[time-nuts] Re: Thunderbolt "good performance" example

[Bob kb8tq]

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 at 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>_______________________________________________
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