[time-nuts] Estimating expected time error using info from manufacturers' data sheets

[Bob kb8tq]

Hi

One issue you will quickly run into is the nature of the data sheet parameters. 
If you are buying a device you can afford, they rarely have a lot of detail. This
is hardly unique to frequency references. The real device may exceed the spec’s
by a very wide margin. It also may just barely (if at all) meet them. 

There also will be “assumptions” built into various specs. Is the number taken after
running for a month (as with some op-amp parameters) or maybe after a day / week? 
At what rate of change is the temperature stability measured? How much does this 
matter in your application? 

Some devices (TCXO’s) will have a very complex F vs T characteristic. Other devices
may be much less dramatic in their behavior. Getting this back to some sort of “change
per degree” number …. yikes. With one device rated over 0 to 50 and another over -40 
to 85, simply comparing their max excursion numbers really does not tell the story.

Lots of nasty problems ….

Bob

> On Nov 28, 2019, at 8:23 PM, BJ <catgirl at bordernet.com.au> wrote:
> 
> Dear fellow timenuts,
> 
> 
> 
> I am looking for some advice and insight from others wiser and more
> experienced (than me) in the following:
> 
> 
> 
> I want to be able to estimate the ability of a variety of (free running)
> time & frequency references (ranging from crystal oscillators to Rb and Cs
> frequency references) to remain synchronised to some hypothetical 'perfect'
> reference over time. I.e. for each device I want to calculate expected time
> error at some time t, given that the device was synchronised to the
> 'perfect' reference at t=0. And I need to do this based on what is provided
> in the datasheets. I thought this should be a straightforward exercise, but
> (probably due to my ignorance) I'm finding this trickier than anticipated. I
> have come up with a possible approach, but wanted to get some feedback from
> anyone else out there that might have already gone down this path.
> 
> 
> 
> The parameters (that appear relevant) in the data sheets are:
> 
> Frequency accuracy
> 
> Frequency stability over temperature
> 
> Aging (per day/week/month/year.)
> 
> Frequency stability (ADEV)
> 
> 
> 
> The question then becomes, how do I combine these figures sensibly to come
> up with the information I seek? For starters, there is some inconsistency in
> how the parameters are recorded in the data sheets. Then there is the fact
> that ADEV is often only given for one or a couple of values of tau. Anyway,
> I have scoured the literature and came up with a couple of equations that
> seemed promising, with the Time Interval Error (TIE) appearing to be the
> most applicable. In particular, I have been using RMS TIE_est(t) as
> specified in IEEE Std1139-2008. 
> 
> 
> 
> Without getting too heavily into the maths, the variables in this equation
> are:
> 
> 1. Uncertainty in initial synchronisation (sigma_x0), which I am setting
> equal to zero, as I am assuming perfect sync at t=0
> 
> 2. Uncertainty in frequency (sigma_y0), which I am using to represent the
> frequency stability over temperature component (although, perhaps I should
> be considering the frequency accuracy here as well?)
> 
> 3. Random frequency instability at time t (sigma_y(t)) after linear
> frequency drift has been removed, which I am equating to ADEV for tau=t 
> 
> 4. Normalised linear frequency drift per unit of time (a), which I am using
> to represent the aging component if applicable
> 
> 
> 
> I have attached a worked example and would like to know if this makes sense,
> or if I am on the wrong track. Note that I have included further questions
> (and concerns) in red text in the worked example. I am quite uncomfortable
> about all the assumptions I am forced to make and all the interpolating and
> extrapolating I am forced to do, due to lack of information in the data
> sheets. But at the end of the day I am just looking at a ballpark figure and
> this is a bit of a learning exercise of sorts for me, to try to understand
> how to interpret the manufacturers' specs and what they really mean in terms
> of how long it might be before a free-running clock becomes too inaccurate
> for certain purposes.
> 
> 
> 
> So, in summary:
> 
> 1.	Does the TIE estimate I am using seem like a sensible choice for
> what I am trying to do? If not, what would be a better approach?
> 2.	Am I implementing the data sheet parameters sensibly in this
> equation? (as per the worked example in the attachment)
> 
> 
> 
> Thanks folks!
> 
> 
> 
> Belinda 
> 
> 
> 
> <TIE_example.pdf>_______________________________________________
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