[time-nuts] GPSDO control system
EWKehren at aol.com
EWKehren at aol.com
Sun Mar 23 14:32:40 EDT 2014
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
In a message dated 3/23/2014 9:02:59 A.M. Eastern Daylight Time,
lists at rtty.us writes:
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
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
8) Is your room temperature representative of the real world? (is mine
9) Do you intend this gizmo to work over a temperature range? Did you test
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 .. ..
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
> and follow the instructions there.
time-nuts mailing list -- time-nuts at febo.com
To unsubscribe, go to
and follow the instructions there.
More information about the time-nuts