[time-nuts] 10811 outer oven controller.

Mark Spencer mspencer12345 at yahoo.ca
Tue Jul 16 00:31:37 EDT 2013


Hi, the recent discussion about 10811's with double oven's caused me to take another look at some of the data I've collected from one of my two GPSDO's that uses a 10811 double oven OCXO.
  
I realize there is much more to the performance of a GPSDO than the OCXO but I can't say I'm unhappy with the performance of this GPSDO and have no complaints about the performance of the OCXO in this application.   I expect the data at tau <40 seconds is skewed by the noise of the HP5370B.    Sorry that data table for these plots shows the data that is more relevant at longer Tau's (it doesn't show the values for the comparison between the BVA and the Z3805) but the plots show the results at shorter Tau's.
 
As a side note, running standalone none of the 8 or so single oven 10811's I own have ever been able to consistently deliver ADEV or MADEV plots in the 13's at tau's that I am able to measure (ie >40 seconds.)   
 
Regards
Mark S
 
https://www.dropbox.com/s/jmpnuos0ei324s4/Composite%20MADEV.png?m
 
https://www.dropbox.com/s/3wm2wb6uj9jei7a/Composite%2520ADEV%5B2%5D.png?m
 
 
 
 


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Message: 6
Date: Mon, 15 Jul 2013 12:16:22 -0700
From: "WarrenS" <warrensjmail-one at yahoo.com>
To: "Discussion of precise time and frequency measurement"
    <time-nuts at febo.com>
Subject: Re: [time-nuts] 10811 outer oven controller.
Message-ID: <8FAFF758C2AB473EBFAC769FA195DB36 at Warcon28Gz>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
    reply-type=response


The Tbolt & LadyHeather plots in my posting are being used as a poor mans 
high resolution TIC tester as discussed at length in other postings, not for 
it's GPSDO output capability.
This is a method that allows a time-nut person that does not have any of the 
high end equipment still the ability to do a lot of the high end state of 
the art time-nut testing.
So in this case, it is valid to compare the results of the EFC changes with 
different types of ovens or even different oscillators such as for finding 
an oscillators tempco and ageing,
as long as the plots are interpreted correctly, because the GPSDO tuning 
settings have very little if any effect on the long term EFC voltage plots.

I have found that one of the largest variables in a GPSDO is the effect that 
temperature change has on the performance of the OCXO being disciplined.
This makes the stability of the OXCO very much a non-constant, in fact 
temperature effect on the OXCO is the largest variable in many setups.
That is why to achieve the best GPSDO performance, or even consistent 
performance between different runs when using a typical single oven GPSDO,
one needs to build a brick house in the basement or put the OXCO under test 
in a temperature controlled environment such as a dual oven or LH 
temperature controlled S/W loop.

All secondary temperature control devices have the same general goal which 
is to minimize or eliminate any fast temperature changes and therefore allow 
the GPSDO to take full advantages of the OCXO's then essentially constant 
intrinsic performance.

Before doing any meaningful comparisons between single and dual oven GPSDOs 
or comparing the difference in optimal tuning settings,
one must first define what the temperature environment is.  If the 
temperature is not allowed to change then there is no difference.
With a good dual oven set up, temperature change will have little or no 
effect, whereas with most time-nut available single oven oscillators 
including the single oven 10811,
temperature variation is the first thing one needs to be consider before 
tuning for optimal performance.

ws

**********************
from Tom Van Baak (lab) tvb at leapsecond.com
Mon Jul 15 12:22:38 EDT 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)
******************
From: "WarrenS"
>>
>> Tom
>>
>> My posting and plot was only meant to show the difference in tempco 
>> 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 
>> overlaid 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 the effective 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
>>
>> Answer2)
>> The 800 sec TC & 0.9 damping was fixed throughout the run and is a 
>> nominal value I often use with good external oscillators on my TBolt
>>  (or a LH temp controlled internal osc).
>> As you said, for this run and set of conditions, the dual oven did not 
>> help that much even though the dual oven oscillator is much more stable 
>> by 60 to 1  with temperature changes.
>> To take advantages of all of the extra stability of the dual oven I can 
>> set the extended TC has high as 3000 to 5000.
>> Also note that at during that run time, the temperature only changed 
>> about 3 deg C. If this test where done when the temperature changed say 
>> 15 deg C over a short time period then you would really be noticing  the 
>> difference between the two oscillators in the disciplined mode.
>>
>> In summery that picture is what I had on hand to show the performance
>> difference in a 10811 dual oven and single oven operation.
>> In this case the TBold is just being used as a TIC substitute to show
>> relative differences in the EFC voltage over time periods >1000 second, 
>> and therefore only the green plot should be used to see what the 
>> oscillator would do if it where in open loop and undisciplined.
>> It is fair to assume, and the other plots verify, that the GPS control 
>> loop is doing it's job good enough and holding the long term freq and 
>> phase of the osc constant enough for valid freq measurements to be made 
>> using mode 2 above from the EFC voltage.
>>
>> ws
> 



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