[time-nuts] Housing LPRO and Thunderbolt together

Arnold Tibus arnold.tibus at gmx.de
Sat Apr 23 12:08:39 UTC 2011


Bill's comments made me ruminative.
Because there seem to be different informations circulating about the
possibility to discipline Rubidium Oscillators, their phase stability
and lifetime.
I would like to have it precised or corrected.

 Dated 22.04.2011 23:37, WB6BNQ wrote:

> True, you can treat a Rubidium like it was a normal crystal oscillator, but it is
> not the same.  The Rubidium has a definite life span, the more you run it the less
> the life.  A high quality crystal oscillator, on the other hand, just gets better
> the longer you leave it on.  Aside from nominal electrical component failures, the
> crystal blank in a properly designed circuit has no short term failure mechanism and
> will last for decades with constant applied power.  The Rubidium’s life span is, at
> best, 10 years.  The question is how long was it running before you got it ?
> ....
> For high quality measurements, the crystal excels for measurement times of less than
> 10 seconds, as the Rubidium is noisier in that time frame.  That is, for taking
> readings on a one second to second basis, such as with a high resolution time
> interval counter, the crystal excels. 
>

What I not understood (yet) is:

Why shall RbDOCXOs be worse in short term stability terms than good OXCOs?

Why shall the output of a HP10811A or a similar high grade OCXO degrade
when properly disciplined by a RbO or a GPS Receiver or both combined?

Some RbOs do use in fact an OCXO as controlled oscillator element.
A good example is the PRS10 from SRS with a very informative manual
containing good informations and functional schemes. Its stability of
about 5 × 10−12 at Tau 10s and SSB-Phase Noise of <−130 dBc/Hz (10 Hz),
Short Term Stability of <1 × 10−11 (10 s) sounds good. As it has an
analog 0-5V Frequency Adjust and a 1 PPS input it can be well controlled
via GPS I think.

Why not try to exchange the Trimble's Thunderbolt OCXO with an external
10811A shown by John Miles or even with a PRS10? The Thunderbolt
temperature corrective function may not be adequate for other
oscillators specially when externally connected.
Can this function not be disabled?

I believe the end of the flagpole is not yet reached here.

The LPRO (DATUM)and the EFRATOM/ Ball FRK, FRM and FRS models as well
the FE5680A series unfortunately do contain only a VCXO inside and not
an OCXO!.

I found some more sources giving informations of interest:

Disciplined Rb Standards are not (yet) in wide use, but they can already
provide a short term stability down to better than 1E-11 according to
http://tycho.usno.navy.mil/ptti/1989/Vol%2021_13.pdf

PTS announce a stability with Allan Variance down to parts of E-13
(GPS10RB).
See GPS10R/RB - 10 MHz GPS Disciplined Rubidium Frequency Standards
http://www.ptsyst.com/GPS10RB-B.pdf
and
Why Rubidium Outperforms OXCO based units
http://www.ptsyst.com/AppNote2.pdf

Using a carrier phase tracking GPS Receiver, special techniques and
algorythms, Quartzlock in UK even claim to have achieved a short-term
stability down to E-13 and E-14 as long term accuracy.
GPS Synchronized Disciplined Rubidium Frequency Standard from TRAK Systems
http://tycho.usno.navy.mil/ptti/1998/Vol%2030_20.pdf
http://www.quartzlock.com/products.asp?product=69

Concerning the lifetime of RbOs:
According SRS the PRS10 lamp life is 20 years,
the manual of LPRO says:
The LPRO is designed for long operating periods without maintenance
(MTBF >320000h below and @ 30°C amb.) including a long life Rb lamp with
a goal to exceed 10 years, which will not necessarily be the end of life.

Information from Datum:
Over our 27 years of Rubidium oscillator development and production
experience, we have shown that the lifetime of our physics package is
virtually unlimited. Datum is the only company worldwide, being able to
guarantee a lifetime warranty on lamp and cell. There are no “wear-out”
or “use-up” mechanisms in a Datum Efratom Rubidium oscillator.
(http://www.amtestpl.com/DATUM/GmbH/Rubidium-Note-e.pdf)

So I don't see a big problem with RbO's lifetime.

regards

Arnold







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