[time-nuts] TPLL secret reveled
warrensjmail-one at yahoo.com
Thu Jun 10 04:05:57 UTC 2010
There has been speculation on why the low cost, simple, TPLL tester compares
so favorably with the TSC 5120A over a wide range of taus, even when using
different Oscillators. The theories range from pure luck to some special
secret magic hardware.
If I was that lucky, I'd be doing Lottery and not ADEV.
And If I could build some secret simple Hardware, and was able to adjust it
to give some desired effect on future random noise, I'd be in politics or
the stock market, not making ADEV Breadboards.
There is another more likely explanation that is within my limited
capabilities, that many refuse to consider.
That is this method just works correctly without doing anything very special
The reason that the simple TPLL works so good but is hard for some "experts"
to accept, seems to come down to the fact that this method uses Frequency
and not Phase to make the raw data log used to then calculate ADEV data.
Most if not all other methods depend on Phase differences (i.e time
differences) to calculate the Frequency difference.
The simple analog TPLL method holds the Phase difference to zero (with-in 1
femtosecond) and calculates the average Frequency differences by measuring
the voltage on the EFC of the reference Oscillator. That way no variable
phase is involved.
This filtered Frequency data log can then be used to calculate ADEV without
first needing to convert from or to phase.
The fact that average Frequency differences do not start out as wide
bandwidth Phase, means that all the fancy write-ups that tell how best to do
that, do not apply.
Some have argued that Phase and freq are the same thing, so the fact that
this uses freq would make no difference.
Others have argued that one can not average Freq, so you have to convert to
Both have a basis of truth for some methods, but neither is correct here, as
proven by the end results.
(unless one wants to also assume that I'm either the luckiest or smartest
person in the universal for getting it right the first time.)
While it is true that the TPLL Freq data can be turned into accurate
controlled Bandwidth Phase data, Wide band phase data can not be so easily
turned into controlled limited Bandwidth, zero dead time, integrated Freq
data, which is the thing that is needed to get good ADEV results.
The reason is, to get high resolution, such as 1 femtosecond (1e-15) phase
data, it takes a wide bandwidth, which means high noise and many other
problems. To get 1e-15 freq data from the EFC (limited by the ref osc), this
can be done with the B/W equal to tau0.
The short Over simplified summery is:
At 1sec Tau0, the up to 1e15 to one difference in bandwidth between Phase
and Frequency measurements makes a big difference that some seem to be
missing. (think Phase trigger jitter)
The simple TPLL has No secrets, just some basic differences that seem to of
been forgotten or not considered. The way that frequency is measured is
what is different between the TPLL frequency method and the other more
popular phase methods.
The fact is, that the simple TPLL in spite of it limitations, has been shown
to work just fine in the real world.
If one can not understand or accept that measuring frequency without
needing to first measure phase is the real secret reason why the TPLL works
so good, without needing any special H/W or S/W 'adjustments', then feel
free to propose a better reason besides Luck.
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