[time-nuts] Characterising frequency standards
sar10538 at gmail.com
Fri Apr 10 11:40:37 UTC 2009
2009/4/10 Tom Van Baak <tvb at leapsecond.com>:
> We need to be careful about what you mean by "continuous".
> Let me probe a bit further to make sure you or others understand.
My reference to "continuous" data would be defined as measurements
over a specific sampling period with each sample following directly
after the previous. This seems to be what is generally required for
the calculation of ADEV in the literature and postings on this group.
Such that techniques like the picket fence are suggested as a way to
deduce "continuous" data when using instruments that are unable to
measure sequential cycles of the input.
> The data that you first mentioned, some GPS and OCXO data at:
> was recorded once per second, for 400,000 samples without any
> interruption; that's over 4 days of continuous data.
> As you see it is very possible to extract every other, or every 10th,
> every 60th, or every Nth point from this large data set to create a
> smaller data set.
> Is it as if you had several counters all connected to the same DUT.
> Perhaps one makes a new phase measurement each second,
> another makes a measurement every 10 seconds; maybe a third
> counter just measures once a minute.
> The key here is not how often they make measurements, but that
> they all keep running at their particular rate.
> The data sets you get from these counters all represent 4 days
> of measurement; what changes is the measurement interval, the
> tau0, or whatever your ADEV tool calls it.
> Now the ADEV plots you get from these counters will all match
> perfectly with the only exception being that the every-60 second
> counter cannot give you any ADEV points for tau less than 60;
> the every-10 second counter cannot give you points for tau less
> than 10 seconds; and for that matter; the every 1-second counter
> cannot give you points for tau less than 1 second.
It is certainly true that 1 second sampled data collected at 60 second
intervals cannot be fed into an ADEV calculation as having a tau of 1
sec as the resultant calculation will show incorrect results when
noise like drift is a factor. If the data set is pre-processed and
corrected for such effects as drift, I believe it should be possible
to feed this discontinuous data as "continuous" data for the
measurement of short tau with reasonable accuracy.
> So what makes all these "continuous" is that the runs were not
> interrupted and that the data points were taken at regular intervals.
> The x-axis of an ADEV plot spans a logarithmic range of tau. The
> farthest point on the *right* is limited by how long your run was. If
> you collect data for 4 or 5 days you can compute and plot points
> out to around 1 day or 10^5 seconds.
> On the other hand, the farthest point on the *left* is limited by how
> fast you collect data. If you collect one point every 10 seconds,
> then tau=10 is your left-most point. Yes, it's common to collect data
> every second; in this case you can plot down to tau=1s. Some of
> my instruments can collect phase data at 1000 points per second
> (huge files!) and this means my leftmost ADEV point is 1 millisecond.
I guess it really depends on what level your measurement system is
able to work. For, say, the output of a 10MHz OCXO it would be
desirable to measure the source frequency although that would require
a fast measurement system and significant storage. The benefits of
this is that the input source is not degraded in the process of
division down to a more manageable frequency. We are currently
discussing the effects of the introduction of noise into frequency
standards just with distribution amplifiers and dividers. The ability
to measure such close in noise effects would indeed be a great bonus
and I envy your abilty to perform that.
> Here's an example of collecting data at 10 Hz:
> You can see this allows me to plot from ADEV tau = 0.1 s.
> Does all this make sense now?
Yes, I understand.
>> What I now believe is that it's possible to measure oscillator
>> performance with less than optimal test gear. This will enable me to
>> see the effects of any experiments I make in the future. If you can't
>> measure it, how can you know that what your doing is good or bad.
> Very true. So what one or several performance measurements
> are you after?
Well there are a number of them. The selection of best free-running
OCXOs. The effects of locking an OCXO to GPS and the "tuning" of this.
Running a OCXO in active holdover mode. I'd like to separate the
effects of temperature, rate of change of temperature, aging,
humidity, atmospheric pressure and, possibly, gravity on a
free-running OCXO. By changing just one variable at a time, I'd like
to measure the effects of each one with respect to determining the
correction required from a holdover circuit. Agreed, some of these are
simply defined as frequency change in the oscillator but I will wish
to measure the full system performance and need some form of
yard-stick to work to. Now this may not give the most politically
correct and acceptable results for such measurements as ADEV but it
will give me something for comparison. If I can make it match the
politically correct results of others, I also can make comparisons
with their results too.
Does all this make sense now?
You see I need to understand things in my own mind and have problems
when things do not make perfect sense to me. So if I as stupid
questions or make ridiculous suggestions, it's just my way of learning
by probing for the limits. There was a UK TV science program called
Take Nobodies Word For It, a paradigm I have always lived by so I tend
to prove things to my own satisfaction and probably re-invent the
wheel a lot of times. I never went to uni so my classical education is
limited but I have spent all my life learning off my own back.
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Steve Rooke - ZL3TUV & G8KVD & JAKDTTNW
Omnium finis imminet
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