Principles of Communication Systems — DU B. Sc. (Hons) Electronics Previous Year Question Papers

ANDC assay 3/62 1. (a) Draw the trapezoid pattern for an AM signal with 80% modulation. The carrier amplitude is 5 volts. (3) (b) Convert 26 dB carrier attenuation in linear scale. (3) (c) Write the transmission frequency range for FM entertainment purposes. Also write the corresponding frequency deviation. (3) (d) Draw a circuit to generate PAM in laboratory. (3) (e) Differentiate between internal noise and external noise. (3) (f) Draw an FM generator using 555timer. (3) 2. (a) Draw the block diagram of an electronic communication system. (6) (b) What is the need for performing modulation? Give two reasons. (4) (c) Two resistances of value 2k and 5k are connected in case i) series, case ii) parallel Calculate the rms value of the noise voltage associated with each resistance. Calculate the total rms noise voltages for the case i) and case il) specified above. 3. (a) Draw the transistor circuit for generating AM in laboratory. Draw the unmodulated carrier, modulated carrier and modulating signal waveforms, and the frequency spectrum of modulated carrier for the following parameters: modulating signal : sine wave of amplitude 2 volts and frequency 2 kHz unmodulated carrier wave: sine wave of amplitude 5 volts and frequency 10 (b) Discuss independent side band modulation with suitable block diagram and vestigial side band modulation. (444) 4. (a) What is double spotting? Define sensitivity and selectivity for a receiver. (b) What is 1/f noise? Calculate rms noise current for a resistance of 10 kS. If noise current change? another resistance of 20 kM is connected in series with it, how will the rms (c) Draw the block diagram of filter method of generating SSB. Write the unmodulated sine wave carrier and a modulating sine wave. mathematical equations describing generation of LSB starting from an 5. (a) Write the mathematical expression for an FM signal and explain each term. (b) What is the impact of frequency modulation on power consumption in communication systems? (c) How does pre-emphasis affect the signal-to-noise ratio in FM systems? An FM system uses pre-emphasis and de-emphasis with a time constant (T) of 75 us. The maximum baseband frequency is 15 kHz. Calculate the Cut-off frequency (f) of the de-emphasis filter. 6. (a) A signal is sampled at 8 KHz and is quantized using 8 bit uniform quantizer. Calculate the bit rate. Discuss A-law for non-uniform quantization. (2+4) (b) Using a suitable block diagram, explain Delta Modulation technique. Also give its advantages over PCM. (6) (c) A signal x = 10 cos 2x103t is sampled : (i) using an impulse train of f = 2 × 103 Hz (i) using an impulse train of f = 1.5 × 103 Hz (ii) using an impulse train of f = 2.5 × 103 Hz Draw the frequency spectrum of the sampled signal for the three cases. 7. (a) An FM radio station transmits a carrier signal at 90 MHz with a modulating signal of 8 kHz. If the peak frequency deviation is 40 kHz, determine: (i) The modulation index (B).