NIST Technical Note 1337: Characterization of Clocks and Oscillators

258 A. L. LANCE, W. D. SEAL, AND F. LABAARThe measured value is therefore divided by two to obtain the value for thesingle oscillator. A determination of the noise ofeach oscillator can be madeif one has three oscillators that can be measured in all pair combinations.The phase noise of each source I, 2, and 3 is calculated as follows:Y1U) = 10 log[100g'"U)ilO + IOg'IJ(J);JO - IOg'2J(J)/IO)]. (63):f2(f) = 10 !og[fOOg'I2(JI;iO + l~2J(J)/IO - lOY1J(/)!lo)], (64)y 3 U) = 10 ]og[1{lOYIJ(J)/IO + JOY2J(J)IIO - 10g'12(/jllO)]. (65)B. AUTOMATED PHASE-NoISE MEASUREMENTS USING THETWO-OSCILLATOR TECHNIQUEThe automated phase-noise measurement system is shown in Fig. 9. It iscontrolled by a programmable calculator. Each step of the calibration andmeasurement sequence is included in the program. The software programcontrols frequency slection, bandwidth settings, settling time, amplituderanging, measurements, calculations, graphics, and data plotting. Normally,the system is used to obtain a direct plot ofY(f). The integrated phase noisecan be calculated for any selected range of Fourier frequencies.A quasi-continuous plot of phase noise performance .!f(f) is obtainedby performing measurements at Fourier frequencies separated by the IFbandwidth of the spectrum analyzer used during the measurement. Plots ofother defined parameters can be obtained and plotted as desired.The IF bandwidth settings for the Fourier (offset) frequency-rangeselections are shown in the follOWing tabulation:IF Fourier IF Fourierbandwidth frequency bandwidth frequency(Hz) (kHz) (kHz) (kHz)3 0.001-0.4 I 40-10010 0.4-1 3 100--40030 1--4 10 400-1300100 4--10200 10--40The particular range of Fourier frequencies is limited by the particularspectrum analyzer used in the system. A fast Fourier analyzer (FFT) is alsoincorporated in the system to measure phase noise from submillihertz to25 kHz.High-quality sources can be measured without multiplication to enhancethe phase noise prior to downconverting and measuring at baseband frequencies.The measurements are not completely automated because thecalibration sequence requires several manual operations.1N-209