[time-nuts] GPSDO control system

Chris Albertson albertson.chris at gmail.com
Sun Mar 23 17:26:12 EDT 2014


I'm working on a GPSDO but with different goals.  I want mine to be

1) very low cost, under $50 for everything if I can
2) No PCB required.
3) very easy to replicate by a first time builder
4) Easy to understand.  The parts count is very low, no "exotic" parts
and the software written very clearly so the code reads like the
tutorial for a beginner.

I started with Lars' Arduino based design and I've making slight mods.
 But shipping from China takes a month and I'm waiting on parts.    I
expect only  1E-11 level performance

The next one I build I want to be different. I don't need  yet another
copy of an old design.

1) Can I combine two oscillator technologies to get the best of both?
Perhaps phase lock an OCXO to a Rb and then discipline the Rb's
frequency.

2) is there some good way to control the temperature of the entire
assembly?  Perhaps dunk the entire thing into a container of
transformer oil?  Or use thermal epoxy to connect all the critical
parts to ONE common heat sink and then keep that at constant
temperature.   First I need some way to measure temperature very
accurately.

3) are 10MHz crystals the most stable ones?  I bet there is a sweat
spot frequency that is better.  Would 50MHz or 5Mhz be better?

4) why use a PPS as the communications link from GPS to GPSDO?  Can I
find a way to move the OXCO into the guts of a GPS receiver.  That may
mean I have to build a SDR based GPS receiver.

But first the simple one.  I'll post progress reports and photos

On Sun, Mar 23, 2014 at 11:32 AM,  <EWKehren at aol.com> wrote:
> Hi
> There are many issues when it comes to a GPSO. But what has to  be first
> discussed what is it one wants to accomplish. Last year when we worked  on the
> latest Shera GPSDO we always got better than 1E-11 with a unit lying on
> the bench with no enclosure or thermal management.
> Chasing elusive 1 E-13 and better, allow me to make a couple  of comments.
> In order to get there, the total system has to be under  review.  Since I
> know nothing about writing programs I leave that to  smarter people but be
> clear software and code will not do it by it self. The  most critical part is
> the thermal management of the OCXO or Rb and if analog  control is used the
> DAC.and if used its output amp. We are controlling the back  plate of the
> M100 and FRK to within 0.01 C and the front 0.1 C. The  DAC board and the
> temperature controller are on the front, Voltage regulators on  the back.
> After extensive testing the LTC1655 is our preferred  choice. Take a close
> look specs are great for this application and most  important solderable.
> There are better DAC's out there  but very expensive and I am not able to
> solder. 18 bits would be nicer but 16  bits are for Rb's usable. The DAC part
> has to have its own ground plane because  ground loops can create noise and
> voltage changes it has to be tied as close and  separate to the OCXO or Rb. No
> opto Isolation necessary as long as the  controller and DAC are in the same
> box, sharing the same system ground.  Input  to the DAC can handle wide
> ground variations. Took me years to find  that out.
> Absolute must  how ever is opto  isolation between GPS, controller and PC,
> again found out the hard  way.
> The other part I like to touch on is the GPS input section. I  am not a
> time nut but a frequency nut, but there has been so much talk in the  past and
> more recently about sawtooth. I am disappointed but not surprised that  no
> one has stepped up and offered a solution. The site has deteriorated to a lot
>  of talk very little action. Right now there are still affordable and
> solderable  DS1023's out there. Combined with a 12F629 or 12F1840 a sawtooth
> correction is  possible for much less than $20. I bought last year four DIP and
> 10 in SOIC DS's  average price  below $ 5!  Even though I had a very bad
> experience once with a Dutch so called time nut I am willing to make the
> following offer. For the right person I make a board, PIC and DS1023 available.
> Maybe I just missed it but I do not think that there is something out there
>  readily available.
> Bert Kehren
>
>
> In a message dated 3/23/2014 9:02:59 A.M. Eastern Daylight Time,
> lists at rtty.us writes:
>
> Hi
>
> The real answer is  always "that depends".
>
> 1)  How much does the sensitivity of your OCXO change with a change in EFC?
> 1.4:1,  2:1, 4:1 .... (slope sensitivity not % linearity)
>
> 2) How quiet is your  DAC compared to your OCXO?
>
> 3) How quiet is your reference compared to  your OCXO?
>
> 4) How much do the DAC, reference, op-amps, resistors,  capacitors, ... drift
> with time?
>
> 5) How much test time is enough?  (hours, days, weeks ,.....)
>
> 6) How good is the survey on your GPS this  time?
>
> 7) How much does your room temperature impact your OCXO when you  do this
> or that?
>
> 8) Is your room temperature representative of the real  world? (is mine
> like yours?)
>
> 9) Do you intend this gizmo to work over a  temperature range? Did you test
> that range?
>
> 10) Are you trying for best  frequency or best time? Is your definition of
> time "GPS local  time"?
>
> 11) Are there voltage drops on your real board? Do they change  with
> anything? (or everything ?)
>
> 12) Does your controller generate  spurs inside the control loop and
> modulate the output with them when tuned to  an offset of x.xxxxxxxxxxxxxxxxxx Hz?
>
> 13) How do things respond to  load changes or supply voltage changes?
>
> 14) Are the parts (OCXO,  reference, dac, op amps ...) responses to
> temperature, load, supply, tip,  tune,  linear / immediate or do they have artifacts
> that extend out over  longer time periods?
>
> This is by no means a complete list. A lot of  common GPS issues are
> notably absent.  However, I've seen designs fail or  fall short for problems
> related to every item on that list. Can you put this  all in a model - sure. Did
> you put all this in the model ..  ..
>
> Bob
>
>
> On Mar 23, 2014, at 5:06 AM, Bill Hawkins  <bill at iaxs.net> wrote:
>
>> An idea is struggling to take shape  in my fevered brain.  I'd like to
>> check some foundation  assumptions.
>>
>> 1. The difficulty with disciplining a local  oscillator to a GPS signal
>> is due to variations in the received GPS  signal and the LO.
>>
>> 2. The variations occur slowly, as crystal  aging, and quickly - perhaps
>> sawtooth or crystal crack propagation -  and maybe something in between.
>>
>> 3. The gain of the system, in  degrees of phase angle at 10 MHz (or
>> higher) per microvolt of control  signal, is fairly constant in a
>> controlled environment.
>>
>> 4. The power supply for the device providing the control signal  cannot
>> be regulated to the accuracy required of the system, and so is  a source
>> of variance. (Does anyone put the voltage reference device in  the oven
>> with the crystal?)
>>
>> 5. The principle source  of environmental variation is temperature.
>> Humidity and barometric  pressure are not significant. This may not be
>> true of the received GPS  signal due to atmospheric variations.
>>
>> 6. A digital  computational device is available to calculate the control
>> signal from  various measurements and previous values.
>>
>> 7. There are no  supernatural forces at work, such as the experimenter
>> mentally  influencing the results. :-)
>>
>> That's a start . . .
>>
>> Thanks for any replies.
>>
>> Bill Hawkins
>>
>>
>>
>>
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-- 

Chris Albertson
Redondo Beach, California


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