[time-nuts] GPSDO control system
Tom Miller
tmiller11147 at verizon.net
Sun Mar 23 21:44:13 UTC 2014
You are nine days too early.
:)
Tom
----- 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
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com
>>> To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>> and follow the instructions there.
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>
>
>
> --
>
> Chris Albertson
> Redondo Beach, California
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
More information about the Time-nuts_lists.febo.com
mailing list