[time-nuts] GPSDO control system

Tom Miller tmiller11147 at verizon.net
Sun Mar 23 17:44:13 EDT 2014

You are nine days too early.



----- Original Message ----- 
From: "Chris Albertson" <albertson.chris at gmail.com>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Sunday, March 23, 2014 5:26 PM
Subject: Re: [time-nuts] GPSDO control system

> 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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