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NIST Technical Note 1337: Characterization of Clocks and Oscillators

NIST Technical Note 1337: Characterization of Clocks and Oscillators

NIST Technical Note 1337: Characterization of Clocks and Oscillators

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254 A. L. LANCE. W. D. SEAL, AND F. LABAARG. AM NOISE IN THE FREQUENCY DOMAINThe spectral density <strong>of</strong> AM fluctuations <strong>of</strong> a signal follows the samegeneral derivation previously given for the spectral density <strong>of</strong> phase f1uctations.Amplitude fluctuations & <strong>of</strong> the signal under test produces voltagefluctuations bA at the output <strong>of</strong> the mixer. Interpretating the mean-squarefluctuations & <strong>and</strong> 1> in spectal density fashion, we obtain S6,(f), the spectraldensity <strong>of</strong> amplitude fluctuations bE: <strong>of</strong> a signal in volts squared per hertz:(48)The term m(j) is the normalized version <strong>of</strong> the amplitude modulation (AM)portion <strong>of</strong> SJrf p( v), with its frequency parameter f referenced to the signal'saverage frequency vo, taken as the origin such that the difference frequencyf equals ~' - ~'o' The range <strong>of</strong> Fourier frequency difference f is from minusVo to plus infinity.Theterm men is defined as the ratio <strong>of</strong>the spectral density <strong>of</strong>one amplitudemodulatedsideb<strong>and</strong> to the total signal power, at Fourier frequency differencef from the signal's average frequency Va, for a single specified signal or device.The dimensionality is per hertz. !f(f) <strong>and</strong> m(f) are similar functions; theformer is a measure <strong>of</strong> phase-modulated (PM) sideb<strong>and</strong>s, the later is a correspondingmeasure <strong>of</strong> amplitude-modulated (AM) sideb<strong>and</strong>s. We introducethe symbol m(f) to have useful terminology for the important concept <strong>of</strong>normalized AM sideb<strong>and</strong> power.For the types <strong>of</strong> signals under consideration, by definition the two amplitude-fluctuationsideb<strong>and</strong>s (lower sideb<strong>and</strong> <strong>and</strong> upper sideb<strong>and</strong>, at - ff from \'0' respectively) <strong>of</strong> a signal are coherent with each other. Also, theyare <strong>of</strong> equal intensity. The operation <strong>of</strong> the mixer when it is driven at colinearphase is such that the amplitudes <strong>of</strong> the two AM sideb<strong>and</strong>s are added linearlyin the output <strong>of</strong> the mixer, resulting in four times as much power in the outputas would be present if only one <strong>of</strong> the AM sideb<strong>and</strong>s were allowed to contributeto the output <strong>of</strong> the mixer. Hence, for If I < VO,<strong>and</strong>, using the definitionwe find, in decibels (carrier) per hertz,(49)(50)m(f) = (l/2V6)S.ll f I). (51)TN-205

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