Hi Bob,
Sadly, all I have is what I have. The GSPDOs are in an unvented Hammond
aluminum case. I've got a thermistor on the bottom side of the board.
Actually, it's right below the PIC, so the temperature from the PIC would seem
to be the driver of what I see. Except that I've also done some experiments
with putting a thermistor on the receiver plugin, as well as on the OCXO, and
right next to the EFC amp. I didn't see anything obviously different in the
temperature swings. So, from my POV, since the drift is about the same on each
unit, it's either from the OCXO or the voltage divider; which is a 15K/16K
divider.
Bob
-----------------------------------------------------------------
AE6RV.com
GFS GPSDO list:
groups.yahoo.com/neo/groups/GFS-GPSDOs/info
From: Bob Camp <[email protected]>
To: Bob Stewart <[email protected]>
Cc: Discussion of Precise Time and Frequency Measurement <[email protected]>
Sent: Saturday, November 5, 2016 9:43 PM
Subject: Re: [time-nuts] Thermal impact on OCXO
Hi
If you go back to my previous list, all the things that have a tempco also age.
Holding temperature
over weeks or months is not at all easy …. You have gradients between parts so
it is impossible to
look at temperature (or aging) as a single effect. In the case of a fairly
normal room, temperature may
well peak in one area when it hits minimum in another area. Everything has
hysteresis so it’s a never
ending process of perturbation and response.
The 3456 is not immune to any of this. It’s only rated as an 8 ppm device over
24 hours. Over 90 days
it’s about 3X that. The leads you connect it up to the DUT with are not easy
to do. Coming
up with a “perfect” answer at the 20 bit level is tough. A proper solution
would take the sum of the errors
down below 0.2 ppm. That can be done, but it’s a *lot* of work.
Bob
> On Nov 5, 2016, at 10:06 PM, Bob Stewart <[email protected]> wrote:
>
> Hi Bob,
>
> Could you expound on "so far below a number of issues"?
>
> In any case, your post implies that something else is aging. I'm using an
> ADR4533A as my voltage reference. That feeds an AD8638 op-amp driving an
> MMBT2222A pass transistor. That pretty much leaves the divider resistors on
> an AD8638 in the EFC string. But, aging is aging from my point of view. I
> guess I could prove that by hooking the EFC to my 3456A and plotting that
> over a few days.
>
> Bob
>
> -----------------------------------------------------------------
> AE6RV.com
>
> GFS GPSDO list:
> groups.yahoo.com/neo/groups/GFS-GPSDOs/info
>
>
> From: Bob Camp <[email protected]>
> To: Bob Stewart <[email protected]>
> Sent: Saturday, November 5, 2016 8:46 PM
> Subject: Re: [time-nuts] Thermal impact on OCXO
>
> Hi
>
> My experience is that the aging on these oscillators is so far below a number
> of issues
> that you will not be able to see it in under a year on a > 10 year old unit.
>
> Bob
>
> > On Nov 5, 2016, at 9:06 PM, Bob Stewart <[email protected]> wrote:
> >
> > Hi Bob,
> >
> > But will any of the things you mentioned show up as a more or less linear
> > march downhill at about 1 step per hour over the course of 7 days? In
> > fact, I think Dan's units show about the same -1 per hour, and he's had
> > them running continuously for many months. I believe you use these same
> > OCXOs, Bob. They're Trimble 34310-T units salvaged from China. What's
> > been your experience with them over the long run as far as aging is
> > concerned?
> >
> > Bob
> >
> > -----------------------------------------------------------------
> > AE6RV.com
> >
> > GFS GPSDO list:
> > groups.yahoo.com/neo/groups/GFS-GPSDOs/info
> >
> >
> > From: Bob Camp <[email protected]>
> > To: Discussion of precise time and frequency measurement
> > <[email protected]>
> > Sent: Saturday, November 5, 2016 7:37 PM
> > Subject: Re: [time-nuts] Thermal impact on OCXO
> >
> > Hi
> >
> > 2^20 is roughly 1 ppm. It is about 5 uV on a 5V line or 2.5 uV on a 2.5 V
> > EFC center.
