Bob posted
can you explain it to me?
Don't know, I'll give it ONE try.
I'm not so good at explaining, but it is pretty basic if one does not start assuming that it can not be done at the
start.
It is mostly about averaging lots of those transitions, and the real trick is
that it is not Digital.
Analog has no lower limits except manly for Johnson noise type effects (mostly).
Example with some random picked numbers.
and assuming all analog that has no digital steps in it to limit resolution or
add noise.
Say you have a nice logic gate with 1 ns delay
If you make it all nice and clean, and repeatable such as constant PS, rise
time etc.
Then one can get repeatable results say 100 times better from cycle to cycle in
the short term.
so down to 10ps repeatable.
Now make things even more clean with no variations and assuming random noise.
Now if one is doing this at 10 MHz and only cares about the average over 0.1
sec (10 Hz)
One can average 1,000,000 readings of the 10 ps jitter
If they are truly random, that can give you a 1e-3 improvement (square root of
number of samples averaged)
so now down to 10 fs of average jitter at 10 Hz for a 10 MHZ gate starting with
a 1ns initial delay.
OF course if Anything changes at all, it will drift much more than that, which may or may not mater much depending on
what one is doing.
If you only really care about the difference between any two consecutive 100 ms reading that are next to each other,
as is (mostly) the case in ADEV, then not a big deal.
IF it does matter or you want to do better, the next step is to do it all differential, so you are looking at only the
different of two separate independent but equal circuits. Differential can give, say a 1000 to one or better
improvement in drift due to common things such as temperature etc.
If that helps explain the basics, good, if not you need to ask others to
explain it better.
And yes there all kinds of things that can & do go wrong and many ways to screw
it up.
so as easy as it sounds, it does take a bit of skill and art to do it.
Especially when one realizes that you are measuring things << 0.001 in of distance change will have major effects on
because of the speed of light.
(approx 1ft /ns, 0.01 in/ps, 1 micron/4fs)
Now if one starts out, not with a gate but a phase detector that is made for such things, and averages enough (but not
to long) and is real careful,
1fs resolution is possible in the 100 Hz range with 10 MHz
10 MHz & 1fs at 100 Hz gives 1e-13 freq variation resolution at tau 10ms
The simple BB TPLL is only getting about a tenth of that, (as shown on John's test plots) so it can be made much
better with enough care, if anyone has a ref osc that needs it.
But as I am always so quick to point out, the BB tester was not optimized for any one thing, It's performance was
selected as a compromise for 'KISS' reasons. (KISS = Keep It Simple so the experts can understand.)
please let me know on or off line if I'm wasting my time trying to explain this to the non "nut experts" without the
help of the fancy math papers.
ws
*********************
Warren,
I'm a newbie, so can you explain it to me? Femto anything is something
mostly reserved for a well equipped lab. How do you do it when most digital
logic has jitter several of orders of magnitude greater?
Bob
*********************
[time-nuts] Advantages & Disadvantages of the TPLL Method
Robert Benward rbenward at verizon.net
Fri Jun 18 20:23:40 UTC 2010
Previous message: [time-nuts] Advantages & Disadvantages of the TPLL Method
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
Warren,
I'm a newbie, so can you explain it to me? Femto anything is something
mostly reserved for a well equipped lab. How do you do it when most digital
logic has jitter several of orders of magnitude greater?
Bob
*************************
----- Original Message -----
From: "WarrenS" <warrensjmail-one at yahoo.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Friday, June 18, 2010 3:58 PM
Subject: Re: [time-nuts] Advantages & Disadvantages of the TPLL Method
Charles posted
its operation needs to be characterized if technically oriented folks are
to be expected to take you seriously.
If a simple BB that works as well as that one, does not already do that,
then nothing else I'm able or willing to do or say is going to change
much.
It is a good thing that people can accept things when shown that they
work,
even when they do not fully understand how.
Or else there would no acceptance of much of anything including gravity,
computers, email, women or Windows.
So your point is? and why do you think I care? Maybe you are missing the
point of my intended audience.
After all, I'm not the one that is lost or the one that does not know now
it
works. I have a working unit that, I know how it works and how to test it.
how you determined phase locking down to femtosecond levels, & ...
