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Right choice is (b) False

The explanation: Timing errors increase when smaller DISTANCES between signals in the CONSTELLATION diagram are used. M-ary signalling TECHNIQUES are attractive for use in bandlimited channel, but are limited in their applications DUE to sensitivity in timing jitter.

Correct choice is (b) False

The EXPLANATION: In non-linear modulation, the amplitude of the carrier is constant REGARDLESS of the variation in the modulating signal. Many PRACTICAL mobile radio communication systems use these types of nonlinear modulation methods.

Correct option is (a) Low, low

The explanation is: A desirable modulation scheme provides low bit error rates at low received signal to noise ratios. They PERFORM WELL in MULTIPATH and fading conditions, occupies a minimum bandwidth and is easy and cost effective to IMPLEMENT.

Correct answer is (d) ENVELOPE detection

The explanation: There are various types of detection techniques used for the detection of π/4 QPSK SIGNALS. They include baseband DIFFERENTIAL detection, IF differential detection and FM discriminator detection.

Correct option is (b) False

To explain I would say: FM can improve receiver performance through adjustment of the modulation index at the transmitter, and not the TRANSMITTED power. This is not the CASE in AM since linear modulation techniques do not trade BANDWIDTH for SNR.

The correct answer is (b) Zero crossing detection

Easiest EXPLANATION: Zero crossing detector is sometimes known as PULSE averaging discriminator. The rationale behind this technique is to use the output of the zero crossing detector to GENERATE a pulse train with an average value that is proportional to frequency of the INPUT signal.

The correct choice is (a) True

The best I can explain: Frequency demodulator PRODUCES an output voltage with INSTANTANEOUS amplitude that is DIRECTLY proportional to the instantaneous frequency of the input FM signal. THUS, frequency demodulator is a frequency to amplitude CONVERTER circuit.

Correct ANSWER is (c) Frequency modulation, phase modulation

The explanation: The two most IMPORTANT classes of ANGLE modulation are frequency modulation and phase modulation. They provide the ways in which phase of a carrier signal may be VARIED in accordance with the BASEBAND signal.

Correct answer is (b) False

The explanation is: The above is the case for direct method. In the indirect method, a NARROWBAND FM signal is GENERATED using a balanced MODULATOR, and frequency multiplication is used to INCREASE both the frequency deviation and the carrier frequency to the required level.

Right answer is (a) FEEDFORWARD signal regeneration

For explanation I would say: FFSR stands for Feedforward signal regeneration. If the pilot tone and the information bearing signal undergo correlated fading, it is possible at the receiver to counteract the EFFECTS of fading through signal PROCESSING BASED on tracking of pilot tone. This PROCESS is called FFSR.

Correct choice is (b) False

For explanation I WOULD say: AM systems can use either product detector or envelope detector for demodulation. As a rule, envelope DETECTORS are useful when input signal power is at LEAST 10dB GREATER than noise power, whereas product detectors are able to process the AM signals with input signal to noise ratios well below 0 DB.

Right choice is (d) 100%

To explain I would say: A percentage of modulation GREATER than 100% will distort the message signal if detected by an envelope detector. In this CASE the lower excursion of the signal will drive the carrier amplitude below zero, MAKING it NEGATIVE (and hence changing its phase).

Correct answer is (b) False

Easiest EXPLANATION: AM systems do not EXHIBIT capture effect CHARACTERISTICS because all the interferers are received at once and must be discriminated after the modulation process. FM exhibits this characteristic. Capture effect is a direct result of rapid non-linear improvement in received quality for an increase in RECEIVES power.

The correct CHOICE is (d) Constant

The best I can explain: An FM signal is a constant envelope signal. It is due to the FACT that the envelope of the carrier does not change with changes in the modulating signal. Hence, the TRANSMITTED power of an FM signal is constant REGARDLESS of the AMPLITUDE of the message signal.

Right option is (d) Better

To elaborate: FM offers MANY ADVANTAGES over amplitude modulation which makes it a better CHOICE for many mobile radio APPLICATIONS. Frequency modulation has better noise immunity when COMPARED to amplitude modulation.

