TRUE/FALSE
1. All transmitters produce spurious signals.
ANS: T
Any amplifier will produce harmonic distortion.
ANS: T
3. There is a simple correlation between supply power and output power for transmitters.
ANS: F
Transmitters designed for two-way voice communications must be rated for continuous operation at full power.
ANS: F
5. Baseband spectrum often must be restricted to keep transmitted bandwidth within legal limits.
ANS: T
6. Compression is used to restrict baseband spectrum.
ANS: F
7. The opposite of compression is expansion.
ANS: T
In a transmitter, the ALC circuit prevents overmodulation.
ANS: T
9. In a transmitter, the ALC circuit causes compression.
ANS: T
10. Compression is commonly used in commercial broadcast transmitters.
ANS: T
11. In full-carrier AM, modulation is typically done before the RF power amplifier.
ANS: F
12. If the RF stages are all Class C, then AM must be done with low-level modulation.
ANS: F
13. Commercial broadcast AM stations typically use high-level modulation.
ANS: T
14. Low-level AM modulation requires linear RF amplifiers.
ANS: T
15. For good frequency stability, a crystal oscillator is generally required.
ANS: T
16. A variable-frequency oscillator cannot be crystal-controlled.
ANS: F
17. Vacuum tubes are still used in high-power transmitters, even new ones.
ANS: T
18. In a commercial broadcast AM transmitter, all the power in the sidebands came from the audio amplifier.
ANS: T
19. Typically, some kind of matching circuit is required between a transmitter's output stage and the antenna.
ANS: T
20. For testing purposes, the output of a transmitter should be disconnected from the antenna and left with an open-circuit load.
ANS: F
21. In a transceiver, the same audio circuits are used by both the transmitter and the receiver.
ANS: T
22. In solid-state transmitters, it is common to modulate both the driver and the power RF stage.
ANS: T
23. Mixing and amplitude modulation are essentially the same process.
ANS: T
24. DSBSC is never done at low level.
ANS: F
25. To generate SSB from DSBSC, a mechanical filter can be used.
ANS: T
26. To generate SSB from DSBSC, a crystal filter can be used.
ANS: T
27. It is possible that the LSB and the USB can become swapped during a mixing process.
ANS: T
28. The RF power amplifiers in a SSB transmitter are Class C for better efficiency.
ANS: F
29. In FM, a frequency multiplier will also multiply the deviation.
ANS: T
30. Frequency doublers are usually Class B.
ANS: F
31. Mixing can lower a carrier frequency as well as raise it.
ANS: T
32. Indirect FM uses a phase modulator.
ANS: T
33. Even low-power transmitters can cause an RF burn to the skin.
ANS: T
MULTIPLE CHOICE
1. The ability to change operating frequency rapidly without a lot of retuning is called:
a. | agility | c. | VFO |
b. | expansion | d. | spread-spectrum |
ANS: A
2. The difference between the DC power into a transmitter and the RF power coming out:
a. | is a measure of efficiency | c. | may require water cooling |
b. | heats the transmitter | d. | all of the above |
ANS: D
3. Baseband compression produces:
a. | a smaller range of frequencies from low to high |
b. | a smaller range of amplitude from soft to loud |
c. | a smaller number of signals |
d. | none of the above |
ANS: B
4. ALC stands for:
a. | Amplitude Level Control | c. | Accurate Level Control |
b. | Automatic Level Control | d. | none of the above |
ANS: B
5. In an AM transmitter, ALC is used to:
a. | keep the modulation close to 100% | c. | maximize transmitted power |
b. | keep the modulation below 100% | d. | all of the above |
ANS: D
6. With high-level AM:
a. | all RF amplifiers can be nonlinear | c. | minimum RF power is required |
b. | minimum modulation power is required | d. | all of the above |
ANS: A
7. With high-level AM:
a. | the RF amplifiers are typically Class A | c. | the RF amplifiers are typically Class C |
b. | the RF amplifiers are typically Class B | d. | the RF amplifiers are typically Class AB |
ANS: C
8. With low-level AM:
a. | the RF amplifiers must be Class A | c. | the RF amplifiers must be linear |
b. | the RF amplifiers must be Class B | d. | the RF amplifiers must be low-power |
ANS: C
9. Power amplifiers must be linear for any signal that:
a. | is complex | c. | has variable frequency |
b. | has variable amplitude | d. | all of the above |
ANS: B
10. In high-level AM, "high-level" refers to:
a. | the power level of the carrier | c. | the power level of the final RF amplifier |
