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

Sat Aug 6 01:13:32 UTC 2022

Not that I like spending a lot of money but for $300 or less Gepetto Electronics sells a pretty good 10 MHz GPSO "https://www.tindie.com/products/nsayer/gps-disciplined-ocxo/".  I've had one for about 5 years, very accurate and trouble free!  Maybe not as fun as building your own but just putting it out there.

Les, N1SV

> On 08/05/2022 3:38 PM Carsten Andrich via time-nuts <time-nuts at lists.febo.com> 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 â€“ been busy with work â€“ 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 <500â‚¬ 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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