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
offset of the DAC plot until it looked like the temperature plot.
In the dual oven case because of the lower correlation (aka noise) and
limited run time (under 24 hrs) I could only say the TC was under the
resolution for that measurement period and filter setting which was 1e-12 /
degC.
With longer run times and more care it is posible to measure TempCo with
resolution under 1e-13 / degC, which I've done for the LPRO.
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 contol loop
is doing it's job good enough and holding the long term freq and phase of
the osc constant enough for valid freq measurments to be made using mode 2
above from the EFC voltage.
ws
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----- Original Message -----
From: "Tom Van Baak"
Sent: Sunday, July 14, 2013 6:15 PM
Subject: Re: [time-nuts] 10811 outer oven controller.
Hi Warren,
That's a really nice plot.
One comment about the 60:1 quote, though. The plot shows the improvement is
only 4x when you look at the TI data (what you call "phase" noise). And the
plot shows the improvement is only 1.25x when you look at the OSC data.
Further, if you were to plot the actual output of the TBolt (instead of
plotting internal PLL loop statistics) I think you'd find these ratios get
even less impressive.
Thus the other way to interpret the plot is to say that in spite of the 60x
difference an outer oven makes to the tempco of a stand-alone quartz
oscillator, it falls to just 1.25x when used in a GPSDO. One could rightly
conclude then that outer ovens just aren't that important to the stability
of a locked GPSDO.
Two questions,
1) How did you calculate the two tempco values (1e-12, 6e-11)? From the plot
or with other tests?
2) Was the time constant (800 s, 0.9 damping) optimized for outer oven
turned on case? Was it re-optimized for the outer oven off case?
/tvb
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----- Original Message -----
From: "WarrenS"
Sent: Friday, July 12, 2013 6:04 PM
Subject: Re: [time-nuts] 10811 outer oven controller.
Bob said {the 10811 will run fine without the outer oven}
What I've seen is that a dual oven 10811 will run even **finer** and have
up
to 100 times less sensitivity to normal room temperature changes with a
simple outer oven controller and a few mods.
In 2010 I compared the performance of a TBolt using an external dual oven
10811 Osc with and without it's outer oven being controlled.
There was more than a 60 to 1 improvement in the 10811's freq sensitivity
to
small room temperature changes when the outer oven was active.
Open loop 10811 TC was <1e-12 /C with the dual oven on compared to 6e-11/C
with it off.
The green trace shows how much less EFC correction is needed to be to keep
the 10811's frequency constant when the dual oven is active.
http://www.febo.com/pipermail/time-nuts/attachments/20100223/c94d5eec/attachment-0001.gif
ws
*************************
----- Original Message -----
From: "Bob Camp"
Sent: Friday, July 12, 2013 10:45 AM
Subject: Re: [time-nuts] 10811 outer oven controller.
Hi
The "outer oven" on that version is simply a warmup heater. If it's
operating properly, it drops out in normal operation. Put another way -
the
10811 will run fine without it.
Bob
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