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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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7. PHASE ~OISE AND AM NOISE MEASUREMENTS247root <strong>of</strong>) radio frequency power P. The power <strong>of</strong> a signal is dispersed over thefrequency spectrum owing to noise, instability, <strong>and</strong> modulation. This conceptis similar to the concept <strong>of</strong> spectral density <strong>of</strong> voltage fluctuationsSbv(f)· Typically, Sbv(f) is more convenient for characterizing a baseb<strong>and</strong>signal where voltage, rather than power, is relevant. S.;rfP(v) is typicallymore convenient for characterizing the dispersion <strong>of</strong> the signal power inthe vicinity <strong>of</strong> the nominal carrier frequency Va' To relate the two spectraldensities, it is necessary to specify the impedance associated with the signal.A definition <strong>of</strong> frequency stability that relates the actual sideb<strong>and</strong> power<strong>of</strong> phase fluctuations with respect to the carrier power level, discussed byGlaze (1970), is called !I!(f). For a signal with PM <strong>and</strong> with no AM, !I!(f)is the normalized version <strong>of</strong> S.;rfP(v), with its frequency parameter f referencedto the signal's average frequency Va as the origin such that f equalsV - Va' If the signal also has AM, !I!(f) is the normalized version <strong>of</strong> thoseportions <strong>of</strong> S.;rfP(v) that are phase-modulation sideb<strong>and</strong>s.Because I is the Fourier frequency difference (v - va), the range <strong>of</strong> risfrom minus Va to plus infinity. Since !I!(f) is a normalized density (phasenoise sideb<strong>and</strong> power),(22) *!I!(f) is defined as the ratio <strong>of</strong> the power in one sideb<strong>and</strong>, referred to theinput carrier frequency on a per hertz <strong>of</strong> b<strong>and</strong>width spectral density basis,to the total signal power, at Fourier frequency difference f from the carrier,per one device. It is a normalized frequency domain measure <strong>of</strong> phase fluctuationsideb<strong>and</strong>s, expressed in decibels relative to the carrier per hertz:power density (one phase modulation sideb<strong>and</strong>)!I!(f) = . . (23)carner powerFor the types <strong>of</strong> signals under consideration, by definition the two phasenoisesideb<strong>and</strong>s (lower sideb<strong>and</strong> <strong>and</strong> upper sideb<strong>and</strong>, at - f <strong>and</strong> f from Va,respectively) <strong>of</strong> a signal are approximately coherent with each other, <strong>and</strong>they are <strong>of</strong> approximately equal intensity.It was previously show that the measurement <strong>of</strong> phase fluctuations (phasenoise) required driving a double-balanced mixer with two signals in phasequadrature so the FM-AM conversion resulted in voltage fluctuations atthe mixer output that were analogous to the phase fluctuations. The operation<strong>of</strong> the mixer when it is driven at quadrature is such that the amplitudes<strong>of</strong> the two phase sideb<strong>and</strong>s are added linearly in the output <strong>of</strong> the mixer,resulting in four times as much power in the output as would be present ifonly one <strong>of</strong> the phase sideb<strong>and</strong>s were allowed to contribute to the output., See Appendix <strong>Note</strong> # 31TN-198

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