[time-nuts] Unstable FTS1050A?

[AC0XU (Jim)]

Working my way through testing my collection of FTS1050A oscillators, I find one that erratically has poor time stability for tau > 1 sec. In some multi-day tests it is similar to the others (ADEV well under 1e-12 up to about tau = 1000 sec), but during other tests it is about 4x to 10x worse than the others for tau > 1 sec up to about 50k sec.

If it were consistently bad at higher tau, I would think that the heater has gone. But in about half of the 10 multi-day tests I have done with this unit, the stability appears fine.

Any ideas about what could cause sometimes good and sometimes not-so-good stability?

Thanks-
Jim