Questions on Communication Systems

1. An AM signal has the form x(t) = [ 10 + 3 cos 3000πt +4 cos 6000πt].cos 2πf_ct, where f_c =10⁶ Hz. The modulation index is ------------------

2. A super heterodyne receiver uses an IF frequency of 455 KHz. The receiver is tuned to a transmitter having a carrier frequency of 2400 KHz. High side band tuning is to be used. The image rejection ratio will be (Q=50)

a) 32.715

b) 35.25

c) 25.715

d) 23.715

3. An angle modulated signal is given by s(t) = Cos 2π ( 2 x 10⁶ t + 30 sin 300 t + 40 cos 300t). The maximum frequency and phase deviation of s(t) are

a) F_m = 11.18 KHz, ∆φ =100

b) F_m = 70.24 KHz, ∆φ =50

c) F_m = 11.18 KHz, ∆φ =100 π

d) F_m = 1.1 KHz, ∆φ =100 π

4. A speech signal band limited to 5 kHz and peak voltage varying between +7V and -3V, is sampled at the Nyquist rate. Each sample is quantized and represented by 8 bits. The number of quantization levels required to reduce the quantization noise by a factor of √2 would be

a) 1024

b) 512

c) 256

d) 362

5. An analog signal has spectral components from 2 KHz to 7 KHz. What is the minimum band pass sampling rate?

a) 14 KHz 

b) 5 KHz

c) 9 KHz

d) 10 KHz

6. A communication channel having AWGN has a bandwidth of 6 KHz and SNR of 7. The highest transmission rate that such a channel can support is

a) 18 Kbps

b) 42 Kbps

c) 16 KPbs

d) 20 Kbps

7. The sum of two signals E₁= 3 sin 4 π x 10³t and E₂= 5 sin 2 π x 256 t are sampled at 1024 Hz. 

The sampled signal is passed through a LPF with fc = 2048 Hz. The output of this filter will contain components at 

a) only 1024 Hz

b) 256 Hz and 1744 Hz

c) 256 Hz and 1024 Hz

d) only 256 Hz

8. An angle modulated signal is expressed by F(t) = 2 cos (2 x 10⁸ π t + 75sin 4 x 10³ π t).

 Find the total power and bandwidth of the signal.

a) Pt = 4 Watts; BW = 304 KHz

b) Pt = 152 watts; BW = 4 KHz

c) Pt = 2 watts; BW = 304 KHz

d) Pt = 15 watts; BW = 4 KHz

9. When a radio receiver is tuned to 750 KHz its local oscillator provides the mixer with the input at 1250 KHz. At the output, another signal is also received along with the desired signal. The frequency of the other station is

a) 1750 KHz

b) 1500 KHz

c) 1250 KHz

d) 500 KHZ

10. 25 telephone channels, each band limited to 4 KHz, are to be time division multiplexed(TDM) by using  PCM. Calculate the transmission rate of the PCM system for 64 quantization levels and sampling frequency is 1.25 times the Nyquist rate.

a) Rb= 1200 Kbps

b) Rb =750 Kbps

c) Rb =1750 Kbps

d) Rb = 1500 Kbps

11. Which of the following demodulator can be used for demodulating the signal

X(t) = 3( 1 + 2.5 cos 500 πt). Cos 50 10³πt is 

a) Envelope detector

b) Synchronous demodulator

c) Square law demodulator

d) All the above

12. 14 signals each band limited to 4 KHz are to be transmitted over a single channel by FDM.

If AM-SSB modulation guard band of 0.75KHz is used, then the bandwidth of the multiplexed signal will be

a) 63.75 KHz

b) 121.75 KHz

c) 37.75 KHz

d) 70 KHz

13. A delta modulator is designed to operate on speech signals limited to 3.4 KHz. The sampling rate is 10 times the Nyquist rate. The step size is 100mV. The modulator is tested with 1KHz sinusoidal signal. The maximum amplitude of the test signal required to avoid slope overload distortion is 

a) 1 mV

b) 2.08 mv

c) 2.08 V

 d) 1.08 V

14. A source generates four equiprobable symbols. If the source coding is adopted, the average length of the code for 100% efficiency is --------- bits per symbol.

15. An AM wave is given by s(t) = 10( 1 + 0.4 cos 2 π x 10³t + 0.3 cos 2 π x 10⁴ t). Cos 2π x10⁶t. 

The following frequency will not be present in the spectrum is/are

a) 1 MHz

b) 2 MHz

c) 1010 KHz, 1001 KHz

d) 990 KHz, 999KHz

16. A source generates 3 symbols with probabilities 1/2, 1/4 and 1/4 at a rate of 2500 symbols per second. Assuming independent generation of symbols, the most efficient source encoder would have average bit rate

a) 3750 bits/sec

b) 1666 bits/sec

c) 4500 bits/sec

d) 3500 bits/sec

17. A single tone frequency modulated wave is processed through a non-linear system y(t) = x²(t). (β  and ∆f are modulation index and frequency deviation of original FM wave). The bandwidth required to transmit the output of the non-linear device is

a) 4 ∆f + 4 fm

b) 4 ∆f + 2 fm

c) 2 ∆f + 2 fm

d) 2 ∆f + 4 fm

18. An SSB transmission contains  8kW. This transmission is to be replaced by a standard amplitude modulated signal with same power content. If the modulation index is 0.75, then the upper side band power is -----------------

19. In a multitone modulation, the maximum peak to peak voltage of an AM wave is 14mV and the minimum peak to peak voltage is 6 mV and second modulating wave has modulation index µ = 0.5. The overall modulation index is --------------

20. The frequency deviation of an FM signal is increased from 30 kHz to 60 kHz. For the same sinusoidal modulating signal of 12kHz, the improvement in the demodulated signal to noise power ratio will be ______________

21. To overcome slope over distortion and granular noise problem, which type of integrator is used in Delta modulation?

a) Linear Slope

b) Fixed Slope

c) Companding

d) Variable slope

22. A carrier wave with frequency 8 MHz and peak amplitude value of 10V is DSB-SC amplitude modulated by a 5 kHz sine wave of amplitude 6 V. The amplitude of the side band component is _______

23. The tuned-circuit of the oscillator in an A.M. transmitter uses a 25µH coil and 2.5 nF capacitor. Now if the oscillator output is modulated by audio frequencies upto 5kHz, then the frequency range of side bands would be

a) 631.62 kHz to 641.62 kHz

b) 630 kHz to 645 kHz

c) 630.62 kHz to 646.62 kHz

d) 646 kHz to 656 kHz

24. A 3 V, 1 kHz signal modulates the carrier to cause frequency deviation of 6.5 kHz. On increasing the modulating voltage to 12 V, the frequency deviation is ___________

25. A TDM link has 15 signal channel, each sample is represented by 8 binary bits and it contains an additional bit for synchronisation. If the total bit rate for the TDM link is 1250kbps, then the value of sampling duration (Ts) is ___________

a) 108µs

b) 54µs

c) 125µs

d) 250µs  

  26. For an AM signal, the bandwidth is 8 kHz and the lowest frequency component present is 800kHz. The carrier frequency used for this AM signal (in kHz) is --------------