EC312 Twelve Week Exam Spring 2016
April 08, 2016
United States Naval Academy
Electrical and Computer Engineering Department
EC312 - 12 Week Midterm – Spring 2016
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Do a page check: you should have 6 pages including this cover sheet.
You have 50 minutes to complete this exam.
An FE-approved calculator may be used for this exam.
This exam is closed book and closed notes. You may use two single-sided hand-written pages of notes.
Turn in your two single-sided hand-written pages of notes with your exam.
This exam may be given as a makeup exam to several midshipmen at a later time. No communication is
permitted concerning this exam with anyone who has not yet taken the exam.
Name:
____________________
Instructor:
____________________
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EC312 Twelve Week Exam Spring 2016
April 08, 2016
Question 1. (4 pts) In Annapolis, WNAV broadcasts at 1430 AM and WYRE broadcasts at 810 AM. Both
stations have monopole antennas. Which of the following statements are true? Circle all that apply.
a) The carrier frequency for WNAV is greater than the carrier frequency for WYRE.
b) The antenna for WNAV should be taller than the antenna for WYRE.
c) It is possible that another radio station could be assigned 1430 AM if it is sufficiently distant
from Annapolis.
d) The antennas for AM radio stations should be designed for very high gain in order to best serve
the listener.
Question 2. Note time scale is in µs
8
6
Signal (V)
4
2
0
-2
-4
-6
-8
0
20
40
60
80
100
120
140
160
180
200
Time (µs)
(2 pts) What is Vm?
(2 pts) What is Vc?
(2 pts) What is the modulation index for the given waveform?
(6 pts) If the carrier frequency is 500 kHz, sketch the frequency domain for the transmitted AM signal. Label
the axes and applicable provide values.
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EC312 Twelve Week Exam Spring 2016
April 08, 2016
Question 3. (4 pts) Circle either T (True) or F (False) for each statement:
T / F Ground waves follow the earth, but cannot go beyond the horizon.
T / F Sky waves are used to talk to satellites such as the Hubble Space Telescope
T / F The main radio wave propagation characteristics are: Reflection, Refraction, Diffraction, and Scattering.
T / F Higher frequency waves propagate farther than lower frequency waves.
Question 4. (8 pts) List and briefly describe two advantages of digital modulation over analog systems.
Question 5. (4 pts) The component values for the series RLC circuit shown
here are R=100Ω, C= 10nF, and L=1mH. What is the resonant frequency
for this circuit?
(3 pts) With the input and output connections as shown, what type of filter is this circuit?
(4 pts) If you were to increase the resistance while keeping the other components the same, how would the
following filter parameters be affected?
Resonant frequency:
increase
decrease
stay the same
Bandwidth:
increase
decrease
stay the same
Quality factor (Q):
increase
decrease
stay the same
|Vout/Vin| at resonance:
increase
decrease
stay the same
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EC312 Twelve Week Exam Spring 2016
April 08, 2016
Question 6. The figure below shows a radio receiver designed to filter a received signal with a low-pass filter
(cutoff frequency 5 kHz), and then use a two-stage amplifier to attain a desired output power.
PSignal
Low-pass
filter (cutoff
freq. 5 kHz)
A1
A2
Pout
a) (4 pts) Suppose the input signal has power Psignal = -1dBm, and the required output power Pout is 100
mW. What is the required overall system gain as a unitless ratio?
b) (6 pts) Now suppose that the required overall system gain (including the low-pass filter and the two
amplifiers) is 10dB. If the first amplifier has gain A1=2, what should be the second amplifier gain A2 (in
dB) for a 5 kHz input signal?
Question 7. (8 pts) Draw the frequency spectrum on the provided axis if the signal is run through an ideal high
pass filter with a cutoff frequency of 100Hz.
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EC312 Twelve Week Exam Spring 2016
April 08, 2016
Question 8. (4 pts) Circle all of the following which are true
(i) 8 QAM has twice the bandwidth of 8 PSK
(ii) QAM is a combination of FSK and ASK.
(iii) FSK and BPSK will always have the same bandwidth
(iv) 16 QAM transmits 4 bits per symbol
Question 9. (4 pts) What is the Front Back Ratio of the given beam pattern?
(4 pts) What is the beam width?
Question 10. (6 pts) Your EA-18G Growler is 5000m from a target aircraft and the transmitting tower you’re
attempting to jam is 10,000m from the target aircraft. If the transmitting tower has an EIRP of 20dBW, how
much power, in Watts, do you need to transmit in order to have a 3 dB J/S?
Question 11. (6 pts) What is the maximum bitrate and baud (symbol rate) for 8-QAM that has a bandwidth of
10 kHz? Show all work. Recall BW=2*Rb/N
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EC312 Twelve Week Exam Spring 2016
April 08, 2016
Question 12. The figure below is a graph of the signal
𝑉𝑉𝑠𝑠 (𝑡𝑡) = 10sin( 2𝜋𝜋(10)𝑡𝑡) + 5 sin( 2𝜋𝜋(50)𝑡𝑡 + 20) − 5 cos(2𝜋𝜋(50)𝑡𝑡)
Suppose the signal is sampled at a 50Hz sampling rate and quantized with a 2-bit quantizer with a range of -20V
to +20V.
1) (4 pts) Will this analog-to-digital conversion suffer from aliasing? Explain your answer.
2) (4 pts) What is the bit stream for the first 4 samples? Start at Time=0s.
3) (4 pts) What is quantization error and how can it be reduced?
Question 13. (4 pts) Define spread spectrum.
(3 pts) List two benefits and one drawback of using spread spectrum.
Turn in your equation sheets with your exam!
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