanation: Disadvantage of synchronous detection of AM signal are that it needs an additional system for generation of carrier. It also needs additional system for synchronization of carrier. Moreover receiver is complex and costly.

7. Which devices did we use for AM Demodulation?
a) Ratio detector and Slope detector
b) PLL detector and Foster-Seeley discriminator
c) Envelope detector and Square law demodulator

Explanation: AM signals can be demodulated using Square law demodulator or Envelope detector. Demodulator extracts information from received AM signal by decoding it. Other options are the demodulators for FM signal.

8. What do you understand by low level AM?
a) Output power is low
b) Modulation is done at high power of carrier and modulating signal
c) Collector Modulation Method is low level AM
d) Output power is high

Explanation: In low level AM, modulation is done at low power of carrier and modulating signal therefore output power is low. Therefore, power amplifiers are used to boost the carrier and modulating signal. Collector Modulation Method is high level AM. At high-level AM, output power is high.

9. Square Law modulators ________
a) used for amplitude modulation
b) have non linear current-voltage characteristics
c) have non linear current-voltage characteristics as well as used for Amplitude Modulation
d) used for frequency modulation