Chapter 2: Radio-Frequency Circuits

Transcription

1 Chapter 2: Radio-Frequency Circuits MULTIPLE CHOICE 1. The time it takes a charge carrier to cross from the emitter to the collector is called: a. base time c. charge time b. transit time d. Miller time B 2. A real capacitor actually contains: a. capacitance and resistance only c. capacitance, inductance, and resistance b. capacitance and inductance only d. reactance only C 3. Bypass capacitors are used to: a. remove RF from non-rf circuits c. neutralize amplifiers b. couple RF around an amplifier d. reduce the Miller effect A 4. A resonant circuit is: a. a simple form of bandpass filter c. both a and b b. used in narrowband RF amplifiers d. none of the above C 5. Loading down a tuned-circuit amplifier will: a. raise the Q of the tuned circuit c. "multiply" the Q b. lower the Q of the tuned circuit d. have no effect on Q B 6. The "Miller Effect" can: a. cause an amplifier to oscillate c. reduce the bandwidth of an amplifier b. cause an amplifier to lose gain d. all of the above D 7. The Miller Effect can be avoided by: a. using a common-emitter amplifier c. increasing the Q of the tuned circuit b. using a common-base amplifier d. it cannot be avoided B 8. In a BJT, the Miller Effect is due to: a. inductance of collector lead c. base-to-emitter capacitance b. collector-to-emitter capacitance d. base-to-collector capacitance D

2 9. In RF amplifiers, impedance matching is usually done with: a. RC coupling c. direct coupling b. transformer coupling d. lumped reactance 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 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 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 C 13. The conditions for sinusoidal oscillation from an amplifier are called: a. the loop-gain criteria c. the Bode criteria b. the Hartley criteria d. the Barkhausen criteria D 14. The Hartley oscillator uses: a. a tapped inductor c. an RC time constant b. a two-capacitor divider d. a piezoelectric crystal A 15. The Colpitts VFO uses: a. a tapped inductor c. an RC time constant b. a two-capacitor divider d. a piezoelectric crystal B 16. The Clapp oscillator is: a. a modified Hartley oscillator c. a type of crystal-controlled oscillator b. a modified Colpitts oscillator d. only built with FETs B

3 17. A varactor is: a. a voltage-controlled capacitor c. used in tuner circuits b. a diode d. all of the above 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 D 19. If two signals, V a = sin(ω a t) and V b = sin(ω b t), are fed to a mixer, the output: a. will contain ω 1 = ω a + ω b and ω 2 = ω a ω b b. will contain ω 1 = ω a / ω b and ω 2 = ω b / ω a c. will contain ω = (ω a + ω b ) / 2 d. none of the above 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 C 21. "VFO" stands for: a. Voltage-Fed Oscillator c. Varactor-Frequency Oscillator b. Variable-Frequency Oscillator d. Voltage-Feedback Oscillator 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 A COMPLETION 1. Generally, conductor lengths in RF circuits should be. short 2. At UHF frequencies and above, elements must be considered as instead of as being "lumped".

4 distributed 3. When one side of a double-sided pc board is used for ground, it is called a. ground-plane 4. Interactions between parts of an RF circuit can be reduced by using between them. shielding 5. In high-frequency RF circuits, the placement of wires and can be critical. components 6. A circuit is used to remove RF from the DC voltage bus. decoupling 7. A capacitor is used to short unwanted RF to ground. bypass 8. The bandwidth of a tuned-circuit amplifier depends on the of the tuned circuit. Q 9. A value of or more for Q is required for the approximate tuned circuit equations to be valid In a class C RF amplifier, the extracts one frequency from all the harmonics contained in the device current (e.g. collector current). tuned circuit 11. Using additional feedback to compensate for "stray" feedback is called. neutralization 12. A Colpitts oscillator uses a voltage divider to provide feedback. capacitive 13. Electrically, a piezoelectric crystal has both a and a resonant frequency. series, parallel

5 14. To produce sum and difference frequencies, a mixer must be a non- circuit. linear 15. At some bias point, a diode or a transistor can act as a -law mixer. square SHORT ANSWER 1. What inductance would you use with a 47-pF capacitor to make a tuned circuit for 10 MHz? 5.4 µh 2. What value of Q is required for a 10-MHz tuned circuit to have a bandwidth of 100 khz? 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? What is the advantage of a Clapp oscillator compared to a Colpitts oscillator? 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? 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? 50 khz, 0.05% 7. Two sinusoidal signals, V 1 and V 2, are fed into an ideal balanced mixer. V 1 is a 20-MHz signal; V 2 is a 5- MHz signal. What frequencies would you expect at the output of the mixer? 15 MHz and 25 MHz

6 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? 12