[time-nuts] Thermal impact on OCXO

Bob Camp kb8tq at n1k.org
Sat Nov 5 22:43:55 EDT 2016


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 <bob at evoria.net> 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 <kb8tq at n1k.org>
> To: Bob Stewart <bob at evoria.net> 
> 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 <bob at evoria.net> 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 <kb8tq at n1k.org>
> > To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
> > 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 <bob at evoria.net> 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 <azelio.boriani at gmail.com>
> > > To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
> > > 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 <bob at evoria.net> 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 <hmurray at megapathdsl.net>
> > >>  To: Bob Stewart <bob at evoria.net>
> > >> Cc: Hal Murray <hmurray at megapathdsl.net>
> > >>  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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> > > 
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