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

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 <stewart.cobb@gmail.com> 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 -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-leave@lists.febo.com > To unsubscribe, go to and follow the instructions there.