On 06/05/2010 01:19 PM, Steve Rooke wrote:
So, at best, it's an estimate.
Yes.
How good it is, how fast you get it, how much you pay for it and how
much effort it is to get and operate is the issue.
Getting accurate measurements is hard to prove actually. Getting
sufficiently good relative measurements (for money, effort etc) is
easier most of the times.
So what we have to do is to study various forms of impairments, learn
their effects, learn how to deal with them and learn how various
approaches have benefits and defficiencies. The deeper I study this, and
the more of the things I have initially ignored but forced myself to
follow up, the more complex the issue becomes and things comes in a
different light. You get humbled by learning just how little you knew as
you learn more. It is a time-consuming effort, but I hope some of it
pays of in my contributions to the Allan variance article on Wikipedia.
I have still not delivered a complete view from the things I have
learned recently, even if I hint some of it. There is a number of
statements in there which is unsatisfactory in that they do not have a
complete inline reference, but I think I have done a fairly god job so far.
Few people seems to estimate the values of h_-2, h_-1, h_0, h_1 and h_2
even if both phase noise and time/frequency difference data may be used
for it.
Also, modern cheap programmable TCXOs break the model as they have a
hump in the phase noise due to their locked PLL, which the original
model does not allow for. The autocorrelation function will be quite
different. Notice how this ripples over to other locked oscillators such
as passive masers, GPSDO etc.
There is still basic research to be done and basic research to be
recovered from the archives.
Cheers,
Magnus
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