b. | the power level of the modulation | d. | none of the above |
ANS: D
11. In high-level AM, the power in the sidebands comes from:
a. | the modulating amplifier | c. | the driver stage |
b. | the RF amplifier | d. | the carrier |
ANS: A
12. In an AM transmitter with 100% modulation, the voltage of the final RF stage will be:
a. | approximately half the DC supply voltage |
b. | approximately twice the DC supply voltage |
c. | approximately four times the DC supply voltage |
d. | none of the above |
ANS: C
13. Practical transmitters are usually designed to drive a load impedance of:
a. | 50 ohms resistive | c. | 300 ohms resistive |
b. | 75 ohms resistive | d. | 600 ohms resistive |
ANS: A
14. Which of the following can be used for impedance matching?
a. | pi network | c. | both a and b |
b. | T network | d. | a bridge circuit |
ANS: C
15. When a transmitter is connected to a resistor instead of an antenna, the resistor is called:
a. | a heavy load | c. | a temporary load |
b. | a dummy load | d. | a test load |
ANS: B
16. When a transmitter is connected to a resistor instead of an antenna, the resistor must be:
a. | wire-wound | c. | 1% tolerance or better |
b. | noninductive | d. | all of the above |
ANS: B
17. A Class D amplifier is:
a. | very efficient | c. | essentially pulse-duration modulation |
b. | essentially pulse-width modulation | d. | all of the above |
ANS: D
18. To generate a SSB signal:
a. | start with full-carrier AM | c. | start with a quadrature signal |
b. | start with DSBSC | d. | all of the above |
ANS: B
19. The carrier is suppressed in:
a. | a balanced modulator | c. | a frequency multiplier |
b. | a mixer | d. | none of the above |
ANS: A
20. To remove one AM sideband and leave the other you could use:
a. | a mechanical filter | c. | both a and b |
b. | a crystal filter | d. | none of the above |
ANS: C
21. A direct FM modulator:
a. | varies the frequency of the carrier oscillator |
b. | integrates the modulating signal |
c. | both a and b |
d. | none of the above |
ANS: A
22. An indirect FM modulator:
a. | requires a varactor in the carrier oscillator |
b. | varies the phase of the carrier oscillator |
c. | both a and b |
d. | none of the above |
ANS: B
23. AFC stands for:
a. | Amplitude to Frequency Conversion | c. | Automatic Frequency Control |
b. | Automatic Frequency Centering | d. | Audio Frequency Control |
ANS: C
24. Frequency multipliers are:
a. | essentially balanced modulators | c. | essentially mixers |
b. | essentially Class C amplifiers | d. | none of the above |
ANS: B
25. With mixing:
a. | the carrier frequency can be raised |
b. | the carrier frequency can be lowered |
c. | the carrier frequency can be changed to any required value |
d. | the deviation is altered |
ANS: C
COMPLETION
1. The accuracy and stability of a transmitter frequency is fixed by the ____________________ oscillator.
ANS: carrier
2. In the USA, the ____________________ sets requirements for accuracy and stability of a transmitter's frequency.
ANS: FCC
3. In Canada, _________________________ sets requirements for accuracy and stability of a transmitter's frequency.
ANS: Industry Canada
4. Frequency ____________________ is the ability of a transmitter to change frequency without a lot of retuning.
ANS: agility
5. Power output of SSB transmitters is rated by ____________________.
ANS: PEP
6. Reducing the dynamic range of a modulating signal is called ____________________.
ANS: compression
7. The opposite of compression is called ____________________.
ANS: expansion
8. ALC is a form of ____________________.
ANS: compression
9. High-level modulation allows the RF amplifiers to operate more ____________________.
ANS: efficiently
10. Low-level modulation requires the RF amplifiers to be ____________________.
ANS: linear
11. To isolate the oscillator from load changes, a ____________________ stage is used.
ANS: buffer
12. The peak collector voltage in a Class C RF amplifier is ____________________ than the DC supply voltage.
ANS: higher
13. Most practical transmitters are designed to operate into a ____________________-ohm load.
ANS: 50
14. Transmitters built with transistor RF amplifiers often use a ____________________ network for impedance matching.
ANS: T
15. Matching networks also act as filters to help reduce ____________________ levels.
ANS: harmonic
16. Severe impedance ____________________ can destroy a transmitter's output stage.
ANS: mismatch
17. Transceivers combine a transmitter and a ____________________ into one "box".
ANS: receiver
18. To allow a high modulation percentage, it is common to modulate the ____________________ as well as the power amplifier in transistor modulators.
ANS: driver
19. Pulse-width modulation is the same as pulse-____________________ modulation.
ANS: duration
20. Switching amplifiers are sometimes called Class ____________________ amplifiers.
ANS: D
21. Because the sideband filter in a SSB transmitter is fixed, ____________________ is used to operate at more than one frequency.
ANS: mixing
22. To generate a SSB signal, it is common to start with a ____________________ signal.
ANS: DSBSC
23. Indirect FM is derived from ____________________ modulation.
ANS: phase
24. Using a varactor to generate FM is an example of a ____________________ modulator.
ANS: reactance
25. The modern way to make a stable VFO is to make it part of a ____________________ loop.
ANS: phase-locked
SHORT ANSWER
1. If a 50-MHz oscillator is accurate to within 0.001%, what is the range of possible frequencies?
ANS:
50 MHz ± 500 hertz
2. What is the efficiency of a 100-watt mobile transmitter if it draws 11 amps from a 12-volt car battery?
ANS:
75.8%
3. The power amplifier of an AM transmitter draws 100 watts from the power supply with no modulation. Assuming high-level modulation, how much power does the modulation amplifier deliver for 100% modulation?
ANS:
50 watts
4. If the final RF amplifier of an AM transmitter is powered by 100 volts DC, what is the maximum collector voltage at 100% modulation?
ANS:
400 volts
5. Suppose the output of a balanced modulator has a center frequency of 10 MHz. The audio modulation frequency range is 1 kHz to 10 kHz. To pass the USB, what should be the center frequency of an ideal crystal filter?
ANS:
10.005 MHz
6. Suppose you have generated a USB SSB signal with a nominal carrier frequency of 10 MHz. What is the minimum frequency the SSB signal can be mixed with so that the output signal has a nominal carrier frequency of 50 MHz?
ANS:
40 MHz
7. Suppose you have an FM modulator that puts out 1 MHz carrier with a 100-hertz deviation. If frequency multiplication is used to increase the deviation to 400 hertz, what will be the new carrier frequency?
ANS:
4 MHz
8. Suppose you had an FM signal with a carrier of 10 MHz and a deviation of 10 kHz. Explain how you could use it to get an FM signal at 100 MHz with a deviation of 20 kHz.
ANS:
First, put the signal through a frequency doubler to get a 20-MHz carrier with a 20-kHz deviation. Then mix that signal with an 80-MHz carrier to generate a 100-MHz carrier with 20-kHz deviation.
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