[time-nuts] Re: GPS Control Loop

[ASSI]

On Montag, 25. April 2022 18:27:01 CEST André Balsa wrote:
> A PDF of Shera's article can be found here (many thanks to whomever is
> hosting this file):
> 
> https://www.qsl.net/n9zia/wireless/QST_GPS.pdf
> 
> To me, there is no doubt Shera's original design inspired all the following
> DIY GPSDO designs in one way or another. Also this remark:
> 
> "Figure 6 also suggests that two major causes of frequency
> instability—temperature shift and aging—could be predicted and largely
> eliminated by tracking the performance of the VCXO for a while to estimate
> the aging parameters and by measuring the ambient temperature. The
> predicted corrections could be applied to the VCXO independently of the
> PLL, which might allow much longer loop filtering time constants to be
> used, further reducing GPS jitter. Although this scheme would be ultimately
> limited by sources of crystal frequency instability that are random and
> inherently unpredictable, it might be interesting to explore."

Establishing aging parameters for a modern non-ovenized crystal is a fools 
errand in my experience, at least if you keep the system operational for a 
long enough period of time.  If you don't, then you'll need to learn the aging 
parameters anew or you'll at least have to wait out the retrace before re-
using data from the previous run.  When the initial retrace / aging transient 
has subsided, a linear model is good enough for short timescales (out to 
several days), but the actual logarithmic aging behaviour ensures that the 
slope gets very small.  I have some systems that are going into their fifth 
year of mostly uninterrupted, self-ovenized operation and aging induced 
frequency drift is swamped by other influences at the timescales of a 
reasonably imaginable control loop, although it is still visible on (much) 
longer timescales of course (currently drifting at about 100…200ppb/year).  
Feed-forward compensation of temperature fluctuation does work reasonably 
well, but you can expect only about one order of magnitude performance 
improvement from doing that, maybe two if you manage to get a really close 
coupling of the sensor to the actual crystal temperature. IIRC, some TCXO used 
to have a second quartz platelet with a special cut to act as a temperature 
sensor.  It's also possible to interrogate the crystal temperature by exciting 
multiple harmonics and looking at their frequency difference, but I don't know 
if any commercial applications employ that effect.


Regards,
Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+

Wavetables for the Terratec KOMPLEXER:
http://Synth.Stromeko.net/Downloads.html#KomplexerWaves