Communication Theory
Question 1 a) Explain the concept of power spectral density and write its mathematical description. [6 marks] b) Sketch the block diagram of an FM demodulator and briefly explain the operation of each element. Assuming the carrier channel is subject to white noise, sketch the Power Spectral Density of the noise signal at the output of the following points in your block diagram: (i) IF filter (ii) Phase detector (iii) Discriminator. (iv) Post detection filter. [10 marks] c) For an FM receiver to operate correctly and achieve optimum detected signal-to-noise performance a post detection filter is essential. Why is this? [4 marks] d) Explain how pre-emphasised FM overcomes the need for an ideal post detection filter. [5 marks] Page 3 Turn Over ECS701P/D/U (2015) Question 2 a) A single, square-pulse time-signal of unit amplitude, product modulates a cosine carrier of frequency c ; evaluate and plot I. The spectra associated with this AM signal II. The Power Spectral Density [8 marks] b) Show how demodulation of the AM signal in part a) of this Question is achieved by synchronous detection [6 marks] c) Explain the significance of the term ‘synchronous detection’ and consequently the phenomenon of ‘total fading’, using an appropriate mathematical description. [6 marks] d) Briefly outline in relation to Question 1 why AM is inappropriate for satellite communication links. [5 marks]
