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: http://www.amigazone.fi/files/gpsdo/tbolt-lpro-test.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 -- 73's Esa OH4KJU _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
