NIST Technical Note 1337: Characterization of Clocks and Oscillators

above. The measured noise voltages are scaled usingthe measured gains and the spectral density of phasenoise calculated. The overlapping ranges of thevarious spectrum analyzers allows one the opportunityto compare the measurements on the three spectrumanalyzers, Typically one can obtain S.(f) of theoscillator pair to an accuracy of about ± 0.6 dB atFourier frequency above 100 Hz. The agreement withrepeat measurements is often of order ± 0.2 dE asshown in Fig. 15.7) The noise floor of the system is determined bydriving both sides of the measurement system with oneoscillator haVing a similar power and impedance levelas that used in these measurements. If the noisefloor is within 13 dB of that measured in step 7above, corrections are made to the measurement datato remove the bias generated by the noise floor.Measurements of the amplitude accuracy of the phasemodulation side bands generated by the prototypephase modulators are summarized in Fig. 15. The samemodulator was used for carrier frequencies from 5 to300 MHz. The error in the phase modulation amplitudeis less than 0.5 dB for modulation frequencies fromdc to 10' of the carrier. The 10 GHz modulator alsomaintains an accuracy of better than 0.5 dB out to500 KHz from the carrier. At 1 GHz the modulationamplitude is 1.5 dB high. Once this is measured itcan be taken into account in the calibrationprocedure. The 45 GHz modulator results shown inFig. 15 should be attainable over the entire WR22wavequide bandWidth.The performance that can be obtained with thismeasurement technique is illustrated by actual phasenoise data on oscillator pairs shown in Figs. 15 and16. Typical accuracies are ±0.6 dB with a noisefloor at about - 175 dB relative to 1 radian 2 /Hz.The corrections applied to the raw data at 10 GHz areshown in Fig. 17. At low frequencies the effect ofthe phase-locked loop is apparent while at the higherfrequencies the roll-off of the amplifiers areimportant. These effects have been emphasized herein order to examine the ability of the calibrationprocess to correct for instrumentation gainvariations.V, ConclusionWe have analyzed several traditional approaches tomaking phase measurements and found that they alllacked some element necessary for making phase noisemeasurements from essentially dc out to 10, of thecarrier frequency with good phase noise floors and anaccuracy of order 1 dB. By combining several of thetechniques and adding a phase modulator which isexceptionally flat from dc to about lOt of thecarrier frequency, we have been able to achieveexcellent phase noise floors, bandwidths of at leastlOt of the carrier, and accuracies of order ±0.6 dB.1.5m Ll 1.0.!:...g 0.5wc:.Q oI--__.::=----------=:iii"5-g -0.5:ECDtil~ -1.011.-1.5!::--1-_~~-I-_...J--;!;-..l_-L-:_.L---l~:I::-....L._.L-~0,001 0.01 0.1 1 10 100Modulation Frequency (I) in % of Carrier FrequencyFig. 14 Measurement of the amplitude error ofmodulation signal versus Fourier frequencies f. forthese prototype phase modulators. Curves labeled 5,100, and 300 KHz were obtained with the modulatorused in 5 to 1,300 KHz test set. The curve labeled10 GHz was obtained with the modulator for the 2 to26 GHz test set. The curve labeled 45 GHz wasobtained with the WR22 test set.N -136I-­NU~2 -138IV.... 11dBal'Uc::- ":9­--140tf)Two Oscillator Calibration Test @ 5MHz• System Ax System BA System B + M410 Hz 100 Hz 1kHzI...og Fourier Frequency (HzlFig. 15 Demonstration of calibration accuracy fortwo oscillator concept. The curve labeled System Ashows the measured phase noise of a pair of 5 MHzOSCillators using the test set shown in Fig. 13. Thecurve labeled System B shows the measured phase noiseof the same pair of 5 MHz oscillators using a totallyseparate measurement system with the oscillators heldin phase quadrature with the measurement test set ofA. The curve labeled System B + ~/4 shows the phasenoise of the same pair of oscillators using test setB with and extra cable length of ~/4 inserted intoeach signal path. The agreement between the threecurves is in the worse case ± 0.15 dB.TN-137440