**B.E. (fourth semester) EXAMINATION, June, 2002**

**(Information technology)**

**ANALOG AND DIGITAL COMMUNICATION**

**(CS-4504)**

Note: Attempt any five questions. All question carry equal marks. Make suitable assumption wherever necessary.

(a) What is synchronous detection? Explain the effect of phase and frequency error in synchronous detection. Show that an over modulated signal can be detected by synchronous detection.2,3,3

(b) Show that AM signal with large carrier can be detected by squaring it and than passing it through a loss pass filter.

2. (a) Explain the effect of bandwidth in case of frequency modulation (FM) and phase modulation (PM) when the modulating frequency is doubled.

A 500 Hz audio sine wave modulates a 28 MHz carrier. If the carrier voltage is 5 V and the maximum deviation is 10 kHz. Write the equation for (i) FM, and (ii) PM wave. Now if modulating frequency is changed to 1 kHz write the new equation for (i) FM and (ii) PM.

(b) Draw the schematic diagram of a delta modulation system and explain its operation. What are its limitation and how are the they improved?

3(a) Plot the entropy for two message and show that is maximum for equally likely message. An analog signal band limited to 100 Hz is sampled at nyquist rate samples are quantized into four levels these four levels are assumed to be purely independent with probabilities of occurrence as:

P1 = p4 = 28 ; p2 = 18 ,p 3 38

Find the information rate of the source.

(b) With the help of the block diagram explain the principle of an encoder for generating convolution codes. How convolution

codes signals are decoded?

(a) Explain the merits and demerits of PCM system. In a PCM system 24 telephone channels each band limited to 3.4 kHz are multiplexed. Calculate the bandwidth of PCM system for 128 Quantization levels and 8 kHz sampling frequency.

(b) Determine the relation between power spectral density and autocorrelation of a function. Hence find the auto correlation of low pass filtered (LPF) white noise with power spectral N2 assume cut-off frequency of LPF is BHz.

(a) The noise figure of an IF amplifier is 15 lb. the amplifier is processed by a preamplifier of low with 10 db gain and noise figure of 6db. Find the overall noise figure. Derive the formula used if any.

(b) Define the transfer function of a system in time domain. Show that an ideal low pass filter cannot be physically realized.

State and prove parseval’s theorem for energy signal and power signals. A signal with power spectral density Df is applied to the input of linear system shown Fig. Determine the ratio of average power of the response and excitation, if Df is given as:

(a) Draw the block diagram of a super heterodyne receiver and explain each. Block gives the criteria for Selection of intermediate frequency.

(b) Define the following with example.

(i) Cumulative distribution function

(ii) Probability density function

(iii) Variance

(iv) Standard deviation

(8) Write notes on any two of the following:

(a) Matched filter

(b) Error probability comparison of ASK , PSK and FSK

(C) Bandwidth –SN trade off

(d) Noise figure of merits of FM receiver