[time-nuts] Another "atomic" clock question

Bob Camp lists at rtty.us
Wed Mar 5 22:02:39 EST 2014


The disconnect is that there is no need at all to correct for the delay with some sort of delay line setup. The sawtooth correction simply sums into the input of your control algorithm for the corrected oscillator. It’s nothing more than an adder that sums the TDC output with the sawtooth information. No delay lines, nothing fancy to it. You have everything you need in the CPU that reads the sawtooth data from the GPS module. The loop only gets an input once a second. You have *lots* of time for math.

The output of the GPS module, if directly corrected for sawtooth still has a bunch of jitter on it. Even a disciplined TCXO will significantly improve things. 


On Mar 5, 2014, at 8:43 PM, Didier Juges <shalimr9 at gmail.com> wrote:

> Tom and Bob,
> It is not obvious to me that it is "easier" to simply apply a correction in
> nS increments with a range as wide as 100nS. How is this done? Using
> switched delay lines or delay gates?
> In the digital domain, 1nS resolution implies pretty fast clocks. On the
> other hand, processing a message that comes once per second to, say, drive
> a DAC is fairly trivial.
> Now, if we were to simply ignore the problem (an otherwise perfectly valid
> engineering choice under the right circumstances), we would not be
> time-nuts, would we? If you look at the number of posts on this list
> mentioning hanging bridges, it will show you that it is something most
> everyone has shown concern over at one time or another.
> I am still amazed at the simplicity and elegance of the Thunderbolt design
> where the issue of hanging bridges has been purely eliminated (not
> filtered, compensated, corrected or ignored) while making the design
> simpler and cheaper. It is much more robust than having to deal with a
> correction, no matter how well it is implemented.
> Now it is entirely possible that for most applications of interest to
> time-nuts, it is a negligible problem. I would not know but I defer to your
> qualified judgement. I have three Thunderbolts (the minimum number you have
> to have to resolve any discrepancy) and no other GPSDO, therefore I cannot
> comment on the relative merit of the two approaches from a practical
> standpoint. From a design standpoint, eliminating a problem by design
> (particularly when it is associated with a cost saving) is always
> preferable to an after the fact correction or compensation.
> Didier KO4BB
> On Wed, Mar 5, 2014 at 5:48 PM, Tom Van Baak <tvb at leapsecond.com> wrote:
>> I agree with Bob.
>> For casual use, "hanging bridges" are not really a problem, statistically
>> speaking -- so don't worry.
>> Yes, you can apply various techniques to reduce/eliminate the rare effect:
>> forced temperature change, forced Vcc change, 2 or 3 or more shared-antenna
>> receivers, modulating phase, frequency, voltage, temperature, etc. But as
>> you spend too much time engineering this uncertain hack you maybe start to
>> wonder if the real solution is just to apply known digital, numerical
>> correction instead of wishful analog cover-up. Been there, done that.
>> For more serious use, at the tens or unit nanosecond level, the robust
>> solution is simply to apply 1PPS sawtooth correction from the receiver.
>> This issue comes up every now and then as people gradually transition from
>> casual to serious use. I welcome any hard data or plots that demonstrate
>> the difference among all approaches. There *is* a slight difference for
>> sure. It's just that most people throw in the towel and use sawtooth
>> corrections instead of trying to avoid them and cover up with less
>> deterministic methods.
>> /tvb
>> ----- Original Message -----
>> From: "Bob Camp" <lists at rtty.us>
>> To: "Discussion of precise time and frequency measurement" <
>> time-nuts at febo.com>
>> Sent: Wednesday, March 05, 2014 3:03 PM
>> Subject: Re: [time-nuts] Another "atomic" clock question
>> Hi
>> If you are going to decode and use the sawtooth data out of the receiver,
>> there's no need to eliminate the hanging bridges. The sawtooth data does
>> that for you already. Put another way, heating the receiver is *harder*
>> than just using the decoded data....
>> Bob
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