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The CORRECT choice is (d) REMAINS constant

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

Correct answer is (a) True

Easy 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.

The correct option is (a) Sampling signals less than NYQUIST Rate

To explain I would say: 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.

Right answer is (a) Needs ADDITIONAL system for synchronization of carrier

The BEST 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.

Correct choice is (a) Envelope detector and Square law DEMODULATOR

To EXPLAIN: 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.

Right CHOICE is (c) have non linear current-voltage characteristics as well as used for AMPLITUDE Modulation

Easiest explanation: SQUARE law modulators are used for amplitude modulation and have non-linear current-voltage characteristics. The output of Square law demodulator is SAID to vary with respect to square of the input. They are highly linear in LOW voltage region.

Correct option is (a) Output power is low

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.

The CORRECT option is (b) Modulation is done at high power of carrier and modulating signal

Easiest EXPLANATION: For high LEVEL AM, modulation is done at high power of carrier and modulating signal, so power AMPLIFIERS are not used to boost the carrier and modulating signal. Therefore, output power is high. At LOW-level AM, output power is low.

Correct choice is (B) Has a LOW efficiency

Explanation: EMITTER Modulator operates in class A region. It has very low efficiency. The output POWER is low so for MODULATION at high level, it is not suitable.

Right OPTION is (a) CONSISTS of only ONE frequency component

The best I can explain: Single tone modulation consists of only one frequency component in the baseband or message signal. Thus, modulation of carrier wave is done by a single frequency component only.

Right choice is (d) Carrier frequency with both upper and lower SIDEBAND

For explanation I would say: 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.

The correct answer is (d) Avoids Receivers Complexity

For explanation: AM detectors are GENERALLY, square law demodulators or envelope detectors at the receiver. As AM detectors at the receiver end are simple CIRCUITS and AVOID any KIND of complex structure, THEREFORE, AM used for broadcasting.

Correct option is (B) SSB-SC

To EXPLAIN: A signal has two sidebands which are EXACTLY the mirror images of each other. So we can remove ONE side band which further reduces its bandwidth. In SSB-SC modulation TECHNIQUE, the carrier is suppressed and only either of the sidebands is transmitted. Thus, SSB-SC has minimum channel Bandwidth.

Right answer is (a) 816KHZ and 804KHz

Easy explanation: Maximum FREQUENCY = 810 + 6 = 816KHz and Minimum frequency = 810 – 6 = 804KHz. Moreover it has a bandwidth of (816 – 804) = 12KHz.

( Formula:Fmax = fc + fm,

Fmin = fc-fm,

Bandwidth = 2*fm = Fmax = Fmin,

fc = CARRIER frequency, fm = message signal frequency).

The correct OPTION is (d) 888KHz

The explanation: HIGHEST frequency component is 900KHz and BANDWIDTH is 12KHz. So lower frequency component is 900 – 12 = 888KHz.

(Formula: Fmin = Fmax-2*fm = Fmax-Bandwidth,where fm = Message Signal Frequency).

Correct option is (a) Sum of carrier and modulating wave

Easy EXPLANATION: The modulation of a carrier wave by varying its amplitude with RESPECT to amplitude of BASEBAND signal is known as amplitude modulation. It is REPRESENTED as,

s(t) = [1 + MX(t)] c(t),

where, x(t) =Modulating Wave, m=Modulating Index

c(t) = Carrier Wave = Ac (Cos ωc) t

Thus, Amplitude Modulated wave is the Sum of carrier and its product with modulating wave.

The correct answer is (d) 15.84W

Explanation: MODULATION index = 0.6 and Pc = 176W. Power in SIDEBANDS MAY be calculated as

Right answer is (d) 25%

The BEST I can explain: Modulation INDEX = 1. Power in sidebands MAY be CALCULATED as

Correct choice is (a) Distortion

Easiest EXPLANATION: When INSTANTANEOUS LEVEL of modulating signal exceeds the value necessary to provide 100% modulation, the signal is said to over-modulated. In other WORDS, when modulation INDEX is greater than 1, it results in Overmodulation. Thus, Overmodulation results in distortion of the modulating signal.

Correct OPTION is (b) 33%

Easy explanation: EFFICIENCY (ή) = m^2 / (m^2 + 2), m=Modulation Index

 For maximum efficiency m = 1 so, ή = 1/(1+2) = 1/3

 and ή% = (1/3)x100 = 33%.

Correct ANSWER is (a) Better than that of synchronous detector

To explain I would SAY: Process of recovering message signal from RECEIVED modulated signal is called demodulation. It is exactly opposite to modulation. There are two most used AM demodulators: Square LAW Demodulator and Envelope Demodulator. NOISE performance of Square Law Demodulator is far better than that of Synchronous Detector.

Right CHOICE is (b) be equal to signal amplitude

Easiest explanation: Modulation index is the amount of modulation present in a CARRIER WAVE. It is also DESCRIBED as the ration of the amplitude of message signal to that of carrier signal.

Modulation Index (m) = VM/Vc, where Vm is maximum baseband or message signal amplitude and Vc is maximum carrier signal amplitude. So for m = 1, Vm should be equal to Vc.

Right OPTION is (b) 0.066

Explanation: Given equation can be WRITTEN as 30(1 + 0.066 SIN(700πt)).

