[time-nuts] Cheap GPSDO's

Keith Loiselle keith.loiselle at gmail.com
Thu Feb 25 15:41:35 EST 2016

Below is a message Said asked me to forward to the group:


While I haven’t posted here for a while, I have been following Time Nuts.
One recent post caught my attention, and here are my comments.

There are some serious basic design issues with Nick Sayers’ GPSDO hack
mentioned by Attila in the previous post. Since Nick is asking up to $275
for the kit, and there are various ways to get professional-grade brand-new
GPSDO’s for significantly less than that. Here are some comments:

1.       The design is based on a 3.3V switching regulator. The output of
this is used essentially without any significant filtering to power the
DAC, the DAC opamp, the RF output drivers, and the OCXO/TCXO. All of these
items should be powered by a low-noise LDO and the DAC should be driven by
a high-stability voltage reference. The noise floor and spur levels will be
quite bad, and since the switcher is running at 2.5MHz, its fourth harmonic
and higher harmonics (at 10MHz, 20MHz, etc) will beat with the TCXO/OCXO.

Any switching noise on the 3.3V supply will also be directly transferred
into the opamp positive input pin via R17 and will be amplified to find its
way to the EFC pin of the OCXO. There should have been a cap in parallel to
R16 to filter this noise out. Additional low-pass filtering at the output
of the opamp would have also helped.

2.       The opamp positive input is driven by a 50K Ohms equivalent
impedance. This high resistance will cause resistor thermal noise to pass
right through the opamp, and will pass right into the EFC pin and modulate
the OCXO. Same issue with the negative input pin and the high-value
resistors R14 and R15 connected to that input pin

3.       The DAC only has ~200ppt resolution (16 bits). If using a DOCXO
with stability in xE-012, this will cause issues as the LSB resolution is
10x to 100x worse than what the DOCXO could achieve in stability

There are additional issues in this simple hw design. This design is not
professional grade in my opinion, and at $275 not really priced as an
amateur kit either.

For example at $220 in single piece quantities for a brand new LTE-Lite kit
you could also purchase a very good MTI or Morion DOCXO for about $25 on
eBay that can be gluelessly disciplined by the LTE-Lite. That solution
would still cost a bit less than Nicks kit overall and have DOCXO stability.

There is one big advantage to Nicks design - access to the firmware for
tweaking and expanding. That fw may be useful as a basis to design, or
modify the hardware to follow the above design suggestions.



On Thu, Feb 11, 2016 at 12:34 AM, Attila Kinali <attila at kinali.ch> wrote:

> On Thu, 11 Feb 2016 00:13:20 -0700
> Joseph Gray <jgray at zianet.com> wrote:
> > I searched the list archives and found some discussion mid-to-late
> > last year about several inexpensive GPSDO's made by bg7tbl. It seems
> > that all of the better models (according to discussion on the EEVBlog
> > forum) are gone.
> Have a look at [1] especially at last two paragraphs.
> If you want something cheap but ok GPSDO, get one of Nick Sayers [2].
> He sells them on tindie [3]. The TCXO version goes for $175 and the
> OCXO version for $275. If you already have a good OCXO you can
> modify it to get the EFC voltage out. Maybe you'll have to adjust
> the firmware for it, which you can find on github [4].
>                         Attila Kinali
> [1] http://www.ke5fx.com/gpscomp.htm
> [2] https://hackaday.io/project/6872-gps-disciplined-tcxo
> [3] https://www.tindie.com/products/nsayer/gps-disciplined-ocxo/
> [4] https://github.com/nsayer/GPS-disciplined-OXCO
> --
> It is upon moral qualities that a society is ultimately founded. All
> the prosperity and technological sophistication in the world is of no
> use without that foundation.
>                  -- Miss Matheson, The Diamond Age, Neil Stephenson
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