# Electronics and Communication Engineering - Communication Systems

1.

24 telephone channels, each band limited to 3.4 kHz are to be time division multiplexed using PCM. If sampling frequency is 10 kHz and number of quantization levels is 128, the required bandwidth of PCM is

 A. 1.68 MHz B. 240 kHz C. 81.6 kHz D. 3.072 MHz

Explanation:

Bandwidth ≈ 24 (ln2 128) 10 kHz = 1.68 MHz.

2.

Resonant circuits are used in

 A. audio amplifiers B. RF amplifiers C. both audio and RF amplifiers D. none of the above

Explanation:

RF amplifiers use resonant loads (L and C in parallel).

3.

The bandwidth of DSB suppressed carrier modulation system when the modulating frequency varies between 500 Hz and 5 kHz is

 A. 555 kHz B. 505 kHz C. 500 kHz D. 9 kHz

Explanation:

fm = 5 kHz - 500 Hz 4.5 kHz

Bandwidth = 2 fm 2 x 4.5 kHz = 9 kHz.

4.

Linear diode detector utilizes

 A. linear portion of static characteristics of diode B. linear portion of dynamic characteristic of diode C. square law portion of dynamic characteristics of diode D. rectification property of diode

Explanation:

Detection (also called demodulation) is due to rectification property of diode.

5.

In the equivalent circuit of a moving coil cone type loudspeaker, the mass of the moving system is represented by

 A. a resistance B. a capacitance C. an inductance D. a conductance