[time-nuts] GSP clock stabilitiy, Rb vs Cs

WarrenS warrensjmail-one at yahoo.com
Sun May 5 12:40:21 EDT 2013

>"All the data is in an adev plot... The cross overs will happen... you have 
>to measure them."

True, but then what do you do?
It is not quite as simple or easy as it may sound.

Although it is a good place to start,
for best results in a GPSDO you can not just compare the ADEV crossover 
points of the two frequency sources.
The problem I've seen is that long term ADEV plots generally show a turn-up,
often around the 1000 sec range.
The turn-up in the plot, more often than not, is caused by systematic 
errors, not random noise,
so the turn-up may be scaled incorrectly by the ADEV plot.

The things I've seen that can cause 'premature' turn-up on an ADEV plot are:
Not allowing enough time for the osc to stabilize after turn on,
room temperature variation, outliers and fixed rate ageing.
With careful attention to many details, the turn-up can often be 
significantly reduced.
The effect that each of these errors types have on various disciplined 
control loops varies greatly.

The problem is when the effect that each of these errors has on a 
disciplined control
loop such as a GPSDO is not the same as the effect that they have on the 
ADEV plot,
you can not just use the crossover point of the two plots.

The most extreme example is **fixed** ageing rate of the frequency source 
that is to be disciplined.
A fixed ageing rate drift causes a slope of one turn-up on an ADEV plot
(but has little or no effect on a Hadamard plot).
On the other hand a fixed ageing rate error, which is often the major error 
of a good DOCXO,
has no effect on frequency stability in a basic fixed time constant
disciplined control loop such as used in a TBolt.
It does cause a constant fixed phase error that is a function of the control 
loop's time constant and damping settings,
but that can be removed completely if desired by just changing the control 
loop's cable length setting.
On other types of disciplined control loops, the effect of a fixed ageing
rate error may vary and depends on the type of advanced control loop used.

The effect of temperature variation on a disciplined control loop is another
big variation that can effect ADEV plots and disciplined control loops 
In the case of a TBolt, delta temperature correction is only applied when 
the unit is in Holdover,
so its effect has to be considered when setting up the GPSDO.
This is why the best way to fix that error source is to not let the 
temperature change or to use a external DOCXO.
Advanced control loops can greatly reduce the effect of changing temperature 
with feed-forward control,
so they may not be nearly as sensitive to temperature variation.


>All the data is in an adev plot. In this case short is < 100 seconds, and
>long is > 10,000 seconds. Those are rough numbers, since a really good Rb
>(like Corby's) may cross over a bit earlier. A really crummy Cs (low beam
>current) might not cross over for a couple of days against a well 
>Rb or Maser. A good BVA OCXO will give the Rb a bit more of a run for it's
>money ..
>The cross overs will happen. Where is going to depend entirely on the
>specific individual standards you happen to have. If you are making
>decisions about which of your boxes to use, you have to measure them.

On May 5, 2013, at 12:53 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
> tvb at leapsecond.com said:
>> Rule of thumb: quartz is best short term, Rb or H-maser mid-term, and Cs
>> by far the best long-term.
> What is short, medium, and long?
> Radio astronomers use H-masers.  Can I assume that they are mid-term and
> that H-masers are better than Rb (at mid-term)?
> Does the classic ADEV graph contain all the information, or is it making
> an assumption that is valid in most cases that allows it to compress/hide
> lots of information that is interesting for only a few obscure types of
> applications?

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