[time-nuts] FLL errors

Magnus Danielson magnus at rubidium.se
Sun Aug 30 07:23:42 EDT 2015


On 08/30/2015 07:45 AM, Bill Hawkins wrote:
> Ah, Magnus, source of so much solid and useful information, I don't see
> a time loop as valid.
> Frequency, yes, social time, no.

You say social time, but I didn't, you extrapolated my words.
The main point of a Time Locked Loop is to use a enumeration beyond the 
cycles, so that you lock to the right cycle. This enumeration may be the 
code-phase of a GPS for instance. Some call this Delay Locked Loop while 
others call it a Time Locked Loop.
You can also have a time-scale such as TAI or GPS time if you like, and 
there it makes sense.
It's essentially a PLL, but aware of the enumeration so that the 
phase-detector has a much wider range such that false-lock cannot occur. 
Making the distinction from a normal PLL makes sense, and that's why 
both DLL and TLL is used.

> A clock (purveyor of time) consists of an oscillator and a counter. In
> olden times the oscillator was a pendulum and the counter was a set of
> gears driven by the tick-tock of the escapement. Today we have
> electronic local oscillators providing one pulse per second (or whatever
> is needed) to electronic counters and displays. As an old timer, I
> prefer neon Nixie tubes.
> The problem we struggle to solve is to relate our local oscillator to
> some widely recognized standard frequency, preferably derived from an
> inordinately expensive generator based on the bouncing of atoms under
> controlled conditions. The very best way to transfer the standard (not
> "At the tone, the time is ...") is to use an electronic phase
> comparator, error amplifier, and filter time constant that will cause
> the local oscillator to track the standard *frequency* usually
> propagated by GPS.
> The remaining problem is to get the counter to agree with our preferred
> version of time display (UTC, TAI, etc.). If the display electronics
> permit adjustments such as adding a second at a predetermined time, or
> adjusting by an hour for summer or winter time, then our needs for
> social time can be satisfied.
> I don't see the need to yank the oscillator around for social time with
> a "time loop."

If you don't need it to, then don't do it. It doesn't mean others don't 
feel it being an adequate solution. I'm ot forcing anyone to any 
solution, I'm only want to show that there is cases where it does make 
sense to use one.


> Best regards,
> Bill Hawkins
> P.S. We're moving to a life care community that has no room for a time
> lab. The Junk Genius truck arrives at 10300 Colorado Road, Bloomington,
> MN 55438, at 11 AM on 1 September. If you can get here before that you
> can have anything you see. There are only antiques, except possibly the
> HP 3335A synthesizers and Racal Dana 1882 counters. I've tried to sell a
> few times but have had no takers. I won't ship (no time) but you can
> hire someone to pick it up.
> -----Original Message-----
> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Magnus
> Danielson
> Sent: Saturday, August 29, 2015 9:47 PM
> To: Discussion of precise time and frequency measurement
> Cc: magnus at rubidium.se
> Subject: Re: [time-nuts] FLL errors
> Hi,
> On 08/29/2015 11:24 AM, Neville Michie wrote:
>> A PLL locks on to the nearest cycle,
>> is a Time Locked Loop different?
> Yes and now. In a signal conveying time, rather than letting a rising
> edge denote "0 degrees of phase" you have some even time measure
> occuring, of some known nominal rate. You know what "time" it was on the
> time-scale, so that you know how much your local replica time-scale is
> off when compared. This time difference does go beyond the nearest
> cycle, but typically for locked situations is the nearest cycle.
> Don't ask how I know, I just know.
>> If the decoded time from a GPS system is used discipline an oscillator
>> then leap seconds would have to have a frequency transient to maintain
>> lock.
> No, as GPS time in itself does not have leap-seconds, it's nominally the
> TAI time-scale offset. GPS signal conveys the difference between GPS
> time and UTC, and thuse the UTC can be conveyed.
>> If you use the output to say drive a radio telescope monitoring a
>> distant object you would want Earth's rotation to be phase or sidereal
>> Time locked. I realise that for such a task far more complex
>> computation would be required.
>> So is a time locked loop a valid concept?
> Yes, whenever the enumeration of cycles to some time-scale is relevant.
> Cheers,
> Magnus
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