Hi

I have a 740 as do others on the list. The big gotcha is the OCXO in the device.
It has various quirks that limit the performance of the device. The ADEV plot they
show for the OCXO is a “sometimes / maybe / might” sort of thing. One of the
“get to it eventually” projects here is to see if it can be replaced with something
that has better ADEV performance.

Be wary of folks showing up in dirt parking lots in the middle of Montana with one
in the back of their car …. :) Doubly so if the car shows signs of having hauled
a dozen Cs standards up various mountains….. If they are named Tom, simply
give up and accept that once you see one you will eventually buy one…

The process goes sort of like: Gee, that’s a lot of money. Later on, … I need a few
more standard lines at this or that frequency … Then it goes to If only I could come
up with that signal … hmmm … 740 would do that … hmmmm…. off you go to
get one.

It’s a handy box to do a variety of things with. The only issue I’ve seen is the OCXO
performance.

Bob

I think that before one places much faith in the ADEV data, it's important
to understand the thermal environment in which the tests were conducted,
compared to your own thermal environment.  The tests were likely to have
been performed in a well-stabilized ambient temperature, to make them
look good.  But your own results may be a lot worse due to HVAC cycling
etc.

One way to reduce such effects is to make the GPS locking loop quite fast,
but doing so is at the cost of the disciplines oscillator's being yanked
around
in phase by "GPS noise", which is really quite awful.  At the other
extreme,
(long locking loop time constant) one largely escapes GPS noise, but finds
that his system has essentially become an expensive thermometer.  So I favor
the use of a GPS-disciplined Rb for critical work (or an active H-maser if
you're
really rich).

I'm presently crutching along with a manually-disciplined surplus PRS-10,
but
am looking for a good excuse to get a factory new lockable unit.

Aha:  KB8TQ's note just came in;  we're saying about the same thing- I'm
just
wordier.

Dana  K8YUM

On Tue, Nov 16, 2021 at 11:59 PM Matt Huszagh huszaghmatt@gmail.com wrote:

Bob kb8tq kb8tq@n1k.org writes:

Good to know. Have you tried disciplining the PRS-10 in one of these
units? Based on Dana's comments, it sounds like this may not exhibit the
same ADEV susceptibility to the thermal environment.

Matt

I have been looking at it for years had an email conversation with the designer and Bob. This box is not intended for time nuts and does sell very well, price has gone up over 20 percent.  It would be nice if one could use an external OCXO or Rb and using the Internet interface and have SRS download a Filter Time Constant for a fee based on data submitted. No deal.                                                                                                                                               Bert Kehren                                                                                                                                                                                                                                                                                          

In a message dated 11/17/2021 9:09:42 AM Eastern Standard Time, k8yumdoober@gmail.com writes: 
I think that before one places much faith in the ADEV data, it's important
to understand the thermal environment in which the tests were conducted,
compared to your own thermal environment.  The tests were likely to have
been performed in a well-stabilized ambient temperature, to make them
look good.  But your own results may be a lot worse due to HVAC cycling
etc.

One way to reduce such effects is to make the GPS locking loop quite fast,
but doing so is at the cost of the disciplines oscillator's being yanked
around
in phase by "GPS noise", which is really quite awful.  At the other
extreme,
(long locking loop time constant) one largely escapes GPS noise, but finds
that his system has essentially become an expensive thermometer.  So I favor
the use of a GPS-disciplined Rb for critical work (or an active H-maser if
you're
really rich).

I'm presently crutching along with a manually-disciplined surplus PRS-10,
but
am looking for a good excuse to get a factory new lockable unit.

Aha:  KB8TQ's note just came in;  we're saying about the same thing- I'm
just
wordier.

Dana  K8YUM

On Tue, Nov 16, 2021 at 11:59 PM Matt Huszagh huszaghmatt@gmail.com wrote:

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Hi

In this case it’s not thermal. I’ve tested a ton of parts in the same environment ( maybe
not 2,000 pounds but several hundred OCXO’s ).

The PRS-10 gives you that hump at 2 seconds no matter what you do. It’s part of the
basic design of the unit ….

Bob

But if you can stand the ADEV hump around 2 sec, the PRS-10 is delightful.

BTW, the ordinary factory new PRS-10 does provide for direct locking to 1
PPS,
but many of the available surplus units had that feature removed.  Tne new
PRS-10's sold for years for $1595, but I see that it has gone up to $1695
since
I had last checked.

Dana

On Wed, Nov 17, 2021 at 9:52 AM Bob kb8tq kb8tq@n1k.org wrote:

Dana Whitlow k8yumdoober@gmail.com writes:

Have you had luck steering the PRS10 with GPS? One individual mentioned
to me that despite repeated efforts, he was unable to discipline the
PRS10 without significantly degrading the short-term stability and phase
noise.

However, if the performance as reported by SRS isn't just for some
impossible, ideal environment than one solution would just be to
replicate the circuitry they use for the loop filter and divider to
generate the 10 MHz PPS signal. Or, to just purchase the FS740.

