[time-nuts] DC Voltage Ramp?

[Tom Van Baak]

> Any decent ovenized oscillator, should deliver stability of 10e-12,
> typically flat from 1-100 seconds, but likely getting worse outside of 100
> seconds due to drift.  A really good oscillator, like the dewarized (??)
> 1000B may hold 10^(-12) out to 1000 seconds (but I wouldn't count on it).

I pretty much agree with this, athough a well
behaved 1000B or a BVA will easily stay under
1e-12 out to 1000 seconds or more. See:
http://www.leapsecond.com/museum/datum-1000b/
http://www.leapsecond.com/museum/osa8607/

> With a one-way code receiver, the best stability you can get out of GPS is
> about 10^(-8)/tau, corresponding to 10 ns jitter at 1 second.   As such,
it
> takes about 10^(4) seconds for it to integrate down to the 1e-12 level, by
> which time your OCXO has wandered above it.  As such, it is tough to take
> full advantage of a good OCXO by steering to GPS.

> Standard-performance cesium, on the other hand, typically delivers
> 1e-11/sqrt(tau), i.e. it integrates down to the level of your super OCXO
in
> 100 seconds (1e-11/sqrt(100) = 1e-12).
>
> For this reason, you will never get "cesium class" stability out of any
> OCXO, no matter how good, by locking it to GPS.

That's a great, concise analysis. Thanks.

> Note that you can do pretty well by locking a rubidium clock to GPS.  In
> this case, you want to set your loop tau of your phase lock to the
crossover
> of the rubidium stability with that of GPS (about 3000 seconds).
Typically
> the phase noise of rubidium is pretty poor, so you may want to use your
> 1000B as a clean-up oscillator by phase-locking it to the rubidium with a
> shorter tau, maybe 100 seconds.
>
> After all this, you may ask why the loop tau of the OCXO in a cesium clock
> is set to 1-2 seconds, rather than 100 seconds, to take advantage of its
> better stability at tau<100 seconds.  The reason is that the quartz may
> experience a frequency "hop," perhaps exacerbated by vibration or shock to
> the instrument, and most users don't want this to persist for several
> hundred seconds.  In a nice quiet environment, like a standards lab, with
a
> well-aged oscillator, some users take the risk, turn down the loop gain,
and
> enjoy the better stability of the OCXO at shorter tau.

Even later models of the old HP 5061 had a LTC
(long time constant) switch. It was interesting to
watch the 5061 lose lock in LTC mode if you
banged on it too hard. In normal mode it would
stay locked.

> Of course, the best of all worlds is cesium locked to GPS (with a loop tau
> of a week or more).  This was once available as an option board to the
> TrueTime XLdc, which steered HP5071 to GPS, or as a package from Datum,
> using the 9390 GPS receiver coupled with the 6801 tracking servo and a
4065
> or 4040 cesium instrument.  There was even a short-lived all-in-one-box
> Datum solution which was called Cesium+ and later GPS+.  All of these
> off-the-shelf GPS-steered cesium solutions are now obsolete, but may
appear
> on Ebay occasionally.

Next month Doug can tell us how his dual frequency
carrier phase GPS locked 5071A is working...

By the way, a photo of your first *Symmetricom* 5071A
rehearsing the 12/31/2005 leap second is at:

http://www.leapsecond.com/notes/leap-watch.htm

>
>
> -RL

/tvb