# Amplitude, Frequency, Pulse Modulation - Electronics Engineering

## Amplitude, Frequency, Pulse Modulation - Electronics Engineering test questions

(1) In SSB the pilot carrier is provided

(A) For stabilizing frequency
(B) To reduce noise
(C) For reducing power consumption
(D) As an auxiliary source of power

(2) For AM wave, the antenna current is doubled when the modulation index is doubled. The AM system being used is

(A) A3H
(B) A5C
(C) A3
(D) A3J

(3) For 100% modulated suppressed carrier AM signal the percentage saving in power is

(A) 80
(B) 66 2/3
(C) 50
(D) 40

(A) Unmodulated RF carrier
(B) Modulated RF carrier
(C) Both A and B
(D) None of the above

(5) In the standard AM radio broadcast band, the frequency tolerance for the RF carrier is

(A) Zero
(B) +25 Hz
(C) + 20 Hz
(D) +1000 Hz

(6) If a 900 KHz carrier is amplitude modulated with a 4000Khz audio tone the lower and upper frequencies are

(A) 800 and 1000 KHz
(B) 450 and 1800 KHz
(C) 4000 and 8000 KHz
(D) 896 and 904 KHz

(7) In an AM signal, the peak antenna current = 12 A, the minimum antenna current = 3A. The percentage modulation is

(A) 60 %
(B) 100 %
(C) 50 %
(D) 40 %

(8) High-level modulation is used in a 5 KW AM transmitter. For 100% modulation the value of audio signal power needed is

(A) 2.5KW
(B) 0.5KW
(C) 1KW
(D) 1.5KW

(9) Vestigial side band is most commonly used in

(B) Telephony
(C) Television transmission
(D) All of the above

(10) The programme called vividh bharati is transmitted in

(A) Very high frequency
(B) Medium frequency
(C) Short wave
(D) None of the above

(11) The Audio frequency range lies between

(A) 20 Hz and 20 KHz
(B) 20 KHz and 200 KHz
(C) 2 MHz and 20 MHz
(D) 20 MHz and 200 MHz

ANSWER: 20 Hz and 20 KHz

(12) What will be the overall modulation when a carrier is simultaneously modulated by two sine waves of modulation indices 0.8 and 0.6?

(A) 0.5
(B) 1.0
(C) 0.7
(D) 0.9

(13) A balanced modulator produces

(A) SSB
(B) AM
(C) VSB
(D) DSB

(14) In case of amplitude modulation if modulation index > 1 then

(A) There will be interference with another signal
(B) The bandwidth will decrease
(C) The wave will get distorted
(D) The efficiency of transmission will improve.

ANSWER: The wave will get distorted

(15) Consider a transmitter that radiates 9kW when the carrier is unmodulated. When the carrier is sinusoidally modulated a transmitter radiates 10.125 KW. What will be the modulation index?

(A) 50%
(B) 40%
(C) 60%
(D) 100%

(16) During amplitude modulation, the phase angle of the modulating voltage changes by

(A) 180°
(B) 90°
(C) 45°
(D) 0°

(17) A FM discriminator changes

(A) FM signal into AM signal
(B) FM signal into PM signal
(C) FM signal into FM signal of high frequency
(D) FM signal into FM signal of low frequency

ANSWER: FM signal into AM signal

(18) In an FM transmitter, the final power amplifier usually operates as

(A) Class A
(B) Class B
(C) Class C
(D) Class D

(19) In FM broadcasting the frequency of the stereo sub-carrier signal is

(A) 48KHz
(B) 78KHz
(C) 19KHz
(D) 38KHz

(20) De-emphasis circuit is used

(A) After modulation
(B) Prior to modulation
(C) For de-emphasizing low frequency component
(D) For de-emphasizing high frequency component

ANSWER: For de-emphasizing high frequency component

(21) Which one of the following is the commercial FM radio broadcast band?

(A) 20 to 80 KHz
(B) 88 to 180 KHz
(C) 520 to 1800 KHz
(D) 300 to 3000 MHz

(22) Let noise phase-modulates the FM Wave. As noise side band approaches the carrier frequency, the noise amplitude

(A) Is decreased
(B) Is increased
(C) Is equalized
(D) Remains constant

(23) Foster seelay discriminator uses a

(A) Single tuned circuit
(B) Double tuned circuit with primary and secondary tuned to the different frequency
(C) Double tuned circuit with primary and secondary tuned to the same frequency
(D) None of the above

ANSWER: Double tuned circuit with primary and secondary tuned to the same frequency

(24) A narrow band FM can be approximated to __________ when bandwidth is the major consideration

(A) SSB
(B) FM
(C) AM
(D) All of the above

(25) Bandwidth of a FM wave is 160KHz and modulation index is 7.5. The frequency deviation is

(A) 75MHz
(B) 75KHz
(C) 16KHz
(D) 16MHz

(26) FM is indirectly generated by

(A) Armstrong modulator
(B) Hartley oscillator
(C) Varactor diode modulator
(D) Transistor modulator

(27) In FM system if the depth of modulation is doubled, the power transmitted

(A) Increases by factor of 2
(B) Increases by factor of √2
(C) Increases by factor of √3
(D) None of the above

(28) For a FM broadcast station the assigned carrier frequency is 99.7 MHz. Its FM signal has a rest frequency of

(A) 99.7 MHz
(B) 99.7 KHz
(C) 99.7 Hz
(D) 100 MHz

(29) The statement that is not true about wide band FM is

(A) Noise suppression is more effective
(B) Pre-emphasis and de-emphasis cannot be used
(C) Bandwidth of frequency required is more
(D) Excessive noise peak

ANSWER: Pre-emphasis and de-emphasis cannot be used

(30) The difference between PM and FM

(A) Lies in the poorer audio response of PM
(B) Is purely theoretical because practically both are identical
(C) Lies in the different definitions of the modulation index
(D) Is too great to make the two systems compatible

ANSWER: Lies in the different definitions of the modulation index

(31) Consider a case where frequency doubler has an FM signal input at 13 MHz with a deviation of 5KHz.The output frequency for the carrier will be

(A) 26.1 MHz
(B) 10KHz
(C) 100 MHz
(D) 26 MHz

(32) Companding is used in PCM

(A) To protect small signals from quantizing distortion
(B) To overcome quantizing noise
(C) Transmitters to allow amplitude limiting in the receivers
(D) Receivers to overcome impulse noise

ANSWER: To protect small signals from quantizing distortion

(33) Which one of the following statement is true for PCM system?

(A) Lower power
(B) Lower Bandwidth
(C) Lower noise
(D) All of the above

(34) To ensure no loss of information in pulse modulation, the number of samples required is given by

(A) Laplace transform
(B) Nyquist theorem
(C) Fourier transform
(D) Parsevals theorem

(35) Pulse communication system that is inherently highly immune to noise is

(A) PCM
(B) PPM
(C) PAM
(D) PWM

(36) To generate PWM signal the technique used is

(A) Differentiating the PPM signal
(B) Integrating the PPM signal
(C) A astable multivibrator is used
(D) A monostable multivibrator

(37)

(A) PCM
(B) PWM
(C) PAM
(D) PFM

(38) The ratio of time on to time off in pulse transmitter is called

(A) Bandwidth
(B) Duty cycle
(C) Efficiency
(D) Base

(39) In PCM the signal to quantization noise ration depends on

(A) Bandwidth of message signal
(B) Sampling rate
(C) Number of quantization levels
(D) None of the above

(40) To transmit normal speech signal, bandwidth required in the PCM channel is

(A) 4 KHz
(B) 2 KHz
(C) 8 KHz
(D) 64 KHz