> >
> > A DAC that does 100 ppm / C is a pretty typical part. 10 ppm / C is
> >unusually good
> > A “good” voltage reference might do 2 ppm / C
> > A very typical room will swing around +/- 2C without much going on.
> > Open the door in the room or open a window and 10C is not unusual.
> > Grabbing a normal banana plug with your fingers will give you > 10 uV,
> >other thermocouples are running around
> > A DAC with < 10 uV of noise in the 0.01 to 1 Hz region is unusually good
> >
> > That’s just looking at the easy to measure part of the system. A few
> > hundred uV delta on
> > the PCB between the OCXO ground pin and “ground” from grabbing the OCXO is
> > harder
> > to measure, but also pretty typical. It sums right into the EFC line.
> >
> > It’s not a rabbit hole so much as the details of the design at the level
> > you have chosen
> > to look at.
> >
> > Bob
> >
> > > On Nov 5, 2016, at 8:10 PM, Bob Stewart <[email protected]> wrote:
> > >
> > > This seems to be turning into a rabbit hole from which there can be no
> > > possible return. With no evidence, you question my competence and cast
> > > aspersions on my testing methodology. I give up.
> > >
> > > Have a nice day.
> > >
> > > Bob -----------------------------------------------------------------
> > > AE6RV.com
> > >
> > > GFS GPSDO list:
> > > groups.yahoo.com/neo/groups/GFS-GPSDOs/info
> > >
> > > From: Azelio Boriani <[email protected]>
> > > To: Discussion of precise time and frequency measurement
> > > <[email protected]>
> > > Sent: Saturday, November 5, 2016 7:02 PM
> > > Subject: Re: [time-nuts] Thermal impact on OCXO
> > >
> > > ...with a 20bit DAC, a suitable voltage reference for that DAC and an
> > > HP3458...
> > >
> > > On Sun, Nov 6, 2016 at 12:39 AM, Bob Stewart <[email protected]> wrote:
> > >> Hi Hal,
> > >>
> > >> With a 20 bit DAC, even a small aging rate is going to show up. I'll
> > >> let the one GPSDO cook for a month or so and see what it shows then.
> > >> I'll also pull the data from the log file and see if I can see any
> > >> correlation between the temperature and the EFC over time. IOW, for
> > >> data points separated by 3.5 days, does the temperature difference
> > >> between the two point seem to play a large part of the change in the
> > >> recorded DAC value.
> > >>
> > >> The graph was made from the logging data sent to the PC. But, the
> > >> firmware doesn't have access to historical data on the PC, so something
> > >> is needed for aging calculations. Tom mentioned here recently about the
> > >> 58503A GPSDO saving 64 hours of data for it's aging calculations. So, I
> > >> just added a few hours to that to get to 3.5 days of history in the
> > >> GPSDO.
> > >> Bob -----------------------------------------------------------------
> > >> AE6RV.com
> > >>
> > >> GFS GPSDO list:
> > >> groups.yahoo.com/neo/groups/GFS-GPSDOs/info
> > >>
> > >> From: Hal Murray <[email protected]>
> > >> To: Bob Stewart <[email protected]>
> > >> Cc: Hal Murray <[email protected]>
> > >> Sent: Saturday, November 5, 2016 6:23 PM
> > >> Subject: Re: [time-nuts] Thermal impact on OCXO
> > >>
> > >>> The OCXO has been hot for a number of months.
> > >>
> > >> Then I don't understand why it is still aging that much.
> > >>
> > >>
> > >>> I changed the firmware to save the DAC voltage every 30 minutes for 3.5
> > >>> day
> > >>
> > >> That's not very long.
> > >>
> > >> How did you make the graph? You had to get the data out to a place
> > >> where you
> > >> can plot it somehow. If you can do that, why are you "saving" the data?
> > >> I
> > >> assume you have a serial port to a PC or something like that.
> > >>
> > >>
> > >>> The fuzz in the temperature line is, indeed, the HVAC cycling
> > >>
> > >> You could try putting a box over the unit to see if that slows down the
> > >> temperature changes.
> > >>
> > >> Mostly, you are trying to block air flow.
> > >>
> > >>
> > >>
> > >> --
> > >> These are my opinions. I hate spam.
> > >>
> > >>
> > >>
> > >>
> > >>
> > >>
> > >> _______________________________________________
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