[time-nuts] FLL errors

Bill Hawkins bill at iaxs.net
Sun Aug 30 01:45:41 EDT 2015

Ah, Magnus, source of so much solid and useful information, I don't see
a time loop as valid.

Frequency, yes, social time, no.

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."

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
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


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.

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