[time-nuts] 10811 outer oven controller.

Tom Van Baak (lab) tvb at leapsecond.com
Mon Jul 15 16:22:38 UTC 2013


Ok, thanks for clarifying. In general the time constant one chooses must reflect both the intrinsic performance of the OCXO (essentially constant) and the realities of GPSDO mechanical, sky-view, and environmental conditions (possibly variable). Disabling an oven during a run is equivalent to a radical change in environment and not re-tuning the loop parameters will lead to sub-optimal or misleading results when plotted.

If you have time, it would be instructive to re-run the experiment. First with double oven enabled and do your best case ws-tuning. Then disable the outer oven and again do a best-case tuning. The phase/freq/adev plots would be revealing, as well as the (major?) difference in optimal tuning values.

/tvb (iPhone4)

On Jul 14, 2013, at 9:19 PM, "WarrenS" <warrensjmail-one at yahoo.com> wrote:

> Tom
> 
> My posting and plot was only meant to show the difference in tempcoef between an undisciplined single and dual oven 10811 osc which in this case is clearly =>  60 to 1.
> Your comments  bring up a different subject which is who needs it and how good does a controlled GPSDO oscillator need to be when not in holdover.
> 
> As you know, the purpose of a GPSDO control loop is to make the oscillator's long term stability relatively un-important.
> The longer the measurement time the less important the stability of the controlled osc is in a GPSDO, and as time increases past the GPSDO control loop time constant, the osc stability matters less and less
> 
> What you are seeing and saying when analyzing the phase and Freq errors plots, is closed loop performance.
> The phase and freq plots of the dual oven osc would pretty look the same even if compared with a 'perfect' osc, because the dual osc plots is already near or at the noise floor of that TBolt setup and antenna.
> 
> One can measure the longer term stability of an oscillator different ways;
> 1) Hold the EFC voltage constant and measure the change in frequency or phase with time.
> 2) Measure the scaled EFC change necessary to hold the oscillator's freq or phase output constant
> When done carefully and with the EFC voltage scaled correctly both ways can give the same answer.
> 
> Answer1)
> The way I measured the two tempco's is by measuring the correlation between the EFC control voltage and the temperature plot
> In the case of the single oven osc, the plot gains are set so that when overlayed the EFC DAC plot looks as close as possible to the temperature plot.
> When the plot time is >24 hr and there is good repeatability, the TC is just the ratio of the two plot gains, i.e theEffective EFC freq change divided by the delta temp.
> In the single oven case DAC plot gain = 1e-10 per division,  temp plot gain = 1.5C per division. Tempco = 1e-10 / 1.5  ==  6.7 e-11 / degC.
> I did the same thing for the dual oven trace by expanding the gain and zero 



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