Analog Communication Test Questions - Set 4

ANSWER: Both a and b are correct

Explanation:


Square law modulators are used for Amplitude Modulation. They have non linear current-voltage characteristics. Square law modulators are highly nonlinear in low voltage region.

ANSWER: Both a and b are correct

Explanation:
AM signals are detected or demodulated using Square law demodulator or envelope detector. The demodulators extract the information from the received AM signal.

Square law demodulators are highly nonlinear in low voltage region so they are used for low level modulated signals. Envelope detectors extract the envelope of the AM wave.

ANSWER: Detection

Explanation:
Detection or demodulation of the received signal is the recovery of information or the original message that was transmitted by the transmitter after modulation. The process is also called demodulation it recovers the original signal from the modulated signal received.

ANSWER: All of the above

Explanation:
Ring modulator is a product modulator used for DSB SC generation. It consists of four diodes connected in the form of ring. In AM , the ring modulator acts as a product modulator for a square wave carrier and modulating signal and generated a Double Side Band-Suppressed Carrier signal.

ANSWER: 33.33%

Explanation:
The transmission efficiency (η) of AM wave is defined as the percentage of total power contributed by side bands of the AM signal. The maximum transmission efficiency of an AM signal is 33.33%, i.e., only one third of the total transmitted power is carried by the side bands in an AM wave. The remaining two third of the total transmitted power gets wasted.

ANSWER: All of the above

Explanation:
In synchronous detection of AM signal, the carrier signal is locally generated and then passed through a low pass filter. At the output of the low pass filter, the original modulating signal is recovered. The synchronous detection requires the generation of carrier at the receiver also. So additional circuitry is required at the receiver.

ANSWER: All of the above

Explanation:
Synchronous detection of AM signal requires local generation of the carrier signal at the receiver. The frequency of the locally generated carrier must be identical to that of transmitted carrier. And the phase of the locally generated carrier must be synchronized to that of transmitted carrier otherwise the detected signal would get distorted.

ANSWER: All of the above

Explanation:
Disadvantages of using synchronous detection of AM signal are that it needs an additional system for generation of carrier. The frequency of the locally generated carrier must be identical to that of transmitted carrier. And the phase of the locally generated carrier must be synchronized to that of transmitted carrier otherwise the detected signal would get distorted so it needs an additional system for synchronization of carrier. This makes the receiver complex and costlier.

ANSWER: All of the above

Explanation:
The modulation scheme Quadrature Amplitude Modulation (QAM) enables same bandwidth used for transmission of two DSB-SC signals. It is also known as Bandwidth Conservation scheme. However it needs synchronous detection so it needs an additional system for generation of carrier. Also known as Quadrature Carrier Multiplexing, the scheme is used in color television.

ANSWER: All of the above

Explanation:
Pilot carrier is a small carrier transmitted with modulated signal from the transmitter. It is separated at the receiver and used to phase lock the locally generated carrier signal generated at the receiver. It provides synchronization at the receiver.

As a weak carrier is transmitted with the DSB signal, it is also referred to as partially suppressed carrier system, as the carrier is not totally suppressed. So it is a case of over modulation.

ANSWER: Both a and b

Explanation:
SSB-SC signals are the signals where only one of the sidebands is transmitted in an AM wave. The carrier is also suppressed while transmitting the AM signal. SSBSC signal may be generated by two methods:

i) Frequency discrimination method or filter method
ii) Phase discrimination method or phase shift method

ANSWER: Both a and b

Explanation:
Limitations of Frequency discrimination method are that as the base band signal must be appropriately related to the carrier signal frequency, the designing of band pass filter is difficult if the carrier frequency is quite higher than the bandwidth of the baseband signal. The system is not useful for video communication purpose.

ANSWER: All of the above

Explanation:
Phase shift method is one of the methods used for the generation of SSB-SC signals. This method includes two balanced modulators and two phase shifting networks and avoids the use of filters. Both the balanced modulators produce side band as an output.

The SSB-SC signals are generated using multiple tone modulating signals as if done with single tone modulating signal, they do not carry any useful information.

ANSWER: Synchronous detection

Explanation:
Vestigial side band signals are detected by Synchronous detection or coherent detection technique. The distortions in a VSB signal may be reduced by reducing the modulation index and increasing the width of VSB.

ANSWER: All of the above

Explanation:
Automatic gain control provides controlled signal amplitude at the output even if there are amplitude variations at the input. This controlled output adjusts the input to output gain to a suitable value so that the system may work properly even for wide range of variations at the input.

The AGC is used in AM radio receivers for controlling the weak and strong sound signals.

ANSWER: All of the above

Explanation:
In an Amplitude Modulation, the amplitude of the carrier varies according to the instantaneous values of the baseband signal. The frequency and the phase of the carrier remain constant.

The envelope of the AM wave has the same shape as the message of the base band signal.

ANSWER: All of the above

Explanation:
Modulation index is the measure of extent to which the parameter of the carrier is varied in accordance with the modulating signal. If modulation index is greater than 1, or the modulation percentage is more than 100%, it is called over modulation.

The baseband signal is not preserved in the envelope of the AM signal and therefore, the recovered signal is distorted at the output of the receiver.

ANSWER: Both a and b

Explanation:
In a Low level Amplitude Modulation system, modulation is done at lower power of carrier and modulating signal. Therefore the output power of modulation is low. So power amplifiers are required to boost the signal up to the desired power level. Examples of low level modulation are

i) Square law diode modulation
ii) Switching modulation

ANSWER: All of the above

Explanation:
Frequency components of an AM wave having m modulation index are

- carrier frequency (ω_c ) with amplitude A

- upper side band (ω_c + ω_m) having amplitude mA/2

- lower side band (ω_c - ω_m) having amplitude mA/2

with these components of AM wave, the frequency spectrum may be plotted as

ANSWER: All of the above

Explanation:
Squelch circuit suppresses the output audio when there is insufficient desired input signal at the receiver. The circuit is used to suppress the unwanted channel noise when there is no reception by the receiver. Squelch circuits may be used in wireless microphones stay away from replicating the noise when enough signal is not received. Regulating squelch is provided in some systems for the receiver.