BLAKE ELECTRONIC COMMUNICATION SYSTEM Chapter 1: Introduction to Communication Systems TRUE/FALSE PDF

Preview

Summary

BLAKE ELECTRONIC COMMUNICATION SYSTEM 22. The hotter a component gets, the more noise it will generate. T 23. "Shot" noise creates a "noise current" in an electronic device such as a transistor. T Chapter 1: Introduction to Communication Systems 24. "Flicker" noise is w...

Description

BLAKE ELECTRONIC COMMUNICATION SYSTEM

22. The hotter a component gets, the more noise it will generate. T 23. "Shot" noise creates a "noise current" in an electronic device such as a transistor. T

Chapter 1: Introduction to Communication Systems 24. "Flicker" noise is worst at radio frequencies. F TRUE/FALSE

25. Signal-to-noise ratio is more important than noise power. T

1. The first electronic communications device was the telephone. F

26. With cascaded stages in a communication system, the noise from the first stage is the least important. F

2. Transatlantic radio communication started in 1901. T 3. A communication system may or may not include a channel. F

27. Due to the frequency, a radio signal requires a "real-time" analyzer to look at the spectrum. F

4. "Baseband" refers to the basic carrier frequency band. F MULTIPLE CHOICE 5. Putting an information signal onto a carrier is called "modulation". T 6. The carrier frequency is higher than the highest baseband frequency. T

1. The theory of radio waves was originated by: a. Marconi c. Maxwell b. Bell d .Hertz

7. A modulated carrier occupies a band of frequencies. T 8. "Detection" is another term for demodulation. T 9. The amount of information per second that can be sent is independent of bandwidth. F 10. It is possible to combine FDM and TDM in the same system. T 11. For radio signals, longer wavelength means higher frequency. F 12. Noise is easily removed from an analog signal by using filters. F 13. As long as a one can be distinguished from a zero, a digital signal corrupted by noise can be restored to its original form. T 14. Audio signals are in the time-domain, but radio signals are in the frequency domain. F 15. Any periodic AC signal contains an infinite series of harmonic frequencies. T 16. Every term in a Fourier series must be used in calculations. F 17. All noise in a system can be eliminated by using good grounding and shielding. F 18. The sparking brushes of a DC motor can cause radio frequency noise. T 19. Signals with fast rise and fall times can cause radio frequency noise. T 20. Digital systems such as computers are not affected by noise. F 21. Every component in an electronic system generates noise. T

2. The person who sent the first radio signal across the Atlantic Ocean was: a. Marconi c. Maxwell b. Bell d. Hertz 3. The transmission of radio waves was first done by: a. Marconi c. Maxwell b. Bell d. Hertz 4. Radians per second is equal to: a. 2π π×f c. the phase angle b f ÷ 2π d. none of the above 5. A complete communication system must include: a. a transmitter and receiver b. a transmitter, a receiver, and a channel c. a transmitter, a receiver, and a spectrum analyzer d. a multiplexer, a demultiplexer, and a channel 6. The bandwidth required for a modulated carrier depends on: a. the carrier frequency c. the signal-plus-noise to noise ratio b. the signal-to-noise ratio d. the baseband frequency range 7. When two or more signals share a common channel, it is called: a. sub-channeling c. SINAD b. signal switching d. multiplexing 8. TDM stands for: a. Time-Division Multiplexing b. Two-level Digital Modulation

c. Time Domain Measurement d. none of the above

9. FDM stands for: a. Fast Digital Modulation b. Frequency Domain Measurement

c. Frequency-Division Multiplexing d. none of the above

10. The wavelength of a radio signal is: a. equal to f ÷ c c. the distance a wave travels in one period b. equal to c ÷ λ d. how far the signal can travel without distortion 11. Distortion is caused by: a. creation of harmonics of baseband frequencies b. baseband frequencies "mixing" with each other c. shift in phase relationships between baseband frequencies d. all of the above 12. The collection of sinusoidal frequencies present in a modulated carrier is called its: a. frequency-domain representation c. spectrum b. Fourier series d. all of the above 13. The baseband bandwidth for a voice-grade (telephone) signal is: a. approximately 3 kHz c. at least 5 kHz b. 20 Hz to 15,000 Hz d. none of the above 14. Noise in a communication system originates in: a. the sender c. the channel b. the receiver d. all of the above 15. "Man-made" noise can come from: a. equipment that sparks b. temperature