You do indeed have many valid technical questions that I'd try and explain
in a way that you could understand,
IF my goal was still to try and educate you, or if I thought there was ANY
chance I'd be able to do so.
You seem to again of missed the point of my last answer,
One with enough technical ability can see what John's data says from his
noise floor measurements.
But
If you do not understand that or how I'm doing it by measuring the PD
output, then
I can not teach or show you how, even if I still wanted to try. My past
failed attempts prove that.
So I'll just continue to do what I do best, and that is to make and test
TPLL BBs for myself.
If you were thinking that I am one that can or am willing to try and teach
you basic or advanced anything, then you are greatly over estimating my
abilities and/or my patience. You need to look else where if you want to
read a fancy math paper on how and why this all works or how to measure
it.
If you can't find one to your liking and still want to know how to measure
fs stuff, I'm sure if you ask nicely in a new thread, others would be more
than willing to help you out with something that you could understand and
accept.
Then again if I missed your point and your only goal is to verify if I now
how to do it, then I can save you some trouble,
Yes the TPLL and I know how to measure fs phase differences, and we both
know how to integrate along with a lot of other basic things including
even
adding two plus two. (and I doubt that you understand that last point
either, & Don't take it so personal, It is likely not just your problem)
BTW, one of the other points you seem to be missing in how I can measure &
test things so easy that others can find so hard to do.
After all I do have a big advantages over most, I can use another one of
my
GP PLL BB as a tester.
As I've point out before, they can do much more than just ADEV.
But then you would not be expected to know that without an advantage list.
****
Seems like it is again 'Time to Push the Reset.Button' on this thread's
subject, cause this has got way off the subject.
As long as it is so far off the subject of advantages & disadvantages,
I'll
add, in response to what others have said both on and off line.
IF others want to build a TPLL using buffer amps, or VFC, or difference
Ref
Osc, or multiple Ref osc, or a digital version or with cross correlation,
or
using different software, or different algorithms, or different
connectors,
or more parts, or more expensive parts or cheaper parts, or that works
better with some imaginary unreal data set, or over a wider freq range, or
over a longer tau, or at a lower level, or with less injection locking, or
any of the other thing's that have been brought up,
by all means, Go for it. I've tried to encourage others to do it their
your
own way. No single solution is best for all situations.
What several of the suggestions show is that many do not yet know how
simple
a TPLL can be made or do they even understand exactly how the 3 basic
parts
work together.
I have consider all the suggestions and tried many of them and so far have
found the variations unnecessary for my applications.
Also I have not heard about any H/W that others have built, only a lot of
criticism from some about what I've done or said or not said.
No problem, If others do not like what I've done or the way I've done it
or
tested it, even when many of them do not know what it is or how it works,
by all means they should do it there own way.
What I've done is to test one of my simple TPLL versions and show that
it's
performance is good enough to be limited by the OCXO.
That is all, Don't read more into my comments than that.
The simple TPLL does not go down to 1e-15. What I said is that is the
limit
of the low cost AMP that I choose, and it insures the amp has no
significant
negative effect.
Of course all is not perfect, nor is this is the best that a TPLL can be
made, not even close. But it is good enough for me.
I know how to made it better, much better, with lower noise, more
resolution, faster, lower tau, smaller or bigger, more costly or lower
cost,
and on & on.
So what?, the achieved goal of 'KISS' was to keep it small, simple and low
cost, and make it good enough to test the high end OXCOs that are
available.
This simple $10 version (my ebay cost) of a TPLL tester using just a
single
powered active part (not counting the Ref or the ADC or the PC) has plenty
of limitations but it has been show to be good enough to closely match a
TSC 5120A over a wide range of signals.
By all means if your needs, skill levels, experiences, desires, goals, and
junk boxes are different than mine, and you want to make something
different
or better, then by all means 'Go For It'.
I'm not trying to tell anyone what method to use, or what they should
build
or how to build one OR how to test it.
I'm just trying to get a list posted to show the many unknown advantages
and
disadvantages of the TPLL methods.
Because so few have had any experience with this method and therefore have
little or no practical knowledge about it.
How can one go about deciding it is something is useful for them without
knowing what the advantages and disadvantages are?
ws
***********************
*************************
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