This set of Digital Communications Multiple Choice Questions & Answers (MCQs) focuses on “Digital bandpass modulation techniques”.
1. Wavelength and antenna size are related as
a) λ/2
b) λ/4
c) 2λ
d) 4λ
View Answer
Explanation: The transmission of Em field in space is done with the help of antennas. Antenna size depends of the wavelength. The length of the antenna is equal to λ/4.
2. The detection method where carrier’s phase is given importance is called as
a) Coherent detection
b) Non coherent detection
c) Coherent detection & Non coherent detection
d) None of the mentioned
View Answer
Explanation: When the receiver uses carrier’s phase as major factor for detection then it is called as coherent detection and when carrier is not given importance it is called as non coherent detection.
3. The coherent modulation techniques are
a) PSK
b) FSK
c) ASK
d) All of the mentioned
View Answer
Explanation: Some of the examples of coherent modulation techniques are phase shift keying, amplitude shift keying, frequency shift keying and continuous phase modulation.
4. The real part of a sinusoid carrier wave is called as
a) Inphase
b) Quadrature
c) Inphase & Quadrature
d) None of the mentioned
View Answer
Explanation: The two main parts of sinusoid carrier wave – real part is called as inphase and the imaginary part is called as quadrature.
5. Antipodal signal sets are those vectors that can be illustrated as
a) Two 180 opposing vector
b) Two 90 opposing vector
c) Two 360 opposing vector
d) None of the mentioned
View Answer
Explanation: In BPSK the two vectors, the signal set can be illustrated as two 180 opposing vector which is called as antipodal signal sets.
6. The FSK signal which has a gentle shift from one frequency level to another is called as
a) Differential PSK
b) Continuous PSK
c) Differential & Continuous PSK
d) None of the mentioned
View Answer
Explanation: In general FSK the signal change from one frequency to another will be abrupt but in continuous FSK the signal change from one frequency to another will be gentle and gradual.
7. Which modulation scheme is also called as on-off keying method?
a) ASK
b) FSK
c) PSK
d) GMSK
View Answer
Explanation: In ASK modulation scheme the signal attains either maximum amplitude or zero point. Thus it is also called as on-off keying.
8. In amplitude phase keying each phase vector is separated by
a) 90
b) 0
c) 45
d) 180
View Answer
Explanation: The combination of ASK and PSK is called as APK. In APK each vector is separated by 45.
9. The term heterodyning refers to
a) Frequency conversion
b) Frequency mixing
c) Frequency conversion & mixing
d) None of the mentioned
View Answer
Explanation: The term heterodyning means frequency conversion and frequency mixing that yields a spectral shift in the signal.
10. The transformation of the waveform into a single point in signal space is called as
a) Vector point
b) Predetection point
c) Preamplification point
d) Transformation point
View Answer
Explanation: The first step of the detection process is to reduce waveform into a single or group of random variable. This first step in the transformation of waveform into a point in signal space. This point is called as predetection point.
Sanfoundry Global Education & Learning Series – Digital Communications.
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