[time-nuts] Strange oscillations

Alberto di Bene dibene at usa.net
Thu Sep 2 08:54:02 EDT 2004

I would like to ask your opinion about some strange oscillations.
I built a GPSDO, not a replica of the Shera's project, a new one,
and monitoring the EFC to check its behavior, I noted that, after
it stabilizes, there are some pseudo periodic oscillations, which
I don't think are caused by an instability of the PI algorithm used
for controlling the EFC.

I have graphed them below, capturing via a serial port the EFC values,
and converting them to frequency deltas, knowing the sensitivity of
the EFC input of the OCXO used, an Isotemp OCXO-134, using Matlab
for the conversion and the graph.
My suspect is that they are caused by a thermal cycling of the oven,
given the period (about 5 minutes), albeit I am pretty sure this
oven has a proportional control system, not bang-bang.
This suspect is also confirmed by the fact that blowing energically
on the oven causes a major disruption of that cycling.

I would know your opinion, especially from those who have a greater
experience than me on this stuff,  thanks.

73  Alberto  I2PHD

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