ECE 342 Midterm

Name:______A. Multiple Choices (30 points, 3 points each)

1. What is the feature of analog communications? ( ) A. Transmission in continuous time and continuous value. B. Analog devices are used in both transmitter and receiver. C. The signal is transmitted through a continuous time channel. D. Noisy communication channels.

2. Which of the following is NOT a feature of thermal noise incurred by motions of free electrons in conductor? ( ) A. Flat frequency spectrum in a very wide band. B. The noise is almost independent in the time. C. The noise power is proportional to the temperature. D. It has many abrupt impulses.

3. Which of the following is a distortion for the signal transmission? ( ) A. Delay B. Scaling of amplitude C. Passing through a linear system with frequency selective response D. Demodulation

4. In a free space transmission of wireless signal, which of the following is correct for the received power (d is the transmission distance)? ( ) 2 A. Pr µ1/ d 3 B. Pr µ1/ d 4 C. Pr µ1/ d 5 D. Pr µ1/ d

5. Which of the following is correct for DSB? ( ) A. There is a carrier component. B. There is not carrier component. C. The modulation index is 1. D. The modulation index is -1.

6. Which of the following picture is D. Middletown? ( )

A. B. C. D.

7. Which of the following AM schemes may not need a coherent demodulation? ( ) A. DSB B. VSB C. SSB D. AM with a small modulation index

8. What is a benefit of carrier component in AM? ( ) A. Provide carrier for the receiver B. Save power C. Save bandwidth D. Save device

9. Which of the following angle modulation is similar to AM? ( ) A. FM of a tone B. PM of a tone C. Narrowband PM D. Wideband FM

10. Which of the following determines the gap between spectrum lines in a single tone FM? ( ) A. Carrier frequency B. Frequency of the modulating signal C. Modulation index D. Amplitude of modulating signal

B. Signal and Noise (30 points)

Consider signal being transmitted in a free space. The transmitted power is 2W and the transmission distance is 1000 meters. It has been measured that the received power at the distance of 100 meters is -95dBW. The temperature at the receiver is 300K and the resistance is 1Om. Then, what is the value of SNR at the receiver? (the Boltzman constant -23 kB =1.3806488´10 ) C. Amplitude Modulation (20 points)

(1) Explain what DSB, SSB and VSB are. (2) Signal x(t) = cos2p100t + 4cos2p 200t is transmitted using DSB with carrier frequency 5000Hz. Sketch the positive-frequency line spectrum. Repeat it when SSB is used. D. Angle Modulation (20 points)

Consider a single tone modulated FM wave with fm = 20kHz, b = 3, Ac =10

and fc = 200kHz . Write the expression for the modulated signal. Sketch the line spectrum. Repeat your solution if the modulating signal is x(t) = cos2p 5000t + 2cos2p10000t. average = 70.6, standard deviation = 16.7