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,
li...@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 <b...@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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