Hi The only thing I would add to that is that the drift due to the OCXO *plus* the DAC *plus* the voltage reference shows up as 0.7 mV. If you dig into it, much of this is due to the DAC + ref.
One possible “fix” on a low tune voltage OCXO would be a resistive attenuator made up of a pair of good ( so $20 each …) resistors. Just how much this would help …. you’d have to try it and see. More or less: If the OCXO tunes on frequency at 0.2V (as some do), put on something in the 10:1 to 20:1 range. The DAC would not center up around 2V to 4V. The gain setting would need to be changed. LH has a very nice tuning feature to measure this and reprogram the TBolt. Bob > On Jul 24, 2021, at 1:44 PM, Stewart Cobb <[email protected]> wrote: > > When one starts trying something new, it can be hard to tell whether one's > results are good or bad. That knowledge comes from experience, which is > exactly what one is lacking when trying something new. > > One solution is to look for results that others have obtained. Time-nuts is > pretty good for this sort of thing. The attached plot is a small > contribution to this knowledge base. This is an example of Thunderbolt > performance with a good antenna installation. > > This screenshot from Lady Heather shows roughly a year of satellite signal > strength data and the most recent 9.5 days of phase and DAC data. The exact > latitude and longitude are not shown, but the lab is located in Silicon > Valley, California, at about latitude +37 and longitude -122. > > The circular plot in the upper right is the signal strength data. By > default, LH "fills in" gaps in the signal strength data. I used the command > S-A-D to disable this fill and show only actual data. The satellite orbits > repeat exactly in sidereal time, which means that they drift slightly in > ordinary time. Over a year, this drift is noticeable and helps to fill in > this plot. Some satellites are stronger than others, and the plot only > shows the most recent data for each pixel, so a lot of stronger signals get > overwritten by weaker ones. > > The antenna is a very good survey-grade choke-ring antenna located on the > roof of the lab. A choke-ring antenna has a sharp rolloff in gain below > about 20 degrees elevation, which helps reduce multipath errors. You can > see this in the "bullseye" shape of the data. You can see the shadow of a > single tree in the data (azimuth 240 degrees, up to about 30 degrees > elevation) but other than that the sky is clear. The usual hole to the > north appears, caused by the orbital inclination of the satellite > constellation. The strongest signals are at C/No of 50 to 51 dB/Hz, and > the weakest trackable signals are below 30. > > Others have recommended setting the elevation mask to 30 degrees, to reduce > multipath errors. While this works, it means the receiver will not even > attempt to track satellites below 30 degrees. I achieved approximately the > same result by setting the elevation mask to 5 degrees and the signal level > mask to 6 AMU (roughly 40 dB/Hz). With these settings, the receiver will > track all satellites above 5 degrees but will not use them in the solution > unless they are stronger than 6 AMU. With my antenna, the plot shows this > signal level corresponds to about 32 degrees elevation. You may need to > choose a different AMU mask for your antenna installation. > > The horizontal plot along the bottom of the screen shows the most recent > 9.5 days of timing data. The yellow line shows the diurnal temperature > variation in a lab which has no HVAC but benefits from Silicon Valley's > temperate climate. This particular Thunderbolt was built with the later, > low resolution temperature sensor. > > The purple line shows that the Thunderbolt's own estimate of its timing > error is within 5 ns of zero. This is optimistic, because the Thunderbolt > is affected by ionosphere and troposphere changes that it cannot measure, > but it is the error input that its timing control loop uses because it's > all it knows. > > The green line shows the tuning DAC response to the measured errors. The > plot legend says the span over 9.5 days is about 0.725 mV. The DAC > calibration (top left) shows that the OCXO tunes 3.717 Hz/Volt, or about > 370 ppb/V. Multiplying these, we find that the OCXO wander due to aging and > temperature variations over 9.5 days is about 270 ppt, which is not too > bad. This Thunderbolt was continuously powered for most of a decade before > this plot was taken, which helps explain the low aging. From several areas > of the plot, we can estimate a short-term change of about 75 uV/C, > corresponding to a tempco of about 27 ppt/C for the OCXO. Again, not too > bad. > > The control loop time constant was set to 300 seconds. For best frequency > performance with this unit, one could set it longer. For best PPS timing > performance, this seems to be optimum for this unit. > > Hope you find this useful. > > Cheers! > --Stu > <LabMainTbolt-2021-04-29.png>_______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send an > email to [email protected] > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
