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MU Computer Engineering (Semester 3)
Elec Circuits & Comm. Fundamentals
December 2013
Total marks: --
Total time: --
INSTRUCTIONS
(1) Assume appropriate data and state your reasons
(2) Marks are given to the right of every question
(3) Draw neat diagrams wherever necessary

1 (a) What do you understand by "Pinch off" voltage and "cut off" voltage as applied to FET?
5 M
1 (b) With neat block diagram explain how PLL can be used to generate large number of frequencies from a single reference frequency.
5 M
1 (c) For an FM radio receiver, when the consumer changes the chanel, the oscillator frequency changes. The frequency of radio signal coming out of the mixer will be always be 10.7 MHz regardless of the channel selected. If the oscillator frequency is 99.2 MHz. what is the frequency of station selected? if a Hartly tuned oscullator is being used with a variable capacitor in the LC tank circuit, what is the value of capacitance if the total inductance is 2.5 µH.
5 M
1 (d) Optical sensor of a package conuting system create 1.5 V DC signal when it detects the seal. This viltage need to be amplifier and sent to a controller with an input resistance of 5K?. Draw block diagram for the system. Choose correct amplifier to drive this low resistance load and select the resistor valves to set the gain. the input voltage range for the controller is +10V to +30V DC.
5 M

2 (a) Determine the VGSQ'IDQ and VDSQ and AV for a voltage divider circuit with VDD=18V' RD=2.2K, R1=2.1M, R2=330K and RS=1.2K, IDSS=10mA, VP= -8V and VDS=40?s.
10 M
2 (b) Explain how OP-Amp can be used as a summing, scaling and averaging amplifier in the Inverting configuration.
10 M

3 (a) sketch an op-amp integrating circuit together with the circuit waveforms.explain in brief the circuit operation.
5 M
3 (b) Explain the fly wheel effect in class c amplifier.
5 M
3 (c) Explain why crystal oscillator are considered to be more stable than other oscillators.
5 M
3 (d) Compare different biasing circuit with respect to Q-point? Comment on which biasing circuit is better in stability.
5 M

4 (a) Draw the block diagram of a phase cancellation SSB generation and explain how the carrier and unwanted side band are suppressed. What change is necessary to supress the other side band.
10 M
4 (b) Draw foster Seeley Descriminator with circuit diagram and explain its working with phasor diagrams.
10 M

5 (a) What is multiplexing in communication system? Draw block diagram of TDM-PCM system and explain?
10 M
5 (b) One input to a conventional AM modulator is a 500KHz carrier with an amplitude of 20 VP. The second input is a 10KHz modulating signal that is of sufficient amplitude to cause a change in the output wave of ± 7.5 VP. Determine
(i) Upper and lower side frequences
(ii) Modulation coefficient and percent modulation
(iii) Peak amplitude of the modulated carrier and upper and lower side frequency voltages
(iv) Expression for the modulated wave
(v) Draw the output spectrum.
10 M

6 (a) What are the drawbacks of delta modulation and how are they overcome by ADM?
5 M
6 (b) Explain in brief the principle of super heterodyne receiver?
5 M
6 (c) Compare PAM, PPM & PWM.
5 M
6 (d) What is meant by Nyquist rate in sampling? What is standard frequency for speech signal.
5 M

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