[time-nuts] Re: GPSDO/GNSSDO project: STM32G4 + u-blox ZED-F9T + TDC7200

[Bob kb8tq]

Hi

Actually John would be the better one to check with since his paper is referenced
as the source of the data :) :): :). My plots of these parts just show up here on
the list ….

The ADEV you get is much better than the “timing” spec on these parts. Indeed
that spec is typically a bit vague in terms of just what it actually is referring to. In
the case of the F9T it certainly isn’t spec’ing 1x10^-11 level ADEV. 

Just what this or that device does in a moving environment is very much a “try 
it and see” sort of thing. There are simply to many variables to come up with a 
single magic number. Moving on a train across a flat / wide open field with 360 
degree sky view to within 10 degrees of the horizon is very different than driving 
a motorbike through the typical urban canyon with very limited sky view. Doing 
either one at 50 degrees north  is different than doing it at the equator. Drones 
have different issues than backpacks…… it goes on and on and on ….

With the F9x’s and a “GPS only” design, you have a limited number of sat’s
to use. That makes everything more difficult. A “multi-GNSS” approach is an
alternative, it brings with it a variety of issues. 

As many have found, dealing with OCXO’s in changing environments is a bit
exciting. They most certainly do go in a variety of settings. Something around
99% live in a fixed / bench sort of install. The spec sheet data on a catalog part 
is inevitably talking about a very stable situation. Performance is always degraded
if things like shock / vibration / acceleration / temperature change are involved. 

Without a target accuracy spec on the device, there really is no way to know if
this or that approach is going to work. Is this a time source? Is it just a frequency
reference? What is the target accuracy in the real world?  What is the “vehicle”
and what are its characteristics? 

Lots and lots of rabbit holes to chase down …. ( and we never even made it
to the problematic filter on the DAC …). 

Bob

> On Aug 5, 2022, at 2:43 PM, WB6BNQ via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hi Carsten,
> 
> I think the F9T relies on being in fixed position for the TIMING it's specs.
> 
> Perhaps Bob kb8tq, could provide more input to that aspec.
> 
> BILL WB6BNQ
> 
> 
> On 8/5/2022 12:38 PM, Carsten Andrich via time-nuts wrote:
>> Hello everyone,
>> 
>> back in April I asked about a 100 MHz GNSSDO for mobile use [0]. As nothing COTS seemed to be available, I've since started developing a prototype based. I finally – been busy with work – got the design to sth. work sharing and am looking for feedback. The following is cross-posted on the EEVblog forum [1]:
>> 
>> I'm in need of a GNSSDO for accurate time synchronization of moving vehicles distributed over a few (dozen) square km. I've been professionally using the James Miller (G3RUH) Ministd [2], Jackson Labs LC_XO [3], and SRS FS740 [4], but none of these deliver satisfactory results when used on the move. That's certainly not a flaw of these devices, because presumably they were not designed with a non-stationary use case in mind. Therefore, I decided to build a suitable GNSSDO myself. This is not a low-cost project. Development priorities are performance, simplicity, and value for money in that order.
>> 
>> I have positive experience with the u-blox ZED-F9P RTK [5] GNSS receivers. As the F9P can achieve <5cm position accuracy with RTK correction data, its timing receiver sibling (ZED-F9T) should enable sub-nanosecond timing accuracy. The F9T supports differential timing [6] via the same correction data used for RTK positioning. I'm not aware of any other GNSS timing receivers supporting this feature. The F9T also exhibits excellent Allan deviation (~5e-11 @ 10s) [7]. Therefore, the F9T is the center piece of my design, relying on the RCB-F9T [8] board for prototyping.
>> 
>> Design aspects/goals:
>> 
>>  * ZED-F9T timing GNSS receiver with RTK correction data
>>  * 10 MHz OCXO as local reference oscillator (special OCXOs with low
>>    g-sensitivity for non-stationary use exist)
>>  * TDC7200 for ~50 ps measurement of GNSS timepulse vs. OCXO
>>  * STM32 32-bit timer capture (~6ns resolution) to resolve
>>    aliasing/ambiguity of TDC measurement
>>  * <10ns relative accuracy within 10 km radius while on the move
>>    (preferably <1ns)
>>  * stabilized pulse output derived from OCXO (not directly from GNSS
>>    receiver)
>>  * configurable pulse properties (not only 1PPS) with fine-tunable
>>    delay (~200ps resolution via STM32G4x4 high resolution timers)
>>  * integrated distribution amplifier for 10 MHz and 1PPS outputs
>>    (capable of driving 50 Ohm loads)
>>  * part and manufacturing costs preferably <500€ per device (excl. OCXO)
>> 
>> My architecture is inspired (among others) by Matthias Welwarsky's "DIY GPSDO project w/ STM32, TDC7200" [9]. However, I hope to improve on these aspects:
>> 
>>  * Minimize digital processing of OCXO output. Feed 10 MHz directly to
>>    TDC7200 without division via 74HC390 to minimize temperature
>>    dependence [10].
>>  * Only use low-jitter clock ICs (e.g., LMK1C110x for fanout). No
>>    74-type components, which AFAIK do not have jitter or phase noise specs.
>>  * Active 1 Hz low-pass filter after tuning DAC to a) minimize noise
>>    (from supply, voltage reference, and DAC) fed to OCXO control
>>    voltage input and to b) enable finer than 16-bit tuning resolution
>>    via PWM. Assuming a 2 ppm tuning range, 16 bit yields 30 ppt
>>    frequency tuning resolution. At 10 seconds, that's already 300 ps,
>>    which is higher than the TDC7200's ~50 ps resolution. Hence, a finer
>>    tuning resolution may come in handy, of course depending on the
>>    actual stability of OCXO and GNSS.
>> 
>> My next step is to build a prototype PCB to put my ideas to the test. I've devised the preliminary architecture and put it into a schematic. The layout is in progress.
>> I've attached the schematic and would appreciate constructive feedback, prior to having it produced.
>> 
>> Thanks and best regards,
>> Carsten
>> 
>> [0] https://febo.com/pipermail/time-nuts_lists.febo.com/2022-April/105583.html
>> [1] https://www.eevblog.com/forum/projects/gpsdognssdo-stm32g4-u-blox-zed-f9t-tdc7200/
>> [2] http://www.jrmiller.online/projects/ministd/manual.pdf
>> [3] https://www.jackson-labs.com/index.php/products/lc_xo
>> [4] https://www.thinksrs.com/products/fs740.html
>> [5] https://en.wikipedia.org/wiki/Real-time_kinematic_positioning
>> [6] https://content.u-blox.com/sites/default/files/ZED-F9T-00B_DataSheet_UBX-18053713.pdf#page=6
>> [7] https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/N8UR_GPS_Evaluation_August2020.pdf#page=25
>> [8] https://www.u-blox.com/en/product/rcb-f9t-timing-board
>> [9] https://www.eevblog.com/forum/projects/diy-gpsdo-project-w-stm32-tdc7200/
>> [10] https://www.eevblog.com/forum/projects/diy-gpsdo-project-w-stm32-tdc7200/msg2921634/#msg2921634
>> 
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