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1. 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

2. 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

3. Which part of a PLL converts voltage changes into frequency changes?

a) Phase Detector
b) Low Pass Filter
c) VCO
d) None of the above

4. How can a technician determine that a PLL is in lock?

a) Check that Vcontrol = Vcc/2
b) Check that Fvco = Fref
c) Check that Fvco = Fref / 2
d) None of the above

5. 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

6. What is the approximate (CARSON'S RULE) bandwidth of an FM transmitter with a deviation of 5 KHz and information frequency of 1 KHz?

a) 10 KHz
b) 2 KHz
c) 15 KHz
d) None of the above

7. What is the maximum deviation permitted for FM broadcasts in the USA?

a) 50 KHz
b) 150 KHz
c) 25 KHz
d) 75 KHz

8. An FM transmitter is producing 45 KHz of devation; the information signal is 5 KHz, 10 Vpk. What is the modulation index?

a) 9.0
b) 4.5
c) 0.6
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. What is the purpose of guard bands?

a) To reduce distortion on received signals
b) To limit the transmitting distance to line of sight
c) To prevent adjacent channel interference
d) None of the above

11. What deviation corresponds to 100% modulation for FM broadcasts in the USA?

a) 50 KHz
b) 150 KHz
c) 25 KHz
d) 75 KHz

12. A certain FM transmitter is operating on 29.600 MHz; the unmodulated carrier voltage Vc = 50 V. How much bandwidth will this transmitter occupy if Fm = 1 KHz and the deviation is 1 KHz? (Use a Bessel table, not Carson's rule).

a) 2 KHz
b) 6 KHz
c) 8 KHz
d) 16 KHz
e) None of the above

13. A certain FM transmitter is operating on 29.600 MHz; the unmodulated carrier voltage Vc = 50 V. The antenna (load) resistance is 50 Ohms. The maximum permitted deviation is 5 KHz. What power will be delivered to the antenna at 100% modulation?

a) 25 Watts
b) 75 Watts
c) 50 Watts
d) 100 Watts

14. 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

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. 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

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. If the frequency at TP "A" in the figure above is 95.300 MHz, what is the MOST LIKELY frequency at TP "D"?

a) 95.300 MHz
b) 93.300 MHz
c) 91.900 MHz
d) None of the above

21. 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

22. 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

23. 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

24. What stage in an FM receiver rejects amplitude disturbances?

a) Detector
b) Limiter
c) Mixer
d) Preselector

25. Which FM detector uses an exclusive-OR gate (phase detector) and a single LC resonant frequency in order to detect frequency changes?

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

End of FREQUENCY MAGIC test!

Grade Exam!