[time-nuts] Thermal impact on OCXO
Bob Camp
kb8tq at n1k.org
Mon Nov 7 20:44:21 UTC 2016
Hi
This is your design, and under your control. Looking at one plot here or there and trying to
guess what is going on is not a really good way to do this. The whole sub-thread we are on
is simply a result of a “if it’s not crystal aging it can’t be anything else” comment way way back.
My point here is that it *can* be something else and the list of things it *can* be is actually quite
long. The voltage measurement stuff is simply one example out of at least a dozen and probably
closer to a hundred possibles. Digging through all of those possible things is a lot of work. It’s work
that can only be done “hands on” with a set of units and test gear. My first step would be to toss
the setup into an environmental chamber and do some 48 hour temperature sweeps.
Aging is a “long term” drift. It goes on for years. Warmup is a very short term effect. It goes on for hours
or days. Retrace is the process of re-settilitg to the previous long term aging line. It can take days / weeks /
months depending on the oscillator. If you were buying the OCXO’s as an OEM, the specs you
bought them to would be a function of what you asked for and how much you were willing to pay. It
is unlikely that as an OEM you would have requirements on retrace and warmup, and aging. If you
did have a spec it would be under fairly specific conditions (on power for three months, measure
frequency, off power at XX C for 24 hours, on power again for 24 hours at YY C, measure delta
frequency. That would cover warmup, but really says nothing about aging or retrace.
Regardless of if an OCXO is spec’d for this or that issue or not, it experiences it. There are a lot of
other things that OCXO’s do that many or may not be spec’d. When using one and digging into it,
you have to deal with all the issues, regardless of if there is a spec or not. That assumes you want
to dig in at this level. A *very* reasonable question is - what level do you want to dig into? That is
why I bring up the good old “what is the goal?” question on a regular basis. Without a goal in mind,
you will keep digging ever deeper. The number of issues you need to look into grows exponentially
as you dig deeper. The time, learning, equipment, experiments, and number of test units each go
up rapidly. It is work, equipment, and experiments that only apply to your design. It is not a piggyback
process as you go deeper.
Yes, that’s a bit of a long winded reply … sorry about that.
Bob
> On Nov 7, 2016, at 2:23 PM, Bob Stewart <bob at evoria.net> wrote:
>
> Hi Bob,
>
> OK, given that, what is your view of the plot I presented? Let's remember that I plotted this because I was trying to discover whether the EFC circuitry has a time-related drift, or whether the DAC value drift I plotted with the GPSDO locked to the GPS was caused by the OCXO.
>
> In one of your posts today, you talked about a long-term retrace lasting some months. I've been using the term "aging". Has my choice of terms caused the discussion to take a turn that it wouldn't have taken if I had used the term "retrace"? The fact is that I don't know whether it's aging or retrace. I'm not really clear on how to tell the difference. I'm especially handicapped by not having the mfg specs for the OCXOs I use. It could be that I simply haven't had the test unit under power for long enough to let the OCXO settle out. And for sure when I added the test lead to the EFC (breaking a trace in the process) there was enough off time to thoroughly insult the OCXO.
>
> Bob
>
> -----------------------------------------------------------------
> AE6RV.com
>
> GFS GPSDO list:
> groups.yahoo.com/neo/groups/GFS-GPSDOs/info
>
>
> From: Bob Camp <kb8tq at n1k.org>
> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com>
> Sent: Monday, November 7, 2016 1:01 PM
> Subject: Re: [time-nuts] Thermal impact on OCXO
>
> Hi
>
> Thermocouples in test leads are nasty because they operate on the temperature
> delta rather than the absolute temperature. It’s the gradient you have to worry about.
> Until you have run out a few dozen sensors around your bench (or test chamber) and
> monitored them for a few days … you won’t believe just how little “point A” has in common
> with “point B”.
>
> Bob
>
> > On Nov 7, 2016, at 1:44 PM, Bob Stewart <bob at evoria.net> wrote:
> >
> > -accidentally omitted timenuts from my reply to bob-
> > Hi Bob,
> > The data is at the 10uV level. As to whether I've plotted "data" or data, I can only report what I have. It seems to be consistent over the long run. I would expect to see something else if it were just noise. If the 4 attachments were strongly thermocoupled, wouldn't the data have a stronger correlation to temperature, rather than just to temperature transients?
> >
> > Bob -----------------------------------------------------------------
> > AE6RV.com
> >
> > GFS GPSDO list:
> > groups.yahoo.com/neo/groups/GFS-GPSDOs/info
> >
> > From: Bob Camp <kb8tq at n1k.org>
> > To: Bob Stewart <bob at evoria.net>
> > Cc: Discussion of Precise Time and Frequency Measurement <time-nuts at febo.com>
> > Sent: Monday, November 7, 2016 11:54 AM
> > Subject: Re: [time-nuts] Thermal impact on OCXO
> >
> > Hi
> >
> > I would suggest you read the papers on the subject. They are out there. Even over a few days
> > OCXO aging is likely to be non-linear ( => curved, not straight line). On an OCXO that is
> > warming up, your are dealing with retrace rather than aging. This can have an impact for a
> > few months after turn on. I have one unit in the basement that took 9 months to come out
> > of retrace after being in storage for “a while”. It was an eBay item so no idea just how long
> > the power off time was.
