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射频知识测验(进阶),附答案和分析

2006-9-29 12:00 3910 0 1 分类: 模拟




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RF Quiz #1


1. What is the impedance of free space?


 


2. What happens to the noise figure of a receiver when a 10 dB attenuator is added at the input?


a) Noise figure increases by 10 dB
b) Noise figure decreases by 10 dB
c) Noise figure doesn't change


3. An RF system has a linear throughput gain of +10 dB and an output 3rd-order intercept point (OIP3) of +30 dBm. What is the input 3rd-order intercept point (IIP3)?


a) +20 dBm
b) +40 dBm
c) +30 dBm


4. Which filter type has the greatest selectivity for a given order (i.e., N=5)?


a) Bessel
b) Chebychev (ripple=0.1 dB)
c) Butterworth


5. Which mixer spurious product is a 5th-order product?


a) 1*LO + 5*RF
b) 6*LO - 1*RF
c) 3*LO - 2*IF


6. A 2.8 GHz oscillator is phase-locked to a 10 MHz reference oscillator that has a single-sided phase noise of -100 dBc at 1 kHz offset. What is the single-sided phase noise of the 2.8 GHz oscillator at 1 kHz offset?


a) -48.6 dBc
b) -100 dBc
c) -51.1 dBc


7. What is the power of a 2 Vpk-pk sine wave across a 50 ohm load?


a) -20.0 dBm
b) +10.0 dBm
c) +19.0 dBm


8. Which 2-port S-parameter is commonly referred to as "reverse isolation" in an amplifier?


a) S21
b) S22
c) S12


9. What are the minimum and maximum combined VSWR limits at an interface characterized by a 1.25:1 VSWR and a 2.00:1 VSWR?


a) 1.75:1 (min), 2.25:1 (max)
b) 1.60:1 (min), 2.50:1 (max)
c) 0.75:1 (min), 3.25:1 (max)


10. An ideal directional coupler has a directivity of 25 dB and an isolation of 40 dB. What is its coupling value?


a) 65 dB
b) 40 dB
c) 15 dB


 


RF Quiz #2 




1. On a Smith chart, what does a point in the bottom half of the chart represent?


a) An inductive impedance
b) A capacitive impedance
c) Power saturation


2. While we're on the subject of Smith charts, what is the impedance of the point at the far left edge of the center horizontal line?


a) Infinite ohms (open circuit)
b) Zero ohms (short circuit)
c) 50 ohm match


3. A single-conversion downconverter uses a high-side local oscillator (LO) to translate the input radio frequency (RF) to an intermediate frequency (IF). Will spectral inversion occur at IF?


a) Yes, always
b) No, never
c) Sometimes


4. What happens to the noise floor of a spectrum analyzer when the input filter resolution bandwidth is decreased by two decades?


a) 20 dB increase
b) 20 dB decrease
c) 40 dB decrease


5. What is a primary advantage of a quadrature modulator?


a) Low LO power required
b) Four separate outputs
c) Single-sideband output


6. What is meant by dBi as applied to antennas?


a) Isolation in decibels
b) Physical size relative to intrinsic antennas
c) Gain relative to an isotropic radiator


7. What is the power dynamic range of an ideal 12-bit analog-to-digital converter (ADC)?


a) 36.12 dB
b) 120 dB
c) 72.25 dB


8. An ideal 10 dB attenuator is added in front of a load that has a 2.00:1 VSWR. What is the resulting VSWR of the load + attenuator?


a) 1.07:1
b) 2.10:1
c) 12.0:1


9. What is the thermal noise power in a 1 MHz bandwidth when the system temperature is 15 degrees Celsius (assume gain and noise figure are 0 dB)?


a) -114.0 dBm (in a 1 MHz bandwidth)
b) -114.0 dBm
c) -114.0 dBm/Hz


10. Two equal amplitude tones have a power of +10 dBm, and generate a pair of equal amplitude 3rd-order intermodulation products at -20 dBm. What is the 2-tone, 3rd-order intercept point (IP3) of the system?


a) +40 dBm
b) +25 dBm
c) +20 dBm


 


RF  Quiz #3 




1. What is a primary advantage to using 90 degree (quadrature) hybrid couplers in amplifier designs?


a) Wider bandwidth possible
b) Low noise figure
c) Input/output impedance not dependent on devices as long as device impedances are equal


2. Why is there a frequency term in the equation for free-space path loss?


a) There is no frequency term
b) Atmospheric absorption
c) Antenna geometry requires it


