Mumbai University Previous year question papers
IV Sem ETC-Examination Dec 2009
Principles of Communication
N.B.: (1) Question NO.1 is compulsory.
(2) Attempt any four questions out of remaining six questions.
.(3) Assume suitbale data if necessary and state ifclearly.
1. Answer the followingany four :-
(a) Why do the amplifiers that follow the modulator circuit inAMDSBFC transmitter have to be linear?
(b) What is the purpose ofAFC loop in FM?
(c) What is gearband when and where is it used?
(d) Explainthe use of limiterin a FMreceiver.
(e) What are the causes of foldover distortion or aliasing? Howcan it be prevented or removed?
2. (a) Draw the block diagram of an AMsuperhetrodyne receiver. Describe its operation and primary functions of eachstage.
(b) List different methods of FM generation. Explain the principle of reactance modulator. Why is direct modulation not preferred for FM generation?
3. (a) Draw the schematic of a ratio detector and describe its operation. 10
(b) One input to a conventional AM modulator is a 500 kHz carries with an amplitude of 20 Vp’ The second input is a 10kHZ modulating signal that is of sufficient amplitude to cause a change in output wave of:t 7.5 Vp’ Determine-
(i) Upper and lower side frequencies
(ii) Modulation coefficient and percent modualtion
(iii) Expression for modulated wave
(iv) Draw output spectrum
(v) Sketch output envelope.
4. (a) Explain the working of balanced ring modulator to generate DSBSC signal.
(b) For an angle modulated carrier Vc = 6 cos (27t110 Mhzt) with 75 kHz frequency deviation due to the information signal and a single frequency interfering signal V n = 0.3 cos (27t 109.985 Mhzt). Determine-
(i) Frequency deviation due to interfering signal
(ii) Voltage.SNR at the output of demodulator.
(c) The noise output of a reistor is amplified by a noiseless amplifier having a gain of60 and a bandwidthof 20 kHz.A meterconnectedto output of amplifier reads 1 mv (RMS). Ifthe bandwidth of amplifier is reduced to 5 kHz with gain constant, what is the reading of meter?
Q.5 (a) Explain the block diagram of Ad~ptive delta modulation with waveforms. How does:it reduce slope overload error?
(b) Explain generation and detection of PWM signal. 10
6. (a) State sampling theorem for bandpass signal. Compare ideal and practical sampling.
(b) Compare AM and FM. 5
(c) The output current of a 60% modulated AM wave is 1.5 A. To what value will the current rise if it is modulated additionally by another wave with 70% modulation index.
7. Write short notes on any four of the following :-
(a) ISB transmission
(b) Pre-emphasis and De-emphasis
(e) Significance of Signal-to-Noise Ratio.