This set of Digital Communications Multiple Choice Questions & Answers (MCQs) focuses on “Spectral Density and Autocorrelation”.

1. Power spectral density function is a ?

a) Real and even function

b) Non negative function

c) Periodic

d) All of the mentioned

View Answer

Explanation: A power signal is periodic signal and its function is a real even and non negative function as per the definition.

2. Energy spectral density defines

a) Signal energy per unit area

b) Signal energy per unit bandwidth

c) Signal power per unit area

d) Signal power per unit bandwidth

View Answer

Explanation: Energy spectral density gives the signal energy equal to the area under the waveform energy spectral density versus frequency curve.

3. Power spectrum describes distribution of _________ under frequency domain.

a) Mean

b) Variance

c) Gaussian

d) None of the mentioned

View Answer

Explanation:Power spectrum gives the distribution of variance of a signal in the frequency domain, sampled into spectral components.

4. How can power spectral density of non periodic signal be calculated?

a) By integrating

b) By truncating

c) By converting to periodic

d) None of the mentioned

View Answer

Explanation: A power signal usually is a periodic signal. But power spectral density of non periodic signal can be calculated by truncating it and observing it in the range of (-T/2,T/2).

a) Spectral density and auto-covariance makes a fourier transform pair

b) Spectral density and auto-correlatioon makes a fourier tranform pair

c) Spectral density and variance makes a fourier tranform pair

d) None of the mentioned

View Answer

Explanation: According to the theorem spectral density of a signal x(t) and auto correlation function makes a fourier tranform pair.

6. According to Parseval’s theorem the energy spectral density curve is equal to?

a) Area under magnitude of the signal

b) Area under square of the magnitude of the signal

c) Area under square root of magnitude of the signal

d) None of the mentioned

View Answer

Explanation: According to Parseval/s theorem the energy spectral density function can be given as equal to the square of the magnitude of the signal x(t).

7. Spectogram is the graph plotted against ?

a) Frequency domain

b) Time domain

c) Both of the mentioned

d) None of the mentioned

View Answer

Explanation: Spectral density function is plotted against frequency domain and spectogram is a graph which is plotted against time domain.

8. Autocorrelation is a function which matches –

a) Two same signals

b) Two different signal

c) One signal with its delayed version

d) None of the mentioned

View Answer

Explanation: Autocorrelation is a function that matches a signal with its delayed version.

9. Autocorrelation is a function of

a) Time

b) Frequency

c) Time difference

d) Frequency difference

View Answer

Explanation: Autocorrelation is a function of time difference as it matches the signal with its delayed version.

a) Unity

b) Origin

c) Infinite point

d) None of the mentioned

View Answer

Explanation: According to its properties autocorrelation is maximum at origin.

11. Autocorrelation function of periodic signal is equal to _______

a) Energy of the signal

b) Power of the signal

c) Its area in frequency domain

d) None of the mentioned

View Answer

Explanation: Autocorrelation function of a real valued signal is equal to energy of the signal and auto-correlatioon function of periodic signal is equal to the average power of the signal.

12. Autocorrelation is a _______ function.

a) Real and even

b) Real and odd

c) Complex and even

d) Complex and odd

View Answer

Explanation: According to properties of autocorrelation function it a even function when the frequency value f is real.

13. Autocorrelation function of white noise will have ?

a) Strong peak

b) Infinite peak

c) Weak peak

d) None of the mentioned

View Answer

Explanation: Autocorrelation function curve of continuous time white noise signal has strong peak.

**Sanfoundry Global Education & Learning Series – Digital Communications.**

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