[time-nuts] Cheap GPSDO's

Nick Sayer nsayer at kfu.com
Wed Mar 2 16:17:40 EST 2016


> On Feb 25, 2016, at 12:41 PM, Keith Loiselle <keith.loiselle at gmail.com> wrote:
> 
> Below is a message Said asked me to forward to the group:
> 
> 
> Gents,
> 
> 
> 
> 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.

The TCXO variant is powered by an LDO rather than a switcher, FWIW.

In my bench experiments with the OH300, its stability did not differ when powered by the switcher as opposed to a massively heat-sunk LDO.

None of the Connor Winfield oscillators I’ve been using have a separate reference voltage output, which would be the best driver for the DAC. In lieu of that, I chose to reference the DAC directly from the oscillator supply voltage, in keeping with Connor Winfield’s own recommendations.


> 
> 
> 
> 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.

There are footprints in the most recent board for an RC filter on the output of the OP amp, and for an additional cap across the feedback resistor.

My experiments have shown no benefit to populating them, but they’re there if you want to have a go.

> 
> 
> 
> 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

The DAC has an ~200 ppt resolution with the DOT050V variant. With the OH300, the resolution is closer to 20 ppt.

Given the stabilities of the two oscillators, I believe this is sufficient.

> 
> 
> 
> 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.

Well, alrighty then.

> 
> 
> 
> 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.

One thing I anticipate adding in the near future is a hardware phase detector (and full credit here goes to Jim Harman). The goal of that is to improve the responsiveness by a couple orders of magnitude, which will have a lot of impact on the 10^1-10^3 ADEV, if all goes according to plan.

Still, most of the items of which you have expressed concern would, in principle, have an impact on low tau ADEV measurements, and my results there are in line with the expectations set forth by Connor Winfield.

> 
> 
> 
> Bye,
> Said
> 
> 
> Keith
> 
> 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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