[time-nuts] Adjusting HP 5065A frequency
EWKehren at aol.com
EWKehren at aol.com
Mon Oct 22 14:46:32 UTC 2012
Tom
A good place to start is with a manual of the Austron 2055 delay if there
is real interest. Does any one have it in PDF form?
As to the disciplining a Rb or specifically a HP5065A, the filter part is
the challenge. Having worked on it for the last ten years always using
Brooks loop and developing work around's it will do every thing except
temperature control and barometric pressure. I have done temperature analog but rely
mainly on holding the Rb temperature stable within a tenth degree C.
Bob recently put things in perspective when there was talk about a 32 bit
DAC. On one side you have GPS and time averaging is required to get full use
of it. That is why a Rb is uniquely qualified for it. The original Shera
input implemented on a 1 $ G/A is more than enough for time capture wether
using a sawtooth corrected GPS receiver or a Tbolt. On the other end a
LTC1655 is again more than enough in an Rb application. You can use it 16 bit
direct or dither two or 4 bits. Filter is easy and do not forget that the
Rb has an additional filter between cell and OCXO. Brooks V402NE will do the
job, how ever others and I have not been able to buy some in the last nine
month. Any one knows what is going on?
What is needed is some one in the group be willing and able to step up to
the plate and develop the filter on a PIC with all the things learned, and
believe me, we are still learning as we adapt a M100/8600 via a Tbolt .
I will be willing to help with every thing except the PIC.
If you bring out the C field coil directly you have full isolation and do
not have to worry about ground loops.
A complete unit would cost less than $ 40 and more important be assembled
by any one. I brought it up before but no one responded. If no response I
will still be able to do every thing with the HP5065A RVFR that was given to
me.
Bert Kehren
In a message dated 10/22/2012 7:33:08 A.M. Eastern Daylight Time,
tvb at LeapSecond.com writes:
Hi Bert,
Not sure about the range/resolution. That would depend on how the standard
is used and what its frequency drift rate is. The stability doesn't have
to be too much better than the standard itself.
The Austron 2055 resolution is 1e-14, IIRC. The Symmetricom AOG is 1e-19
(overkill).
What I've found in some GPSDO and passive atomic standards (e.g., Rb or
Cs) is that as soon as you turn on the DAC and enable the loop, you get more
short-term noise, say in the range of 1 to 100 seconds. That's why for best
stability you always switch off the loop during a sensitive measurement.
Many older Cs had a "Cs off" switch for this. Not only did it conserve
cesium but it also means you're running straight off the high-quality OCXO. This
is also true for GPSDO, like the TBolt which allows you to turn off
disciplining with a s/w command.
In general, when you discipline a OCXO you get that characteristic ADEV
"hump". This is expected, a natural byproduct of combining two unknowns, one
that's assumed to be better at short tau and worse at long tau (e.g., OCXO)
and one that's assumed to be better at long tau and worse at short tau
(e.g., Rb cell, or Cs beam, or GPS receiver). At some point there is a
cross-over and you know/assume that at that point each must be contributing
1/sqrt(2) of the noise.
To see the humps in living color, refer to:
http://www.leapsecond.com/pages/gpsdo/
To answer your question about short/medium/long, I guess in this case
short is tau left of the hump; medium is the hump, and long is tau to the right
of the hump.
/tvb
----- Original Message -----
From: <EWKehren at aol.com>
To: <time-nuts at febo.com>
Sent: Monday, October 22, 2012 4:04 AM
Subject: Re: [time-nuts] Adjusting HP 5065A frequency
Tom
I have two questions what should the range, resolution and stability of
the delay generator be and how much do you think a digital loop driven by
a
Tbolt would degrade short and medium precision. What is your definition of
short and medium?
Bert Kehren
In a message dated 10/22/2012 12:25:08 A.M. Eastern Daylight Time,
tvb at LeapSecond.com writes:
Three companies come to mind for phase microsteppers. A popular one
decades ago was made by Austron (model 2055A). I got mine on eBay but
they are
not as common now as ten years ago.
The current models by Symmetricom and Spectra Dynamics are extremely
high-end (expensive) and overqualified for use with a vintage rubidium
oscillator. If you visit NIST or USNO you will see these impressive units.
It would be a very fun project to make your own. I suspect other group
members could either help you or would eagerly employ your design for
their
own use.
But -- before you decide on a hardware solution see if you can do it in
software.
