On 9/16/19 4:16 AM, pisymbol . wrote:

Do you mean "build the physics package", or are you just talking about
something like getting a commercial rubidium reference (surplus) and
packaging it?

Hi

What are you really trying to do here?

If it’s a “from scratch” atomic standard, then you just aren’t going to get
there. Sorry about that ...

If it’s a wall clock sync’d to an external radio service then indeed you
might get there.

In-between those two lie tings like buying eBay telecom Rubidium’s,
attaching them to a power supply and you have a working standard.

Lots of very different directions this could go and and they all could be
called an atomic clock …. Not at all knock on doing something, just confusion
about what exactly you want to do.

Bob

First off, thanks to everyone who replied.

On Mon, Sep 16, 2019 at 11:00 AM Bob kb8tq kb8tq@n1k.org wrote:

Take over the world - one epoch at a time.

If it’s a “from scratch” atomic standard, then you just aren’t going to get

Ok.

Well, I want something maybe in between. Currently I think I have that: I
have a RPi disciplined to the Adafruit Ultimate GPS HAT via chrony (PPS
etc.). This works well.

So I think this is what I'm talking about. I want something a little bit
more esoteric than a GPS 1PPS. Can you explain a bit about these
prepackaged Rubidium standards? Upside/downside etc. Do I have to
maintain/check these black boxes?

Again, take over the world. Sorry for not being upfront about that. That
would have made things A LOT clearer.

So I guess: How can I get a simple Rubidium standard that outputs a
reference frequency as a discipline to say ntpd or chrony.

Btw, I'm a noob. Please be gentle.

-aps

All of the available Rb standards that I've seen have a 10 MHz output,
Some have a
1 PPS output as well.

The nice thing about a Rb is that its short term stability (seconds to
minutes and perhaps
even longer) is much better than that of a GPS timing receiver.  The bad
news is that Rb
standards exhibit long term frequency drift in the neighborhood of a few
parts in 10^11
per month.  A pretty fair compromise is to use an Rb standard that is
disciplined by GPS
PPS pulses with a loop time constant on the order of a day or so.  The SRS
model PRS-10
seems to be well-regarded and can be had for $1500 factory new.  Many are
on sale by
ebay and similar sources- however these are often left over from telecom
service and
are actually a modified model that has had the PPS locking facility removed
so that the
original buyer could save a few bucks.  My PRS-10 is one of these.

The Rb standards seem to prefer being powered up continuously, and if one
is turned back
on after any significant storage without power, it can cake several days to
really settle down
to its steady state behavior.  Also, because of the long-term frequency
drift, one must
periodically retune the unit back on frequency by referring it to GPS.
Both units I own
(a PRS-10 and an L-PRO) have an analog tuner port, wherein application of 0
to 5V DC
tunes the frequency of the Rb through the range of +/- a few parts in
10^9.  This has been
straightforward but a bit tedious; if I want to get it within a few parts
in 10^12, even a good
10-turn pot is barely adequate to the task, and I'm sometimes turning the
shaft through
an angle so small that I'm not sure I even moved it until I've observed the
phase drift
against  GPS for an hour or more.

So, that's my version of Rb 101.  See what others have to say as well, as
I'm also fairly
new at the game.

Dana  (K8YUM)

On Mon, Sep 16, 2019 at 4:02 PM pisymbol . pisymbol@gmail.com wrote:

On Tue, 17 Sep 2019 at 04:00, Dana Whitlow k8yumdoober@gmail.com wrote:

Is  there any advantage in using a GPS Rb disciplined oscillator vs a GPS
disciplined high quality OCXO like the HP 10811A? I can’t understand why
there should be, as a Rb source would use an OCCO in its output stage
Therefore in each case

• Short term stability depends upon the quality of the OCXO
• Long term stability depends upon GPS.

Perhaps there’s is period over which the the overall stability can be
improved by adding a rubidium oscillator. I would be interested to know if
that is the case or not.