23. SINAD is calculated as: a. signal voltage divided by noise voltage b. signal power divided by noise power c. first add the signal power to the noise power, then divide by noise power d. none of the above 24. Noise Figure is a measure of: a. how much noise is in a communications system b. how much noise is in the channel c. how much noise an amplifier adds to a signal d. signal-to-noise ratio in dB 25. The part, or parts, of a sinusoidal carrier that can be modulated are: a. its amplitude b. its amplitude and frequency c. its amplitude, frequency, and direction d. its amplitude, frequency, and phase angle COMPLETION

c. static d. all of the above

1. The telephone was invented in the year ______.

ANS:

16. Thermal noise is generated in: a. transistors and diodes b. resistors

c. copper wire d. all of the above

17. Shot noise is generated in: a. transistors and diodes b. resistors

c. copper wire d. none of the above

18. The power density of "flicker" noise is: a. the same at all frequencies b. greater at high frequencies

c. greater at low frequencies d. the same as "white" noise

19. So called "1/f" noise is also called: a. random noise c. white noise b. pink noise d. partition noise 20. "Pink" noise has: a. equal power per Hertz b. equal power per octave

22. Signal-to-Noise ratio is calculated as: a. signal voltage divided by noise voltage b. signal power divided by noise power c. first add the signal power to the noise power, then divide by noise power d. none of the above

c. constant power d. none of the above

21. When two noise voltages, V1 and V2, are combined, the total voltage VT is: a. VT = sqrt(V1 x V1 + V2 x V2) c. VT = sqrt(V1 x V2) b. VT = (V1 + V2)/2 d. VT = V1 + V2

1863

2. Radio signals first were sent across the Atlantic in the year _______. ANS: 1901 3. The frequency band used to modulate the carrier is called the ______ band. ANS: base 4. The job of the carrier is to get the information through the _______. ANS: channel 5. The bandwidth of an unmodulated carrier is _______. ANS: zero 6. The 'B' in Hartley's Law stands for _______. ANS: bandwidth 7. In _______, you split the bandwidth of a channel into sub-channels to carry multiple signals. ANS: FDM 8. In _______, multiple signal streams take turns using the channel. ANS: TDM

9. VHF stands for the _______ frequency band. ANS: very high

5. A microwave receiver has a noise temperature of 145 K. Find its noise figure. ANS: 1.5

10. The more information per second you send, the ________ the bandwidth required. ANS: greater, larger, wider

6. Suppose there is 30 µV from one noise source that is combined with 40 µV from another noise source. Calculate the total noise voltage. ANS: 50 µV

11. The VHF band starts at _______ MHz. ANS: 30

7. If you have 100 mV of signal and 10 mV of noise, both across the same 100-ohm load, what is the signal-to-noise ratio in dB? ANS: 20 dB

12. The UHF band starts at ______ MHz. ANS: 300 13. A radio signal's ________ is the distance it travels in one cycle of the carrier. ANS: wavelength 14. In free space, radio signals travel at approximately _________ meters per second. ANS: 300 million 15. The equipment used to show signals in the frequency domain is the ________. ANS: spectrum analyzer 16. Mathematically, a spectrum is represented by a _______ series. ANS: Fourier 17. Disabling a receiver during a burst of atmospheric noise is called ________. ANS: noise blanking or blanking

8. The input to an amplifier has a signal-to-noise ratio of 100 dB and an output signal-to-noise ratio of 80 dB. Find NF, both in dB and as a ratio. ANS: 20 dB, NF = 100 9. Two cascaded amplifiers each have a noise figure of 5 and a gain of 10. Find the total NF for the pair. ANS: 5.4 10. Explain why you could use a diode as a noise source with a spectrum close to that of pure thermal noise. How would you control the amount of noise generated? ANS: When current flows through a diode, it generates shot noise that can be represented as a current source, the output of which is a noise current. The equation for the noise current is very similar to the equation for thermal noise voltage. Since the power in the shot noise is proportional to the diode current, controlling the diode current controls the noise power.

18. For satellite communications, ________ noise can be a serious problem. ANS: solar

Chapter 2: Radio-Frequency Circuits

19. Thermal noise is caused by the random motions of ________ in a conductor. ANS: electrons

TRUE/FALSE 1. In general, components behave the same at 20 MHz as they do at 1 kHz. F

SHORT ANSWER 2. Stray capacitance is important in high-frequency amplifiers. T 1. Name the five elements in a block diagram of a communications system. ANS: Source, Transmitter, Channel, Receiver, Destination 2. Name five types of internal noise. ANS: Thermal, Shot, Partition, 1/f, transit-time

3. Microwave-frequency circuits look much different from circuits designed for 1 MHz. T 4. All electronic devices have both capacitive and inductive properties. T

3. Why is thermal noise called "white noise"? ANS: White light is composed of equal amounts of light at all visible frequencies. Likewise, thermal noise has equal power density over a wide range of frequencies.

