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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