[time-nuts] A philosophy of science view on the tight pll discussion

[Magnus Danielson]

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