This is the Multiple Choice Questions in Chapter 2: Radio-Frequency Circuits from the book Electronic Communication Systems by Roy Blake
Start Practice Exam Test Questions
Choose the letter of the best answer in each questions.
MULTIPLE CHOICE
1. The time it takes a charge carrier to cross from the emitter to the collector is called:
- a. base time
- b. transit time
- c. charge time
- d. Miller time
ANS: B
2. A real capacitor actually contains:
- a. capacitance and resistance only
- b. capacitance and inductance only
- c. capacitance, inductance, and resistance
- d. reactance only
ANS: C
3. Bypass capacitors are used to:
- a. remove RF from non-RF circuits
- b. couple RF around an amplifier
- c. neutralize amplifiers
- d. reduce the Miller effect
ANS: A
4. A resonant circuit is:
- a. a simple form of bandpass filter
- b. used in narrowband RF amplifiers
- c. both a and b
- d. none of the above
ANS: C
5. Loading down a tuned-circuit amplifier will:
- a. raise the Q of the tuned circuit
- b. lower the Q of the tuned circuit
- c. "multiply" the Q
- d. have no effect on Q
ANS: B
6. The "Miller Effect" can:
- a. cause an amplifier to oscillate
- b. cause an amplifier to lose gain
- c. reduce the bandwidth of an amplifier
- d. all of the above
ANS: D
7. The Miller Effect can be avoided by:
- a. using a common-emitter amplifier
- b. using a common-base amplifier
- c. increasing the Q of the tuned circuit
- d. it cannot be avoided
ANS: B
8. In a BJT, the Miller Effect is due to:
- a. inductance of collector lead
- b. collector-to-emitter capacitance d
- c. base-to-emitter capacitance
- d. base-to-collector capacitance
ANS: D
9. In RF amplifiers, impedance matching is usually done with:
- a. RC coupling
- b. transformer coupling
- c. direct coupling
- d. lumped reactance
ANS: B
10. Neutralization cancels unwanted feedback by:
- a. adding feedback out of phase with the unwanted feedback
- b. bypassing the feedback to the "neutral" or ground plane
- c. decoupling it
- d. none of the above
ANS: A
11. For a "frequency multiplier" to work, it requires:
- a. a nonlinear circuit
- b. a linear amplifier
- c. a signal containing harmonics
- d. an input signal that is an integer multiple of the desired frequency
ANS: A
12. A sinusoidal oscillation from an amplifier requires:
- a. loop gain equal to unity
- b. phase shift around loop equal to 0 degrees
- c. both a and b, but at just one frequency
- d. none of the above
ANS: C
13. The conditions for sinusoidal oscillation from an amplifier are called:
- a. the loop-gain criteria
- b. the Hartley criteria
- c. the Bode criteria
- d. the Barkhausen criteria
ANS: D
14. The Hartley oscillator uses:
- a. a tapped inductor
- b. a two-capacitor divider
- c. an RC time constant
- d. a piezoelectric crystal
ANS: A
15. The Colpitts VFO uses:
- a. a tapped inductor
- b. a two-capacitor divider
- c. an RC time constant
- d. a piezoelectric crystal
ANS: B
16. The Clapp oscillator is:
- a. a modified Hartley oscillator
- b. a modified Colpitts oscillator
- c. a type of crystal-controlled oscillator
- d. only built with FETs
ANS: B
17. A varactor is:
- a. a voltage-controlled capacitor
- b. a diode
- c. used in tuner circuits
- d. all of the above
ANS: D
18. Crystal-Controlled oscillators are:
- a. used for a precise frequency
- b. used for very low frequency drift (parts per million)
- c. made by grinding quartz to exact dimensions
- d. all of the above
ANS: D
19. If two signals, Va = sin(ωat) and Vb = sin(ωbt), are fed to a mixer, the output:
- a. will contain ω1 = ωa + ωb and ω2 = ωa – ωb
- b. will contain ω1 = ωa / ωb and ω2 = ωb / ωa
- c. will contain ω1 = (ωa + ωb ) / 2
- d. none of the above
ANS: A
20. In a balanced mixer, the output:
- a. contains equal (balanced) amounts of all input frequencies
- b. contains the input frequencies
- c. does not contain the input frequencies
- d. is a linear mixture of the input signals
ANS: C
21. "VFO" stands for:
- a. Voltage-Fed Oscillator
- b. Variable-Frequency Oscillator
- c. Varactor-Frequency Oscillator
- d. Voltage-Feedback Oscillator
ANS: B
22. A "frequency synthesizer" is:
- a. a VCO phase-locked to a reference frequency
- b. a VFO with selectable crystals to change frequency
- c. a fixed-frequency RF generator
- d. same as a mixer
ANS: A
COMPLETION
1. Generally, conductor lengths in RF circuits should be ____________________.
ANS: short
2. At UHF frequencies and above, elements must be considered as ____________________ instead of as being "lumped".
ANS: distributed
3. When one side of a double-sided pc board is used for ground, it is called a ____________________.
ANS: ground-plane
4. Interactions between parts of an RF circuit can be reduced by using ____________________ between them.
ANS: shielding
5. In high-frequency RF circuits, the placement of wires and ____________________ can be critical.
ANS: components
6. A ____________________ circuit is used to remove RF from the DC voltage bus.
ANS: decoupling
7. A ____________________ capacitor is used to short unwanted RF to ground.
ANS: bypass
8. The bandwidth of a tuned-circuit amplifier depends on the ____________________ of the tuned circuit.
ANS: Q
9. A value of ____________________ or more for Q is required for the approximate tuned circuit equations to be valid.
ANS: 10
10. In a class C RF amplifier, the ____________________ extracts one frequency from all the harmonics contained in the device current (e.g. collector current).
ANS: tuned circuit
11. Using additional feedback to compensate for "stray" feedback is called ____________________.
ANS: neutralization
12. A Colpitts oscillator uses a ____________________ voltage divider to provide feedback.
ANS: capacitive
13. Electrically, a piezoelectric crystal has both a ____________________ and a ____________________ resonant frequency.
ANS: series, parallel
14. To produce sum and difference frequencies, a mixer must be a non-____________________ circuit.
ANS: linear
15. At some bias point, a diode or a transistor can act as a ____________________-law mixer.
ANS: square
SHORT ANSWER
1. What inductance would you use with a 47-pF capacitor to make a tuned circuit for 10 MHz?
ANS:
5.4 µH
2. What value of Q is required for a 10-MHz tuned circuit to have a bandwidth of 100 kHz?
ANS:
100
3. A tuned-circuit amplifier with a gain of 10 is being used to make an oscillator. What should be the value
of the feedback ratio to satisfy the Barkhausen criteria?
ANS:
0.1
4. What is the advantage of a Clapp oscillator compared to a Colpitts oscillator?
ANS:
It is more stable because it "swamps" the device capacitance with large value capacitors in the feedback divider.
5. If a varactor has a capacitance of 90 pF at zero volts, what will be the capacitance at 4 volts?
ANS:
30 pF
6. An oscillator has a frequency of 100 MHz at 20°C, and a tempco of +10 ppm per degree Celsius. What will be the shift in frequency at 70°C? What percentage is that?
ANS:
50 kHz, 0.05%
7. Two sinusoidal signals, V1 and V2, are fed into an ideal balanced mixer. V1 is a 20-MHz signal; V2 is a 5- MHz signal. What frequencies would you expect at the output of the mixer?
ANS:
15 MHz and 25 MHz
8. Suppose the phase-locked-loop frequency synthesizer of Figure 2.39 has a reference frequency of 1 MHz and a fixed-modulus divider of 10. What should be the value of the programmable divider to get an output frequency of 120 MHz?
ANS:
12
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