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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