Comparing it with general AM equation, s(t) = Ac(1 + mAm cos(WMT)) cos(wct),

Where, Ac = Amplitude of CARRIER Signal, Am = Amplitude of Message Signal

 m=Modulation Index

So modulation index(m) = 0.066.

Correct option is (d) ONE and half times as the power of unmodulated SIGNAL

Explanation: TOTAL power, Pt = PC (1 + ^m^2⁄2), where m is Modulated Signal, Pc is Power of Unmodulated Signal or Carrier Signal.

So, for m=1,

 Pt = Pc (1 + 1^2/2)= 1.5 Pc.

The correct choice is (B) 1W

The explanation is: Sideband power i = (m^2* PC)/2 = (PT – Pc) i.e. 451 – 450 = 1W.

Correct ANSWER is (b) shunt capacitance

Best explanation: CAPACITIVE reactance, Xc is low for HIGH frequencies as Xc=(1/wC), where w is the frequency and C is the capacitance. Due to this,the high frequencies are shunted to ground and are not transmitted.

The correct option is (a) [1 + m(t)].C(t)

Best explanation: Amplitude wave is represented by [1 + um(t)].c(t), where c(t) is CARRIER signal, m(t) is message signal, u is MODULATION Index.

Generally, c(t) = Accos(wct), AC = Amplitude of Carrier Signal.

The correct option is (a) upto 600bps

To EXPLAIN: Modem is device that can modulate carrier wave as well as demodulate transmitted modulated SIGNAL to decode the original information. When data rate in bps is upto 600, modem is CONSIDERED having low SPEED.

Right answer is (b) False

The BEST explanation: Modulation is encoding the message SIGNAL for efficient transmission. Whereas, Demodulation is the PROCESS to extract or decode the original message signal from the TRANSMITTED MODULATED signal. Demodulation is also called detection.

Right answer is (d) 516W

Easiest explanation: Total POWER (Pt) = (1+m^2/2)*PC

So, here, m = 0.25, Pc = 500W

Pt = (1+(0.25^2/2))*500 = 516W.

Right option is (a) L = ʎ

Easiest explanation: ANTENNA is used for converting ELECTROMAGNETIC radiation into ELECTRIC currents or vice-versa. If the length of antenna is equal to the whole WAVELENGTH then the radiation at right angles is zero.

The correct answer is (a) 75 KHz

To elaborate: FREQUENCY DEVIATION is the difference between an FM modulated frequency and the carrier frequency. Commercial FM broadcasting uses a maximum frequency deviation of 75 KHz and maximum modulating frequency is approximately 15KHZ.

The correct answer is (c) x(t) = COST + 0.7

For EXPLANATION I would say: x(t) = Cost + 0.7does not FOLLOW Dirchlet’s conditions. FOURIER series of a periodic signal will only be applicable if it satisifies Dirchlet’s conditions firstly. Dirichlet integral states that a finite NUMBER should have finite number of discontinuities and also have finite number of extrema. x(t) doesn’t satisfy any of the criteria, whereas other options satisfy Dirchlet’s conditions. Thus, x(t) = Cost + 0.7, cannot be the Fourier series of a periodic signal.

The correct option is (a) Square-law MODULATOR

Best explanation: AM signals are generated by Square-law modulators. The input for generating AM SIGNAL should be of type (A + m(t)) instead of m(t). Square-law modulator sums CARRIER and INFORMATION signal, then passes them through a non-linear device.

Correct option is (a) CLASS C amplifier

The best I can explain: 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. THUS, Class C amplifier is used.

Correct OPTION is (c) 0.58

The explanation is: Given that m1 = 0.3 and m2 = 0.5. Total modulation index will be equal to . By SUBSTITUTING VALUES we have which is equal to 0.58.

Right answer is (b) change the antenna orientation

For explanation I would SAY: According to MODULATION, length of antenna should be one fourth of wavelength. If DISTORTION occurs whether in television or at any other PLACE, simplest way to eliminate this is to change the orientation of antenna.

The correct answer is (B) 100m

For EXPLANATION: Cordless telephone is one in which the handset is PORTABLE and communicates with the body of the phone by radio waves instead of being connected by a CORD. The range DEPENDS on the quality of instrument and is about 90m to 100m.

Right answer is (b) False

The explanation: It is a high frequency electro-magnetic wave. A CARRIER wave does not have any information. One of the properties LIKE amplitude, frequency or phase of the carrier are modulated with RESPECT to an INPUT signal for the purpose of conveying information.

Correct OPTION is (c) 25W

Explanation: Modulation index, m = 100% = 1. Power in SIDEBANDS is (Pcx^m^2⁄4). By substituting the values, we have (100 x ^1⁄4)which is equal to 25W.

Modulation index, m = 100% = 1.

Power in sidebands = (Pcx ^m^2/4), PC = Power of Carrier

By substituting the values, we have (100 x 1/4) =25W.

Correct answer is (B) Monostable multivibrator

Best explanation: PDM (Pulse Duration Modulation) is used to produce an ANALOG signal with the help of a digital source. Monostable multivibrators are used to GENERATE a single output pulse of a particular width. Its output is EITHER high or low when a suitable EXTERNAL pulse is applied.

Right CHOICE is (d) Demodulation

Easiest EXPLANATION: Process of EXTRACTING the ORIGINAL message signal from the received modulated signal is called demodulation. Demodulation is also known as detection.