ECT261 Practice Test III

Instructions: There are 25 questions on this test. It is written in a style very similar to the REAL exam #3. Choose the best answer from among those given. Click the GRADE button on the bottom of the form when you want the exam graded.

Note: You can practice this test as many times as you desire; the system will cook you up a fresh batch of questions each time you access this page.

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1. Given that N=1593, R=1024, and F1 = 10.240 MHz, what will the output frequency Fout of the PLL synthesizer above be?

a) 10.240 MHz
b) 4.9152 MHz
c) 15.930 MHz
d) None of the above

2. What is the function of a VCO?

a) Converts voltage to frequency
b) Converts frequency to voltage
c) Converts voltage to phase
d) None of the above

3. In the PLL pictured above, the following measurements were made:

F1 = 10.000 MHz
"A" = 0 KHz (Steady +100 mV DC signal)
"B" = 55 KHz (5 Vpp square wave)
VCO Control Voltage = 0.3 V (Steady DC); Vcc = 8.0 V
"D" = 55.055 MHz (Unstable; this is an average).

It is known that R = 100 and N = 1001 from the service documentation. What is the most likely fault in the loop?

a) The N divider is defective (Not triggering correctly on VCO signal)
b) The R divider is defective (Not producing correct frequency at TP "A")
c) The VCO is defective (Not oscillating)
d) The phase detector is defective (Not comparing "A" and "B")
e) The loop is operating correctly

4. Which portion of a PLL is primarily responsible for determining the capture range?

a) VCO
b) Phase Detector
c) Low-Pass Filter
d) Reference Divider

5. When the REFERENCE and VCO frequencies are equal, what state is a PLL in?

a) Free-Running
b) Capture
c) Lock
d) None of the above

6. An FM transmitter is operating on 49.860 MHz. Fm = 5 KHz and the deviation is 5 KHz. The unmodulated carrier voltage is 1V. If the output of this transmitter is viewed on a spectrum analyzer, what voltage will be visible at the frequency 49.860 MHz?

a) 440 mV
b) 770 mV
c) 880 mV
d) None of the above

7. What is the total bandwidth allocated for an FM broadcast station in the USA (Including guard bands)?

a) 200 KHz
b) 150 KHz
c) 75 KHz
d) None of the above

8. What type of the transmitters convert input (information) amplitude changes into output carrier frequency changes?

a) FM
b) AM
c) PM
d) None of the above

9. A certain FM transmitter operates on 100.1 MHz. The maximum frequency from the transmitter is 100.175 MHz, and the minimum frequency is 100.025 MHz. What is the deviation?

a) 50 KHz
b) 150 KHz
c) 25 KHz
d) None of the above

10. How many significant PAIRS of sidebands will be produced by an FM transmitter that is producing 1 KHz of deviation with a 1 KHz deviation rate (information frequency)?

a) 2
b) 1
c) 3
d) 4

11. An FM transmitter is operating on 49.860 MHz. Fm = 5 KHz and the deviation is 5 KHz. The unmodulated carrier voltage is 1V. If the output of this transmitter is viewed on a spectrum analyzer, what voltage will be visible at the frequency 49.865 MHz?

a) 440 mV
b) 1000 mV
c) 880 mV
d) None of the above

12. An FM transmitter is producing 20 KHz of deviation with 1Vpk, 10 KHz information signal. What will happen to the deviation if the information signal is changed to 2Vpk?

a) The deviation will increase to 75 KHz
b) The deviation will decrease to 10 KHz
c) The deviation will increase to 40 KHz
d) None of the above

13. An FM transmitter is producing 45 KHz of deviation; the information signal is 5 KHz, 10 Vpk. What will happen to the deviation if the information signal is changed to 5 Vpk?

a) The deviation will become 37 KHz
b) The deviation will become 22.5 KHz
c) The deviation will become 75 KHz
d) None of the above

14. An FM transmitter is operating on 49.860 MHz. Fm = 5 KHz and the deviation is 5 KHz. The unmodulated carrier voltage is 1V. The information voltage is 3Vpk. What MINIMUM information voltage will cause the 49.860 frequency component (as viewed on a spectrum analyzer) to become ZERO?

a) 3.24 Vpk
b) 5.00 Vpk
c) 7.20 Vpk
d) None of the above

15. What is the percentage modulation of an FM broadcast transmitter producing 50 KHz deviation?

a) 67%
b) 50%
c) 25%
d) None of the above

16. Ideally, an FM transmitter converts:

a) Voltage to Frequency
b) Voltage to Current
c) Frequency to Voltage
d) None of the above

17. What type of FM transmitter is pictured above?

a) Armstrong WBFM
b) PLL
c) Crosby Frequency-Locked-Loop
d) None of the above

18. A 5 MHz crystal oscillator is being driven so that it has 1000 Hz of deviation. This signal is fed into an X3 frequency multiplier. What is the resultant output frequency and deviation?

a) Fout=3 MHz, Deviation=2 KHz
b) Fout=5 MHz, Deviation=3 KHz
c) Fout=15 MHz, Deviation=1 KHz
d) None of the above

19. What type of FM transmitter is pictured above?

a) Armstrong WBFM
b) PLL
c) Crosby Frequency-Locked-Loop
d) None of the above

20. Why is it possible to amplify FM signals in a class C amplifier?

a) The maximum power is no more than twice the unmodulated power
b) The power in an FM signal is constant
c) The bandwidth of FM signals quite small, less than 10 KHz
d) None of the above

21. Where is the demodulated signal taken out of a PLL FM demodulator?

a) At the reference oscillator
b) At the VCO output
c) At the loop filter output
d) None of these

22. An FM detector can be ideally viewed as a/an:

a) Frequency to voltage converter
b) Voltage to frequency converter
c) Amplitude to current converter
d) None of the above

23. Which of these FM detectors rely on a closed loop that "follows" the instantaneous frequency changes of the incoming carrier signal?

a) Quadrature
b) Foster-Seeley
c) Ratio
d) PLL
e) None of these

24. What is the name of the amplifier that operates at a constant frequency in an FM superheterodyne receiver?

a) The audio amplifier
b) The RF amplifier
c) The IF amplifier
d) None of these

25. Why is AFC generally unnecessary in modern FM receivers?

a) Tolerances of LC components have become very small
b) Synthesized local oscillators are used
c) High power amplifiers are used in the audio sections
d) None of the above

End of FREQUENCY MAGIC test!

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