射频工程师面试宝典.docx
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射频工程师面试宝典.docx
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射频工程师面试宝典
RF Quiz
1. What is the impedance of free space?
377Ohm
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
Theformulaforcascadednoisefigureis:
NF(Mstages) =10*log[nf1 +(nf2-1)/(gain1)+(nf3-1)/(gain1*gain2)+...+(nfM-1)/(gain1*gain2*...*gainM-1)];
whereeach"nf"and"gain"valueisexpressedasaratioratherthanindB,andMisthetotalnumberofstages.
Thenoisefigureofanattenuatorisequaltoitsinsertionloss(10dBinthiscase).Notethatpertheequationthatthenoisefigureofthefirstelementinthechainisnotmodifiedbythegainofprecedingstages-asarethesubsequentstages'noisefigures.Therefore,anynoisefigureaddedtothefrontendaddsdirectlytotheoverallsystemnoisefigure-inthiscaseanincreaseof10dB.
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 singlesided phase noise of the 2.8 GHz oscillator at 1 kHz offset?
a) -48.6 dBc
b) -100 dBc
c) -51.1 dBc
Whenanoscillator(2.8GHzinthiscase)isphase-locked(PLO)toareferencesource(10MHzinthiscase),thephasenoiseisincreasedinamplitudebyanamountequalto20*log(fPLO/fRef)+2.5dB,wheretheadditional2.5dB(ruleofthumb)isduetophasenoiseaddedbythephaselockingcircuitry.ThisexplainswhyanextremelylowphasenoisereferenceoscillatorisrequiredwhenbeingusedwithamicrowavefrequencyPLO.
-100dBc+[20*log(2800/10)+2.5]dB=-48.6dBc
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
Commonnamesforeachofthefour2-port
S-parametersare:
S11:
inputreturnloss
S21:
forwardgain
S12:
reverseisolation(orreversegain)
S22:
outputreturnloss
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)
VSWR(max)=[VSWR1*VSWR2]:
1
VSWR(min)=[VSWR1/VSWR2]:
1,whereVSWR1>VSWR2.
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
Adirectionalcouplerischaracterizedbyfivemainparametersasfollows:
1. Frequencybandofoperation.
2. PowercouplingexpressedasdBdownfromtheinputpowerlevel.
3. Isolationofthecoupledportfromtheoutputport(essentiallycouplingfactorfromtheoutputporttothecoupledport)
4. Directivity,which ismathematicallythedifferencebetweenthemagnitudesoftheisolationandthecoupling.Ifthecouplerinthiscasehad0dBmsignalsappliedtoboththeinputandoutputports,thecoupledportwouldsee-15dBmfromtheinputportand-40dBfromtheoutputport,hence,anisolationof25dB.
Coupling=40dB-25dB=15dB
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
Spectralinversionoccurswhenhighfrequencieswithintheinputsignalbandwidtharetranslatedtolowfrequenciesintheoutputbandwidth,andviceversa.Sinceadownconversionisbeingperformed,thelowersidebandofthemixingprocessisextracted,hencethedifferencebetweentheLOfrequencyandtheRFfrequencyisdesired.Considerthefollowingparametersandhowspectralinversionoccurs.
RFinputfrequencyband:
fc=1250MHz,BW=100MHz(1200-1300MHz).
LOfrequency:
1600MHz.
IFoutputfrequencyband:
fc=350MHz,BW=100MHz(300-400MHz).
WhenthelowerfrequencyoftheinputbandissubtractedfromtheLOfrequency(1600MHz-1200MHz=400MHz)alargerfrequencyisobtainedthanwhenthehigherfrequencyoftheinputbandissubtractedfromtheLOfrequency(1600MHz-1300MHz=300MHz).Thismeansthattheoutputspectrumisthemirrorimageoftheinputspectrum.
Howtoavoidspectralinversion?
Alwaysusealow-sideLO(LOfrequencybelowRFinputfrequencyband)formixing,orensurethatanevennumberofspectralinversionsareperformedintheconverter(i.e.,twostagesofconversionwithhigh-sideLO's).
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
Theinputfilterbandwidthdeterminestheamountofpowerthatwillbepresentatthedetectorcircuitry.Sincethedetectorperformsapowerintegrationfunction,itsumsalloftheincidentpoweracrosstheband.Decreasingthebandwidthbyafactorof100(twodecades)allowsoneone-hundredthoftheamountofpowertoreachthedetector,whichintermofdecibelsis:
10*log(1/100)=-20dB.
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
Anideal12-bitADCcanassume212 (4,096)uniquevoltagelevels.Sincepowerisproportionaltothesquareofthevoltage,themaximumpowersamplevalueis40962 (16,777,216)timestheminimumpowersamplevalue.Thereforethedynamicrangeis10*log(16,777,216)=72.25dB.
Aruleofthumbis6dBperbit.
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
VSWRisrelatedtoreturnloss(RL)accordingtoVSWR=[10^(RL/20)+1]/[10^(RL/20)-1].ItfollowsthatincreasingthereturnlosswillresultinalowerVSWR.TheRLofa2.00:
1VSWRis9.542dB.Addthe10dBattenuatorforatotalRLof2*10dB+9.542dB=29.542dB.ConvertbacktoVSWRusingthegivenformulaforavalueof1.07:
1.
Whyaddtwicetheattenuatorvaluetothereturnloss?
Returnlossisthetotaldecreaseinsignalstrengthinpassingthroughtheattenuatorandbeingreflectedbackthroughtheattenuator.Hence,thesignalisdecreasedbytwicetheattenuatorvalue.
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
Thermalnoisepowerdensityisgovernedbytheequation10*log(k*T*B*1000)dBm,wherekistheBoltzmannconstant.TisthetemperatureindegreesKelvin,andBisthebandwidthinHertz.Multiplicationby1000istoconvertwattstomilliwatts.Aruleofthumbfortemperaturesnear15°Cistobeginwithathermalnoisedensityof-174dBm/Hz,andscaleaccordingly(add10dBperdecadeofincreasedbandwidth).
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, 3rdorder intercept point of the system?
a) +40 dBm
b) +25 dBm
c) +20 dBm
2-tone,3rd-orderintermodproductsincrease3dBinpowerforevery1dBincreaseintonesthatproducethem.Thatmeanstheintermodsincreaseinpoweratarateof2dBper1dBrelativetothetonepower.The2-tone,3rd-orderinterceptpointisdefinedasthetheoreticalpointwherethetwooriginaltonesandthetwo3-rd-orderproductswouldhaveequalpower(notpossibleinrealsystemsduetosaturationlimits).
Ifthetwooriginaltoneshaveapowerof+10dBmandthe3rd-orderproductshaveapowerof-20dBm,thentheinterceptpointwillbeat+10dBm+[(+10)-(-20)]/2dB=+10dBm+15dB=+25dBm.
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
Duetothephysicalconstructionofthequadraturecoupler,aslongasthetwodevicesbetweenthecouplersexhibitidenticalimpedancestheinputandoutputimpedanceswillexhibittheintrinsiccouplerimpedance.Forexample,ifmatchedtransistorswithinputimpedancesof12-j5Ωareconnectedbetweentoquadraturecouplersthathaveanintrinsicimpedanceof50+j0Ω,thena50+j0Ωimpedancewouldbeexhibitedatthecircuitinput(similarfortheoutput).
2. Why is there a frequency term in the equation for free-space path loss?
a) There is no frequency term
b) Atmospheric absorptio
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