射频工程师面试宝典Word文件下载.docx

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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 

system 

has 

linear 

throughput 

gain 

+10 

and 

an 

output 

3rd-order 

intercept 

point 

（OIP3） 

+30 

dBm. 

input 

（IIP3）?

+20 

dBm

+40 

4. 

Which 

filter 

type 

greatest 

selectivity 

for 

given 

order 

（i.e., 

N=5）?

Bessel

Chebychev 

（ripple=0.1 

dB）

Butterworth

5. 

mixer 

spurious 

product 

5th-order 

product?

1*LO 

+ 

5*RF

6*LO 

- 

1*RF

3*LO 

2*IF

6. 

A 

2.8 

GHz 

oscillator 

phase-locked 

MHz 

reference 

that 

single-sided 

phase 

-100 

dBc 

1 

kHz 

offset. 

singlesided 

offset?

-48.6 

dBc

-51.1 

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. 

power 

2 

Vpk-pk 

sine 

wave 

across 

50 

ohm 

load?

-20.0 

+10.0 

+19.0 

8. 

2-port 

S-parameter 

commonly 

referred 

as 

"

reverse 

isolation"

in 

amplifier?

S21

S22

S12

Commonnamesforeachofthefour2-port

S-parametersare:

S11:

inputreturnloss

S21:

forwardgain

S12:

reverseisolation（orreversegain）

S22:

outputreturnloss

9. 

are 

minimum 

maximum 

combined 

VSWR 

limits 

interface 

characterized 

1.25:

2.00:

VSWR?

1.75:

（min）, 

2.25:

（max）

1.60:

2.50:

0.75:

3.25:

VSWR（max）=[VSWR1*VSWR2]:

1

VSWR（min）=[VSWR1/VSWR2]:

1,whereVSWR1>

VSWR2.

10. 

ideal 

directional 

coupler 

directivity 

25 

isolation 

40 

dB. 

its 

coupling 

value?

65 

15 

Adirectionalcouplerischaracterizedbyfivemainparametersasfollows:

Frequencybandofoperation.

PowercouplingexpressedasdBdownfromtheinputpowerlevel.

Isolationofthecoupledportfromtheoutputport（essentiallycouplingfactorfromtheoutputporttothecoupledport）

Directivity,which 

ismathematicallythedifferencebetweenthemagnitudesoftheisolationandthecoupling.Ifthecouplerinthiscasehad0dBmsignalsappliedtoboththeinputandoutputports,thecoupledportwouldsee-15dBmfromtheinputportand-40dBfromtheoutputport,hence,anisolationof25dB.

Coupling=40dB-25dB=15dB

#2 

On 

Smith 

chart, 

what 

does 

bottom 

half 

chart 

represent?

inductive 

impedance

capacitive 

Power 

saturation

While 

we'

re 

on 

subject 

charts, 

far 

left 

edge 

center 

horizontal 

line?

Infinite 

ohms 

（open 

circuit）

Zero 

（short 

match

single-conversion 

downconverter 

uses 

high-side 

local 

（LO） 

translate 

radio 

frequency 

（RF） 

intermediate 

（IF）. 

Will 

spectral 

inversion 

occur 

IF?

Yes, 

always

No, 

never

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）.

floor 

spectrum 

analyzer 

resolution 

bandwidth 

decreased 

two 

decades?

20 

increase

decrease

Theinputfilterbandwidthdeterminestheamountofpowerthatwillbepresentatthedetectorcircuitry.Sincethedetectorperformsapowerintegrationfunction,itsumsalloftheincidentpoweracrosstheband.Decreasingthebandwidthbyafactorof100（twodecades）allowsoneone-hundredthoftheamountofpowertoreachthedetector,whichintermofdecibelsis:

10*log（1/100）=-20dB.

primary 

advantage 

quadrature 

modulator?

Low 

LO 

required

Four 

separate 

outputs

Single-sideband 

output

meant 

dBi 

applied 

antennas?

Isolation 

decibels

Physical 

size 

relative 

intrinsic 

antennas

Gain 

isotropic 

radiator

dynamic 

range 

12-bit 

analog-to-digital 

converter 

（ADC）?

36.12 

120 

72.25 

Anideal12-bitADCcanassume212 

（4,096）uniquevoltagelevels.Sincepowerisproportionaltothesquareofthevoltage,themaximumpowersamplevalueis40962 

（16,777,216）timestheminimumpowersamplevalue.Thereforethedynamicrangeis10*log（16,777,216）=72.25dB.

Aruleofthumbis6dBperbit.

front 

load 

VSWR. 

resulting 

attenuator?

1.07:

1

2.10:

12.0:

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.

thermal 

temperature 

degrees 

Celsius 

（assume 

0 

dB）?

-114.0 

dBm 

（in 

bandwidth）

dBm/Hz

Thermalnoisepowerdensityisgovernedbytheequation10*log（k*T*B*1000）dBm,wherekistheBoltzmannconstant.TisthetemperatureindegreesKelvin,andBisthebandwidthinHertz.Multiplicationby1000istoconvertwattstomilliwatts.Aruleofthumbfortemperaturesnear15°

Cistobeginwithathermalnoisedensityof-174dBm/Hz,andscaleaccordingly（add10dBperdecadeofincreasedbandwidth）.

Two 

equal 

amplitude 

tones 

have 

dBm, 

generate 

pair 

intermodulation 

products 

-20 

2-tone, 

3rdorder 

system?

+25 

2-tone,3rd-orderintermodproductsincrease3dBinpowerforevery1dBincreaseintonesthatproducethem.Thatmeanstheintermodsincreaseinpoweratarateof2dBper1dBrelativetothetonepower.The2-tone,3rd-orderinterceptpointisdefinedasthetheoreticalpointwherethetwooriginaltonesandthetwo3-rd-orderproductswouldhaveequalpower（notpossibleinrealsystemsduetosaturationlimits）.

Ifthetwooriginaltoneshaveapowerof+10dBmandthe3rd-orderproductshaveapowerof-20dBm,thentheinterceptpointwillbeat+10dBm+[（+10）-（-20）]/2dB=+10dBm+15dB=+25dBm.

#3 

using 

90 

degree 

（quadrature） 

hybrid 

couplers 

amplifier 

designs?

Wider 

possible

figure

Input/output 

not 

dependent 

devices 

long 

device 

impedances 

equal

Duetothephysicalconstructionofthequadraturecoupler,aslongasthetwodevicesbetweenthecouplersexhibitidenticalimpedancestheinputandoutputimpedanceswillexhibittheintrinsiccouplerimpedance.Forexample,ifmatchedtransistorswithinputimpedancesof12-j5Ωareconnectedbetweentoquadraturecouplersthathaveanintrinsicimpedanceof50+j0Ω,thena50+j0Ωimpedancewouldbeexhibitedatthecircuitinput（similarfortheoutput）.

Why 

there 

term 

equation 

free-space 

path 

loss?

There 

no 

term

Atmospheric 

absorptio