The correct CHOICE is (d) Linear

To elaborate: In amplitude modulation SCHEMES, there is a linear relationship between the quality of the received SIGNAL and the power of the received signal. AM signals superimpose the exact relative AMPLITUDES of the modulating signal onto the carrier.

The correct choice is (b) False

The best EXPLANATION: The BANDPASS signal is called the modulated signal and the baseband message signal is called the modulating signal. MODULATION is done by varying the amplitude, phase or FREQUENCY of a high frequency carrier in accordance with the amplitude of the message signal.

Correct answer is (B) Less

The explanation is: FM signals are represented as frequency VARIATIONS rather than amplitude variations. THEREFORE, FM signals are less susceptible to ATMOSPHERIC and impulse NOISE.

Right choice is (C) 50%

The explanation is: The MODULATION is also expressed in percentage. It is also CALLED percentage modulation. The signal is SAID to be 50% modulated if the peak message signal amplitude is HALF the peak amplitude of the carrier signal.

Right choice is (b) False

To explain I would say: Single sideband (SSB) AM SYSTEMS transmit only one of the sidebands (EITHER upper or lower) about the carrier. Hence, they OCCUPY only HALF the bandwidth of conventional AM systems.

The correct answer is (c) Frequency

The explanation is: FM signal have all their information in the PHASE or frequency of the carrier. This provides a NONLINEAR and very rapid improvement in reception quality once a CERTAIN minimum RECEIVED signal level, called FM threshold is ACHIEVED.

The correct answer is (c) Wider frequency BAND

The best I can explain: FM systems have MANY ADVANTAGES over AM systems. They ALSO have certain disadvantages. FM systems require a wider frequency band in transmitting media in order to obtain the advantages of reduced noise and capture effect. It is generally several TIMES as large as that needed for AM.

Correct choice is (c) CONSTANT, varied

To explain: In FREQUENCY modulation (FM), the amplitude of modulated carrier signal is kept constant while its frequency is varied by the modulating message signal. FM is the most POPULAR analog modulation technique used in MOBILE communication.

The correct answer is (a) OCCUPY less bandwidth

To explain: AM signals are able to occupy less bandwidth as compared to FM signals, since the transmission system is linear. Small SCALE FADING causes rapid fluctuations in the RECEIVED signal. Thus, FM OFFERS superior qualitative performance in fading when compared to AM.

Right option is (a) MODULATION

To EXPLAIN I would say: Modulation is the process of encoding information from a message source in SUITABLE manner for transmission. It translates the baseband message signal to a bandpass signal at FREQUENCIES that are very higher compared to the baseband frequency.

Right answer is (b) Peak MESSAGE signal amplitude, Peak carrier amplitude

To explain: The modulation index k of an AM signal is defined as the RATIO of the peak message signal amplitude to the peak carrier amplitude. The modulation index is OFTEN EXPRESSED as a percentage. It is also called percentage modulation.

The correct answer is (a) Amplitude

To EXPLAIN I would say: AM signals have all information in the amplitude of the CARRIER. Because, AM signal SUPERIMPOSE the EXACT relative AMPLITUDES of the modulating signal onto the carrier.

The correct choice is (c) C, A

For explanation: The constant envelope of the transmitted signal ALLOWS efficient class C amplifiers. However in AM, it is critical to MAINTAIN linearity between the applied message and amplitude of transmitted signal. Therefore, class A or AB are USED for AM systems.

Right answer is (b) False

Easiest explanation: UNLIKE AM, FM has a modulation index. Hence, bandwidth occupancy can be VARIED to obtain greater signal to NOISE performance. THUS, it is possible to tradeoff bandwidth occupancy for IMPROVED noise performance.

Correct option is (b) False

The explanation is: If the FM received SIGNAL is above the FM threshold, BURST noise does not affect FM SYSTEM PERFORMANCE as much as AM systems. It is due to the fact that message amplitude variations do not CARRY information in FM.