[time-nuts] Home made GPS disciplined atomic clock

Esa Heikkinen scifiscifi at sci.fi
Sun Jan 25 09:37:09 EST 2009

Hello everyone

Has anyone tried to replace Tbolt's OCXO with Efratom LPRO
Rubidium oscillator?

I just did that it seems to work, but I have no idea is it really better
or worse (or same) as original Thunderbolt. No ideas how to measure it.
Is it possible to measure allan deviation or such things by using the
logs generated by Tboltmon.exe?

So it would be nice to know if anyone has tried that before and done
some measurement against cesium or such standard to get exact results.

I think that it should be a much better (in theory) than OCXO which
comes short therm stability (what I'm actually seeking for). It should
be much more accurate with long holdovers also.

This is very simple modification by the way. Infact my original plan was
to use the 1PPS to synchronize the LPRO C-field with separate control
electronics (based own design with PIC and 24-bit DAC). But then I
noticed with spectrum analyzer measurements that the Tbolt's OCXO has
almost exactly same output level than LPRO: about +7 dBm and of course
they both have 10 MHz output frequency. So it would be possible to
replace OCXO with LPRO without any level matching or so... Also, with
changing the disciplining settings (DAC  voltages) even the control
voltage of OCXO can be fed directly to LPRO's C-field without any extra
electronics. This enables to use Tbolt's advanced steering algorithms
for C-field control without any own programming work here.

About the settings: Ko (Hz/V) for LPRO is about 0.006 Hz/V (if
calculated correctly from datasheet values) and minimum value what
Thunderbolt accepts for this is 0.01. However even 0.1 seems to work
(may be even better, causes smaller changes of DAC voltage). Min voltage
must  be set to 0.0V, because it's not allowed to feed any negative
voltages  to LPRO's C-field. However the LPRO manual says that it will
not break  with negative voltages under 8V. Initial DAC Voltage should
be set  somewhere about 2.5 volts, I run it some time with GPS lock and
noticed that good initial voltage with my LPRO was 2.56. Loop Dynamics
is big mystery. I'd like to think that very slow loop should be good for
  rubidium. So I tried 1000 secs as a test. But if anyone has tried this
  before could give some advice about loop dynamics?

There are even places for SMA-connectors of Thunderbolt PCB. One is the
10 MHz and another is the control voltage. After doing the modification
it may be a good idea to start with factory settings to reset any
"learned" data for OCXO and right after that reset change the DAC
voltage settings.

So... it seems to work. But it's quite difficult to tell anything of
it's performance. I just logged it for couple of hours (started after it
has been couple of hours up). Within that very short test time it seems
that at least the PPS offset value stays now inside same ns value at
longer times than with OCXO. With OCXO it changed many ns between
readings, quite randomly - but with rubidium the change is usually
couple of hundred picoseconds. Could this be a sign of better short
therm drift / random walk performance?

Here's a link for the log:
(Log format: TOW, PPS offset, DAC voltage, Disciplining mode & activity)

The log was created with following settings:

Time Constant:    1000
Damping factor:   1.2
Ko:               0.01
Initial DAC volt: 2.558V


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