Hallo all the Time Nuts experts, I need some help to find the understanding for a optimized setup with Trimble's Thunderbolts with original and other OCXOs like HP's 10811.
At the first glance the output of the TB does look quite good if I trust the values given by LH adev etc. But how good are the data gained without an external reference, may I believe them? (Ask a baker if his bread is good...!) How do they judge the quality, what they refer to? But I observe some things which makes me thinking and raising questions. If I look to the graph of Mark Sims' and John Miles' Lady Heather Program I see eg. up to 500µV EFC jumps due to SATs phasing in or out (where every µV does count!) resulting in PPS changes of around 7ns and frequency changes of 100ppb and more within several minutes or sudden jumps. I thought that this should be eliminated by processing and filter routines. I have adapted a HP-10811 oscillator (Z3801-Model, outer heater not connected) to one of my TBs as shown by John Miles on his homepage. Was not difficult at all. To get a base I did run it undisciplined in the TB system with a fixed EFC (set to get as close as possible 0.00ppb) and watched the output of LH. Setting different voltages I could easily derive the gain factor. Astonishing for me, I found that the 10811 does behave better in undisciplined mode for a limited time: After a warming up time of several weeks this oscillator does actually show a very low drift with the EFC Voltage set to around -80mV (-0.079724V actually, drifting about +1mV per week) with a gain set to -0.7425V/Hz. (I used LH with discipl. mode disabled to determine these values.) The adev values for the undisciplined 10811: pps oadev over 2311 pts 1 tau 5.5578e-011 2 tau 4.0426e-011 5 tau 3.6200e-011 10 tau 4.6242e-011 20 tau 5.4782e-011 50 tau 3.1047e-011 100 tau 1.9876e-011 200 tau 1.4337e-011 500 tau 8.0590e-012 1000 tau 9.0174e-012 osc oadev over 2311 pts 1 tau 3.5775e-009 2 tau 1.8014e-009 5 tau 7.1510e-010 10 tau 3.7145e-010 20 tau 1.8131e-010 50 tau 7.5577e-011 100 tau 3.6502e-011 200 tau 1.9246e-011 500 tau 8.0409e-012 1000 tau 4.5730e-012 I stopped there because the pps line began to drift away. The frequency curve, beside the jitter of max. +- 50ppb peaking, was smooth and varied along the straight center line about to +- 20ppb. At this point the efc should already have started pulling very smooth the pps back to zero +- a tolerance. But when I run the TB in normal mode with disciplining on then the oscillator curve starts to be a bit nervous as the efc and the pps lines do surf slowly up and down with possible jumps as already mentioned. I tried to modify the damping factor, the time constant and as well the gain. I never got both, the pps and the osc, calm. When I run it with a time constant above 500s, the osc seem to be more stable but the pps starts to behave worse. The best values I could get that way for the disciplined TB. w. a 10811 ocxo after trying a lot of setup combinations are: pps oadev over 10810 pts 1 tau 2.4865e-010 2 tau 1.7802e-010 5 tau 1.1912e-010 10 tau 9.3686e-011 20 tau 7.6403e-011 50 tau 4.9101e-011 100 tau 3.5379e-011 200 tau 2.0019e-011 500 tau 1.1316e-011 1000 tau 7.6236e-012 2000 tau 4.6755e-012 5000 tau 3.2676e-012 osc oadev over 10810 pts 1 tau 6.0651e-009 2 tau 2.9834e-009 5 tau 1.2081e-009 10 tau 6.0071e-010 20 tau 3.0075e-010 50 tau 1.2337e-010 100 tau 6.3227e-011 200 tau 3.2970e-011 500 tau 1.3461e-011 1000 tau 6.8074e-012 2000 tau 3.4545e-012 5000 tau 1.4172e-012 (I see just now, I should have waited for 1 step more to get 10811 pts. ;-) If this would stay stable during days and weeks I do not know yet. So, what do I wrong, what can be done better, what is the way to get the maximum out from the TBs? The values gained with the original ocxo from trimble are even worse.What I am looking for is to get more stability at lower tau steps. Why do I observe a crossing of the pps and osc oadev curves, which I did not see with the original oscillator? Btw. the bottom side of the OCXO I unmounted is marked with 'Phase Noise'. It looks like that this series was equipped with selected OCXOs for low phase noise...! I would appreciate it very much to get comments and advices. many thanks i.a. and regards Arnold _______________________________________________ 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.