Matt

Hi Dana:

My PRS10 is a couple decades old and I use it in the 1PPS locking mode.  But that may not have been the default.  Maybe
the surplus units just need the correct jumper change.
https://prc68.com/I/PRS10.shtml

--
Have Fun,

Brooke Clarke
https://www.PRC68.com
axioms:

1. The extent to which you can fix or improve something will be limited by how well you understand how it works.
2. Everybody, with no exceptions, holds false beliefs.

-------- Original Message --------

Dana Whitlow writes:

The PRS-10 switches the polarity of the C-field solenoid at 5Hz to
cancel out varying external magnetic fields.

If that is not a concern for you, for instance because you use it
in a stationary application, it can be disabled with the "MS0" command.

It's all in the manual.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

"Poul-Henning Kamp" phk@phk.freebsd.dk writes:

How does the 5Hz switching relate to the 2s hump in the ADEV plot?

Matt

Matt Huszagh writes:

The manual says:

"[...]the current in the coil is switched at a 5 Hz rate."

You can either read that as:

"There are five positive and five negative periods every second"

or
"The sign changes five times per second"

It is not entirely obvious which reading is the correct one.

When I experimented with it ages ago, I concluded the latter fit
my data best, but that was a pretty early firmware version, with
quite a number of variances from the manual.

If your ambient magnetic field is stable, and it should be for
time-nuts purposes, modulating the hyperfine transition is a bad
idea, no matter the frequency.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

Poul,

You did not really answer the basic question: "Does the 2.5 Hz
(or 5 Hz if applicable) C-field reversal cause the 2-sec bump in
the ADEV plot for  the PRS-10?".

BTW, some radio hams are involved with microwave and even
MMW communications between mobile stations, and I'd bet they
benefit quite a bit from any scheme that reduces magnetic field
sensitivity of their frequency references.  Therefore, the C-field
reversal thing cannot be all bad.

Dana    K8YUM

Dana

On Thu, Nov 18, 2021 at 2:41 AM Poul-Henning Kamp phk@phk.freebsd.dk
wrote:

Dana Whitlow writes:

I cannot answer that question as stated.

My PRS10 died years ago, all I can say is that when I commissioned
it, I disabled the MS because it improved things.

In any mobile application, or other application where the magnetic
field is not nearly contant (for instance behind a steel door in a
outdoor base-station cabinet, I would absolutely keep MS on.

But for a time-nut installation, where the location is fixed,
temperature controlled and, I would assume, due consideration
is given to also keep the magnetic field from slamming around,
I suggest MS be disabled.

But it's a really trivial experiment to perform, so do it, measure
and decide what is right for you.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

Hi

The 5 Hz (or whatever) switching and the hump definitely both
originate in the same “feature” of the design. Does disabling the
switching nuke the hump? If one has a PRS-10 I’d say it’s well
worth trying.

The “filter” in the PRS-10 is not really set up for a GPS sort of
signal. It’s designed to handle a nice clean lab generated 1 pps.
It’s there to lock the device up as part of a calibration routine.
The 1 pps sync setups on other Rb’s are designed with the same
target in mind.

Bob

Bob kb8tq writes:

It is particularly bad at handling GPS's because of the "hanging bridge"
phenomena, and the better the GPS, the worse the result...

I tried injecting the "negative sawtooth" via the serial port to my
PRS10 but firmware features/bugs prevented that.

The offset could only be changed permanently in the saved configuration,
you could not change the running value on a second to second basis.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

Bob, I seem to remember reading in the manual on the PRS-10 where they
expounded
at length about how they took great pains to make the unit work well with
typical noisy
GPS PPS input.

I'm using a CNS Clock II as my primary GPSDO, checking phase slippage with
respect
to my PRS-10 to make decisions about when (or when not) to manually tweak
the PRS-10.
The CNS seems awfully noisy to me, even though the mfr says that it has
strong hanging
bridge correction.  I run the two 10 MHz signals through a quadrature
demodulator whose
I & Q outputs go into a 2-channel DSO running in extremely low "roll
mode".  I take a glance
at it several times per day, with each glance showing me the most recent
~4-hour history.
When I do tweak, I strive to set the PRS-10 about 1E-11 low in frequency,
which yields a
couple of months of hands-off operation before it drifts to 1E-11 on the
high side.  For
now, that has to suffice.  But I'd rather not have to pay so much attention
to it, which is
why I dream about buying a new PRS-10 with PPS locking capability.

Dana

On Thu, Nov 18, 2021 at 8:47 AM Poul-Henning Kamp phk@phk.freebsd.dk
wrote:

Hi

This is one of those classic examples of what marketing writes and what
(once the beer is poured) engineering actually knows. If you dive into the
grubby details of the filtering. Been there / done that with a number of
these guys.

It’s very much not set up for GPS. The hanging bridge stuff PHK mentions
in a previous post is one example. The need to adapt the filter to the monitored
results is another weak area. Just how and when this or that pops up varies,
but it will pop up.

Bob