Year: 2004
Ludvík Šojdr, Ph.D.; Jan Čermák, MSc.
The most stable frequency sources at the averaging interval around 1s are 5 MHz BVA quartz oscillators that provide frequency stability better than 1×10–13 in terms of Allan deviation (ADEV). To measure these (and the future) BVA oscillators a comparison system is needed with background stability of 1×10–14 at 1 s or better. The best-suited comparison method is based on dual-mixer time-difference (DMTD) multiplication. The method makes use of mixing quasi-synchronous signals at frequency ν0 from the reference and measured oscillators with a signal from a common oscillator at frequency ν0 + Δν. The dual mixing provides two beat-note signals at Δν with a time difference (represented by the zero crossings) multiplied by ν0/Δν with respect to the input.
We started working on the DMTD multiplication in 2002 with the goal to develop a measurement system that would allow us to compare the best BVA oscillators. We studied the noise sources in electronics of the DMTD multiplier and also the instability caused by interference including power-line pick-ups, environmental effects, and construction impacts. By optimizing our DMTD system (in collaboration with the BNM-SYRTE, Observatoire de Paris) we have achieved the results shown in the figures below. The background instability is ADEV(τ) ≈ 7×10–15/τ and the phase flicker floor of the corresponding time deviation, TDEV, is about 2 fs (with 15 Hz cut-off frequency). To our knowledge this is the best stability performance at 5 MHz ever reported.