5. As frequency increases, capacitive effects decrease. F

4. What is "pink noise"? ANS: Light is pink when it contains more red than it does other colors, and red is at the low end of the visible spectrum. Likewise, pink noise has higher power density at lower frequencies.

7. An "unstable" amplifier is one that oscillates, or is close to oscillating. T

6. At some frequency, a capacitor will self-resonate with the inductance of its leads. T

8. Base-to-emitter capacitance can cause a common-emitter amplifier to oscillate. F 9. "Distributed Constants" refers to a mathematical operation to calculate gain. F

10. "Shielding" prevents RF signals from coupling between components in a system. T 11. A "ground-plane" is a type of shielding. T 12. A "gimmick" is a type of shielding. F 13. Removing any RF signals off the Vcc lines is an example of "decoupling". T 14. Decoupling usually involves a "bypass" capacitor. T 15. Basically, radio-frequency amplifiers can not be distinguished from other amplifiers. F 16. Resonant circuits are common in RF amplifiers. T 17. Increasing the load on a tuned amplifier increases its Q. F 18. In a common-emitter amplifier, the collector-base capacitance "looks" bigger than it is. T 19. The Miller Effect only occurs in common-base amplifiers. F

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

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

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 7. In a BJT, the Miller Effect is due to: a. inductance of collector lead b. collector-to-emitter capacitance

c. base-to-emitter capacitance d. base-to-collector capacitance

8. 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

20. The Miller Effect can reduce the bandwidth of an amplifier. T 21. The Miller Effect can be reduced using neutralization. T

9. In RF amplifiers, impedance matching is usually done with: a. RC coupling c. direct coupling b. transformer coupling d. lumped reactance

22. A piezoelectric crystal behaves like a very low-Q tuned circuit. F 23. The frequency of a crystal oscillator is much more stable than an LC oscillator. T 24. Mixers must be nonlinear in order to work. T

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

25. A mixer will produce "sum and difference" frequencies. T 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 2. A real capacitor actually contains: a. capacitance and resistance only b. capacitance and inductance only c. capacitance, inductance, and resistance d. reactance only 3. Bypass capacitors are used to: a. remove RF from non-RF circuits b. couple RF around an amplifier

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 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 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

c. neutralize amplifiers d. reduce the Miller effect

14. The Hartley oscillator uses: a. a tapped inductor b. a two-capacitor divider

c. an RC time constant d. a piezoelectric crystal

15. The Colpitts VFO uses: a. a tapped inductor b. a two-capacitor divider

c. an RC time constant d. a piezoelectric crystal

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

17. A varactor is: a. a voltage-controlled capacitor b. a diode

c. used in tuner circuits d. all of the above

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 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 ω = (ωa + ωb ) / 2 d. none of the above 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 21. "VFO" stands for: a. Voltage-Fed Oscillator b. Variable-Frequency Oscillator

4. Interactions between parts of an RF circuit can be reduced by using ________ between them. ANS: shielding 5. A ________ circuit is used to remove RF from the DC voltage bus. ANS: decoupling 6. In high-frequency RF circuits, the placement of wires and _________ can be critical. ANS: components 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

c. Varactor-Frequency Oscillator d. Voltage-Feedback Oscillator

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 COMPLETION

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

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

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%

11. Overmodulation produces "splatter". T 12. Modulation index can be derived from a time-domain view of the AM signal. T 13. Modulation index can be derived from a frequency-domain view of the AM signal. T 14. In AM, the lower baseband frequencies are in the LSB and the higher baseband frequencies are in the USB. F 15. The upper sideband contains the same information as the lower sideband. T 16. In AM, most of the power is in the sidebands. F 17. In plain AM, the job of the carrier is to allow simple demodulation. T 18. The upper sideband has twice the power of the lower sideband. F

7. Two sinusoidal signals, V1 and V2, are fed into an ideal balanced mixer. V1 is a 20MHz 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

19. In AM, only one information signal can be sent on any given frequency. F 20. It is possible to transmit stereo audio signals using AM. T 21. SSBSC is derived from DSBSC. T 22. SSB AM requires twice the bandwidth of plain AM. F 23. SSB AM is much more efficient than plain AM. T

Chapter 3: Amplitude Modulation

24. To see an envelope in SSB AM, at least two modulating audio tones are required. T 25. AM has inherently worse fidelity than FM. F

TRUE/FALSE 1. It is easy to modulate and demodulate a carrier using AM. T

MULTIPLE CHOICE

2. AM makes efficient use of transmitter power. F

1. AM stands for: a. Audio Modulation b. Amplitude Modulation

3. AM signals are affected by relatively low levels of electrical noise. T 4. The "envelope" of an AM signal resembles the modulating signal. T 5. AM works by varying the carrier power. F 6. Modulation is not a linear process. T 7. In ...