[time-nuts] Adjusting HP 5065A frequency
Sarah White
kuzetsa at gmail.com
Mon Oct 22 16:24:30 UTC 2012
(( replied at beginning to be consistent with this thread ))
Thanks, this has done a good job at furthering my understanding about
"error" (one way of describing adev as I understand) vs short and long
tau times. I don't have anything to add on that subject, but but I have
a question about a page you linked...
http://www.leapsecond.com/pages/gpsdo/
section: Trimble Thunderbolt GPSDO (OCXO version)
The text "This version of the Thunderbolt has an OCXO (unlike the model
shown on the Trimble web page). These are also recently available on the
surplus market."
just before chart image titled "Free vs. Locked: GPSDO [#a20]"
So when I did a quick fact check, I was immediately confused:
(re-quote) ...version of the Thunderbolt has an OCXO (unlike the model
shown on the Trimble web page)...
Fact checking with today's site:
http://www.trimble.com/timing/
The TBoltI can even find is the "Thunderbolt® E GPS Disciplined Clock"
Which in turn, if I follow the link:
Under "Key features" section:
"Ovenized quartz oscillator provides stable 10 MHz and 1 PPS output to
maximizes bandwidth"
Leaves me wondering. Does trimble even offer a non-ovenized TBolt at
this point? I can logically infer / assume that at one point, they did,
but I don't know what to think now.
New and curious,
Sarah
On 10/22/2012 7:32 AM, Tom Van Baak wrote:
> 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
>
>
>
>
>
>
> Información anexa:
>
>
>
>
>
>
>
>
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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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