[time-nuts] Re: GPS Control Loop

[André Balsa]

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."

On Fri, Apr 22, 2022 at 3:20 AM ew via time-nuts <time-nuts at lists.febo.com>
wrote:

>
> 25 years ago Brooks Shera developed a GPSDO that had a Frequency accuracy
> of 1 E-11. A year later in  1998 he published his work in July 11998 in
> QST. The total package included circuits,   PC board and in depth analysis
> of every part of his design including his filter.. I was one of the first
> buying the board, GPS receiver and working with an other Time Nut building
> a couple of units. Using my Tracor 527E,, HP5345A and aHP5061A Cs in
> parallel to an Austron Loran C system using multiple HP 10811's in all
> cases observed better than 1 E-11 frequency.  All 10811's had previously
> been tested to have an AV below E-12. Subsequent tests with Austron 1150
> and Datum 1000 showed even better results. Over time Brooks added features
> but the design remained the same. His last effort was to make other DAC's
> available, but at that time his Cancer had spread to his brain.
>
> Lets remember this was the time of S/A and it is safe to say GPS has
> improved by a factor of 100. DAC's compared to the Audio DAC are also
> improved by a factor 50.
>
> Using later circuits we still use his design and Richard McCorkle did a
> PIC based on his Filter for us.
>
> Very little else has changed. Reviewing commercial units like End Run
> short term is a function of the OCXO or Rb, long term GPS.
>
> There was a time when Time Nuts had access to quality surplus OCXO's and
> Rb's at a 'reasonable" price, that tine is gone. Before the time I did get
> an OSA 8607 I did look at a Morion OCXO, their best with an AV of 2 E-13 at
> 1 second. The would not respond to my RFQ but thanks to a Time Nut that
> buys from them regular the answer was 6000 Euro and 1 year delivery after
> order!
> To us Shera is the Gold Standard. worth a review for any one doing their
> own GPSDO, we are presently focusing on the best GPS receiver, my problem
> is Ublox 9T is not an option because at 80 I am not able to get safely on
> the roof. Hire some one is an option but liability may be a problem
>                                                                      Bert
> Kehren
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