3. If an amplifier has a noise temperature of 60K, what is its noise figure for an ambient temperature of 290K?


a) 8.0 dB
b) 80 dB
c) 0.82 dB


4. What is a primary advantage of offset-quadrature-phase-shift-keying (OQPSK) over standard QPSK?


a) Greater data rates possible
b) Greater spectral efficiency
c) More constant envelope power


5. A mixer has the following input frequencies:  RF = 800 MHz, LO = 870 MHz. The desired output frequency is 70 MHz. What is the image frequency?


a) 940 MHz
b) 1670 MHz
c) 140 MHz


6. What is the spurious-free dynamic range of a system with IP3 = +30 dBm and a minimum discernible signal (MDS) level of -90 dBm?


a) 80 dB
b) 120 dB
c) 60 dB


7. A spectrum analyzer displays a component at 10 MHz @ 0 dBm, 30 MHz @ -10 dBm, 50 MHz @ -14 dBm, 70 MHz @ -17 dBm, and all of the other odd harmonics until they disappear into the noise. What was the most likely input signal that caused the spectrum?


a) A 10 MHz square wave (0 Vdc)
b) A 10 MHz triangle wave (0 Vdc)
c) A 10 MHz cosine wave (0 Vdc)


8. On which side of a rectangular waveguide is an E-bend made?


a) The long dimension
b) The short dimension
c) The inside


9. During a network analyzer calibration, why are both a short circuit and an open circuit used?


a) They average to 50 ohms in an RF system
b) To determine the characteristic impedance of the measurement system
c) Both are easy to produce to at high accuracy


10. What is the first harmonic of 1 GHz?


a) 1 GHz
b) 2 GHz
c) 10 GHz


 


RF  Quiz #4




1. Which of the following can cause frequency intermodulation products in a system?


a) Only semiconductor junctions like diodes and transistors (amplifiers)
b) Cable connectors, bolted or riveted antenna panels, isolators and circulators
c) Both a) and b)


2. What is the melting temperature of standard 60/40, tin/lead solder?


a) 100°C (212°F)
b) 250°C (482°F)
c) 186°C (386°F)


3. What is the frequency band for the 900 MHz GSM cellular band?


a) Tx: 880-915 MHz / Rx: 925-960 MHz
b) Tx: 824-849 MHz / Rx: 869-894 MHz
c) There is no 900 MHz GSM band


4. What does GSM stand for?


a) General System for <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />Mobile phones
b) Greater Spectrum for Mobile phones
c) Global System for Mobile Communication


5. What does POTS stand for (in communications)?


a) Plain Old Telephone System
b) PersOnal Telephony System
c) Personal Orbital & Terrestrial  Satellite


6. Which of these pairs of materials in the triboelectric series have the greatest charge transfer potential?


a) Silk & Wool
b) Rubber Balloon & Celluloid
c) Glass & Hard Rubber


7. Along which side of rectangular waveguide is an "E" bend made?


a) Longer side
b) Shorter side
c) "E"ither side


8. What is the lowest modulation index at which an FM carrier is suppressed?


a) 2.40
b)
p (3.1416)
c) Only AM carriers can be suppressed


9. How much current is required through the human body to cause an onset to muscular paralysis during electrocution?


a) Voltage is the culprit during electrocution, not current
b) 21 mA
c) ½ amp


10. At what frequency is electromagnetic energy maximally absorbed due to oxygen in the atmosphere?


a) 22 GHz
b) Only water in the atmosphere absorbs electromagnetic energy
c) 63 GHz


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maxconn 2006-10-16 20:56

1,答案是377欧,自由空间的阻抗是

z0 = sqrt [m0 / e0] = sqrt [4 p * 10-7 * 36 p * 109] = sqrt [144 * 102 * p2] = 120 p = 376.991 (377) W.

2.答案是增加10dB,选择A,按信号系统的噪声计算公式可得,直接在前级增加衰减器会使其噪声直接叠加在系统噪声之上.

3,对于IIP3来说IIP3=OIP3-Gain="30dBm"-10dB="20dBm".选择A

4,切比雪夫滤波器在阶数一定的情况具有最好的选择性.选择B.

5,a) 1*LO + 5*RF  b) 6*LO - 1*RF  c) 3*LO - 2*IF这三个式子中,哪个混频产物是5次谐波产物,这个问题解释用中文我表达不上来,不过关于这个问题有如下英文解释:The order of any product is (±j*LO ±k*IF) simply the sum of the harmonic orders of the two signals that create it. 在这个题目中本振LO的3次谐波和中频IF的二次谐波一起作用产生一个五次谐波产物.而Order = | j | + | k |,由引可见,Order = | 3 | + | -2 | = 5.所以选C.

 

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