Dave

Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@kirkbymicrowave.co.uk
https://www.kirkbymicrowave.co.uk/
Telephone 01621-680100./ +44 1621 680100

Registered in England & Wales.
Company number 08914892.
Registered office:
Stokes Hall Lodge,
Burnham Rd,
Althorne,
Chelmsford,
Essex,
CM3 6DT,
United Kingdom

Dr. Kirby, et al,

Remember that a GPS-disciplined Rb has two loops, hence two time constants,
to
consider:

a) the loop which locks the Rb's internal OCXO to the atomic transition, and
b) the GPS disciplining loop.

I surmise that the internal loop (a) is fast enough to thoroughly suppress
drifts of
the unit's internal OCXO in the face of local temperature variations etc.

But the other loop (b) can be, and should be, pretty slow (time constant of
hours
or more), so as to prevent the disciplined source being jerked around by
GPS "noise".
This is why the Rb can advantageous over the OCXO- the Rb is inherently
stable enough to permit using a very slow loop for disciplining by GPS.

In my own experience to date, the one GPSDO I have with user-adjustable loop
bandwidth suffers greatly from ambient temperature variations when I make
the
disciplining loop TC large enough to effectively remove GPS noise.  I can
easily
see HVAC cycling, for example, when I use a long time constant.  I've played
with numbers ranging from 5 sec up to 500 sec, and think the best compromise
is around 40 sec.  This with an old Trimble T'Bolt which was apparently
built
around 2004 or thereabouts.

My other GPSDO is a CNS Clock II, purchased new about a year ago.  It does
not have provisions for user adjustment of the disciplining loop, which
is fixed
at some rather short time constant (I'm estimating roughly 5 sec).  So its
OCXO
is kept pretty honest in the face of ambient temperature variations; however
GPS noise jerks it around very considerably. in the short term.  I'm
finding this
device's greatest utility in looking at phase drift in my Rb sources over a
period
of several hours at a time, mainly for purposes of frequency setting.

My two Rb's are an old (telecom-modded) PRS-10 and an old L-PRO, neither
of which has GPS disciplining capability.  I continue to agonize over
whether or
not I should buy new (standard-featured) PRS-10- I've been trying to make
this
decision for about 2.5 years so far :-)

I do not have access to the revered HP 10811A, so can't speak to its value
for my interests.  I'd dearly love to borrow one, as it would undoubtedly
provide
a good learning experience for me.

Dana

On Tue, Sep 17, 2019 at 3:11 AM Dr. David Kirkby <
drkirkby@kirkbymicrowave.co.uk> wrote:

On 9/17/19 3:33 AM, Dr. David Kirkby wrote:

There are a bunch of interesting tradeoffs in choosing a frequency
reference.

Any Rb (except the HP 5065A which is in a different class as a lab
instrument vs. the small telecom units) will be worse at short tau than
a good OCXO, and is also likely to have much worse phase noise.  A
typical telecom Rb will be around 1e-11 at 1 second while a good OCXO
can be one or even two orders of magnitude better.

At medium tau (say a few thousand seconds) the Rb will likely be in the
mid to upper 13s, which is better than any but a very good OCXO.

At long tau, the Rb should show at least an order of magnitude less
drift than even a very good OCXO.

A Cesium at short tau will typically be worse than either an OCXO or an
Rb.  The Cs only wins (a) at long tau since there is zero drift; and (b)
for absolute accuracy.  But at anything shorter than around 10K seconds,
it's not the best choice.

A good GPSDO is really the overall performance winner -- short term
stability and phase noise limited only by the quality of the OCXO, and
very good long term stability and accuracy due to the GPS lock.  It's
only in the mid range of a few hundred to to a couple of thousand
seconds, where the OCXO drift kicks in before the GPS discipline takes
over, that a GPSDO will underperform a telecom Rb.

In short, the GPSDO takes much of the fun out of time-nuttery. :-\

John

On Mon, Sep 16, 2019 at 11:00 PM Dana Whitlow k8yumdoober@gmail.com wrote:

How does this compare with say a used Accubeat AR133/60 substance? I also
see they have a Nano unit that looks awesome (but I have no idea what the
price is and if they are ever sold used?).

-aps