NIST Technical Note 1337: Characterization of Clocks and Oscillators

228 SAMUEL R. STEIN9.3 GHzOSCILLATORUNDERTESTx 1838COUNTERFIG. 12-24 Use of frequency synlhesis to measure oscillators whose frequency differssignificantly from the available low-noise reference. h may be necessary to use a frequencymultiplier to bring the signal into the range of the available synthesizer or to overcome thesynthesizer's phase noise.appropriate range and to enhance the oscillator noise compared to the shorttermphase noise of the synthesizer. Figure 12-24 demonstrates both aspectsof the technique_The initial mixing stage from the microwave frequency to the rf results in asubstantial heterodyne factor, 77.5 for the example chosen. The output of thefirst conversion stage lies within the range of low-noise commercial frequencysynthesizers, which makes it possible to obtain a fixed, low beat frequencyover a wide range of input frequencies. The initial mixing stage also reducesthe frequency synthesizer's contribution to the measurement-system noise.Figure 12-25 shows the typical phase excursions of a high-quality commercialsynthesizer operated near 5 MHz.Under some circumstances a frequency divider may be used to providethe signal for the second mixing stage, as shown in Fig. 12-26. This techniquehas the disadvantage of requiring a custom divider but results in muchlower measurement noise than the direct use of a synthesizer with a singleheterodyne stage.• (t)O~ \-10-.1 !o 80,000TIME (SEC)FIG. 12-25 Typical phase excursions of a commercial frequency synthesizer.IN-98