LNA - A Low Noise Amplifier For Phase Noise Measurement

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1、ALowNoiseAmplifierforPhaseNoiseMeasurementsCharlesWenzelThephasenoiseoflownoiseoscillatorsandsignalsourcesisusuallydeterminedbymeasuringtheaudionoisevoltageattheoutputofthephasecomparatorinaphase-lockedloop.Thephasecomparatoristypicallyalownoisedoubly-b

2、alancedmixerwithaphaseslopefromafewtenthsofavoltperradiantoafewvoltsperradian.Thelowconversiongainofthistypeofphasedetectoryieldsasignalinthelownanovoltperroot-hertzrangeforverylownoisesourceswhichisalevelbelowthenoisefloorofmostspectrumanalyzers.Suitab

3、lelownoisepreamplifiersarereadilyconstructedfromdiscretecomponentsorfrommodernlow-noiseop-amps.Exceptforthenoisevoltage,theamplifierrequirementsarenotparticularlydemanding.Sincetheoutputimpedanceofthephasedetectorislow,theinputimpedanceofmostamplifierci

4、rcuitsisadequate.Afrequencyresponsefromafewhertzto100kHzisusuallyadequateandtheoutputloadisusuallyahighimpedancespectrumanalyzerandoscilloscope.Thephasedetectoroutputimpedanceisquitelowsothenoisecurrentofordinarybipolartransistorsissufficientlylow.Forex

5、ample,anordinary2N4403transistorexhibitsanoisevoltagebelow1nanovoltat10HzwhenthesourceisatypicalSchottkydiodemixer.Severalop-ampsareavailablewithnoisevoltagesbelow3nanovoltsandafewareavailablewithnoisevoltagebelow1nanovolt.Simpleamplifiersbuiltfromanyof

6、thesepartswillperformwellinmostapplications.Bulk-metal,wirewound,andmetalfilmresistorsexhibitlittleexcessnoiseandshouldbeusedinsteadofcarbonfilmorcarboncompositiontypes.Mostpotentiometersshouldbeavoidedsincefilmandcermettypesarequitenoisy.Althoughtheamp

7、lifierrequirementsareminimal,thereareseveralfeatureswhichmaybeaddedtoenhancethemeasurementsystem.Thephaseslopeofthephasedetectorisoftenmeasuredbyobservingthebeat-noteofthefree-runningoscillators.Mostsimpleamplifiercircuitswilldistortthisbeat-notewhenthe

8、yoverloadsotheymustbedisconnectedwhencheckingthephaseslope.Insomeinstances,thequestionofwhetherreconnectingtheamplifierchangesthephaseslopewillarise.Forexample,thenormallowimpedanceterminationusedattheoutputofthephasedetectormayb