An analogy is what we do with GPS 1PPS sawtooth errors. There are two
ways
to deal with this. One is to capture the correction message over RS232,
measure the DUT vs. GPS 1PPS with a TIC, and then numerically apply the
sawtooth correction with one line of code. Several of the popular GPS
monitor
programs do this automatically for you (TBoltmon and TAC32, for example).
The
software solution is perfect to the granularity of the sawtooth message,
typically 1 ns.
The hardware implementation usually involves a PIC and a programmable
delay generator. The PIC listens for the correction message over RS232
and then
has plenty of time (up to one second) to program the delay chip. When the
hardware 1PPS arrives it is delayed to compensate for the aforementioned
sawtooth error. The result is a hardware 1PPS that's quite close to the
ideal
1PPS, limited again by the granularity of the message, as well as offset
or linearity errors in the delay chip.
So that's the analogy. To apply this to your rubidium, ask yourself which
instruments or measurements or users are downstream of your 5065A 10 MHz
output. Can they deal with daily software corrections to a stable but
slightly imprecise frequency, or do they really need the frequency to be
as
accurate as possible at all times.
There's a third alternative as well. You might consider using your 5065A
as the LO in a GPSDO. This will sacrifice some short- and mid-term
precision
due to additive noise, but it will guarantee the best possible long-term
accuracy.
/tvb
----- Original Message -----
From: Edgardo Molina
To: Tom Van Baak ; Discussion of precise time and frequency measurement
Sent: Sunday, October 21, 2012 8:49 PM
Subject: Re: [time-nuts] Adjusting HP 5065A frequency
Dear Tom,
Good evening. In relation to your last comments on this and other
subjects,
I am sharing some thoughts and experience about it. I took the liberty to
separate the topics as to ease the interested parties to follow up
accordingly. TNX.
a. Information you kindly provided and the index for newbies:
Thank you! You just provided me with lots of new ideas and information on
the subject. You have very valuable information in your web site. As Hal
was saying, an index should be done anywhere so it could be easier for
the
rest of us to locate the information. I am planning soon to build a web
page
for my lab. In english of course for everybody to share my experiences. I
could work on an index to point out to the various sources of information
and topics that are difficult to find. That I think could expedite things
a
little bit.
b. Phase Micro steppers:
I saw the phase micro steppers working at CENAM time scale. I was
wondering that the technique could be translated to my 5065As and not
trying to
touch them so often. If I am assuming correctly and the technique could
be
used with the HP Rb standards. Are those phase micro steppers easy to
find? I
mean, affordable in the second market? If there is one of course. I saw
the
ones used at CENAM are produced by SpectraDynamics in Colorado. According
to Mike Lombardi it is a small highly specialized company with a small
market to serve. I could translate it as "expensive and exotic" : ) Am I
correct?
c. Thunderbolt and my will to share initial experiences:
I am gathering a lot of information on the Thunderbolts as I am using
them
in my thesis work. I bought a couple of them. If my information or novice
experience with these receivers is good for anybody, I would be more than
glad to share it.
Thank you.
Kind regards,
Edgardo Molina
Dirección IPTEL
www.iptel.net.mx
T : 55 55 55202444
M : 04455 20501854
Piensa en Bits SA de CV
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On Oct 21, 2012, at 7:29 PM, "Tom Van Baak" <tvb at LeapSecond.com> wrote:
Hi Edgardo,
What you'll find is that many labs do not periodically adjust the C-field
of their 5061A or 5065A at all.
Instead, any phase or frequency adjustment is done with phase
microsteppers or simply done in software with time and rate adjustments to
the raw
data. These methods avoid all possible physical side-effects of changing
voltages, currents, and fields. It also makes it possible to gather
long-term
data to show how the standard is operating (if you make mechanical rate
adjustments it complicates data that you have already collected).
The other point is that when making stability measurements, there is no
requirement that the reference (e.g., 5065A) be perfectly on-frequency.
So
this removes motivation for physically touching and possibly perturbing
the
operation of the reference.
Please also take the time to read these pages.
"HP 5065A Rubidium C-Field Resolution"
http://leapsecond.com/pages/hp-5065a-cfield/
"Rubidium Oscillator Stability"
http://leapsecond.com/images/4rb.gif
"Stability and Noise Performance of Various Rubidium Standards"
http://www.ke5fx.com/rb.htm
"Performance of Low-Cost Rubidium Standards"
http://febo.com/pages/oscillators/rubes/
"A close look at a drifting HP 5065A Rubidium Frequency Standard"
http://leapsecond.com/pages/doug-rb/
/tvb
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