> >
> > Bob
> >
> >
> >> On Nov 7, 2016, at 12:47 PM, Bob Stewart <bob at evoria.net> wrote:
> >>
> >> Hi Bob,
> >>
> >> When you say that OCXOs don't age in a linear fashion, what does that mean? IOW, is this a case where it's almost linear over a week, and the non-linearity is only detectable when you consider the longer term? If that's the case, then it seems reasonable to use the past 3 or so days of data collection to project the near-term behavior; where near-term is less than 3 days into the future.
> >>
> >> Bob
> >>
> >> -----------------------------------------------------------------
> >> AE6RV.com
> >>
> >> GFS GPSDO list:
> >> groups.yahoo.com/neo/groups/GFS-GPSDOs/info
> >>
> >>
> >> From: Bob Camp <kb8tq at n1k.org>
> >> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com>
> >> Cc: Scott Stobbe <scott.j.stobbe at gmail.com>
> >> Sent: Monday, November 7, 2016 6:08 AM
> >> Subject: Re: [time-nuts] Thermal impact on OCXO
> >>
> >> Hi
> >>
> >>
> >>> On Nov 5, 2016, at 10:43 PM, Bob Stewart <bob at evoria.net> wrote:
> >>>
> >>> Hi Scott,
> >>> D'oh. Thanks for the correction! Like I said, I don't do these calculations often.
> >>>
> >>> If as Bob Camp implies, the aging isn't from the OXCO, then I'm a bit stumped. I do have an op-amp in the EFC string with a voltage divider for gain. The resistors are Panasonic ERA-6AEDxxxV resistors. Mouser says they're temperature stable to 25PPM/C, but of course they don't mention an aging rate. I don't really see anything else, other than the OCXO, that is likely to be prone to a linear type of aging.
> >>
> >> OCXO’s don’t age in a linear fashion. At least 90% of them don’t. If you dig into the FCS papers there are various
> >> curves proposed as models. Mil-O-55310 has one of them as the “official” approach. All of them have the basic
> >> issue of mistakenly fitting to to short a time constraint.
> >>
> >> Bob
> >>
> >>> The aging rate appears to be stable from unit to unit, so naturally I considered the OCXO first.
> >>>
> >>> There is one other bit in the EFC string that might be controversial, but I don't see that it would be a candidate for the symptoms of aging.
> >>>
> >>> Bob
> >>> -----------------------------------------------------------------
> >>> AE6RV.com
> >>>
> >>> GFS GPSDO list:
> >>> groups.yahoo.com/neo/groups/GFS-GPSDOs/info
> >>>
> >>> From: Scott Stobbe <scott.j.stobbe at gmail.com>
> >>> To: Bob Stewart <bob at evoria.net>
> >>> Cc: Discussion of Precise Time and Frequency Measurement <time-nuts at febo.com>
> >>> Sent: Saturday, November 5, 2016 9:19 PM
> >>> Subject: Re: [time-nuts] Thermal impact on OCXO
> >>>
> >>> If your DAC spans the full EFC range than 1LSB is 1/2^20 ~ 1 PPM of the EFC range, and the EFC tuning range is 8/10E6 ~ 1 PPM full scale, so 1 LSB is ~1PPT. So, if everything else is stable the DAC code reflects changes solely due to the OCXO, which would be an aging of 24 PPT/day.
> >>> On Sat, Nov 5, 2016 at 9:57 PM, Bob Stewart <bob at evoria.net> wrote:
> >>>
> >>> Hi Scott,
> >>> The 20 bits span about 6 volts. The EFC range spans about 8Hz (+/-4Hz). I don't do these calculations every day, but that's about 4.5PPT?
> >>> Bob ------------------------------ ------------------------------ -----
> >>> AE6RV.com
> >>>
> >>> GFS GPSDO list:
> >>> groups.yahoo.com/neo/groups/ GFS-GPSDOs/info
> >>>
> >>>
> >>>
> >>> From: Scott Stobbe <scott.j.stobbe at gmail.com>
> >>> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com>
> >>> Sent: Saturday, November 5, 2016 8:38 PM
> >>> Subject: Re: [time-nuts] Thermal impact on OCXO
> >>>
> >>> I think that's a nice plot, it looks like you have stepped 160 LSB over 7 days or roughly 1 LSB per hour. With a 20bit dac you are trimming maybe 1 ppt/LSB to 4 ppt/LSB? In allan devation terms, the case of 1ppt/LSB, solely due to drift, you're at 1E-12 at 3600*sqrt(2) = 5000 s, in the case of 4ppt/hour your at 1E-12 at 1280 s. Seems reasonable.
> >>> On Sat, Nov 5, 2016 at 2:47 PM, Bob Stewart <bob at evoria.net> wrote:
> >>>
> >>> Oh dear. I attached the wrong file. Here's the correct one.
> >>> ----------------------------- ------------------------------ ------
> >>> AE6RV.com
> >>>
> >>> GFS GPSDO list:
> >>> groups.yahoo.com/neo/groups/ GFS-GPSDOs/info
> >>>
> >>>
> >>>
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