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

[Anders Wallin]

FWIW meinberg's chart ends up at 1.1 us with similar assumptions:
https://www.meinbergglobal.com/english/specs/gpsopt.htm
you could check that your model gives similar values for the lower quality
oscillators, and longer free-run periods also?


On Fri, Nov 29, 2019 at 3:32 AM 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
>
>
>
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