[time-nuts] Re: GPSDO/GNSSDO project: STM32G4 + u-blox ZED-F9T + TDC7200
Carsten Andrich
carsten.andrich at tu-ilmenau.de
Thu Aug 11 06:57:38 UTC 2022
I've updated the schematic. Added an IMU (TDK InvenSense IIM-42652 on a
separate breakout board) and pushed the active low-pass filter cutoff
frequency to 10 Hz. Also preliminarily finished the layout. BOM is still
open, so maybe a few capacitor/resistor footprints will change (to
minimize assembly costs by picking parts without setup cost), but apart
from that I consider the prototype design finalized. Will let it sit for
a few days to review everything with some distance.
I don't want to flood your inboxes with attachments. You can find
updated schematic, layout, and 3D renderings in the EEVblog thread:
https://www.eevblog.com/forum/projects/gpsdognssdo-stm32g4-u-blox-zed-f9t-tdc7200/msg4351894/#msg4351894
Best regards,
Carsten
On 05.08.22 21:38, 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 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:
>
> Â * ZED-F9T timing GNSS receiver with RTK correction data
> Â * 10 MHz OCXO as local reference oscillator (special OCXOs with low
> Â Â g-sensitivity for non-stationary use exist)
> Â * TDC7200 for ~50 ps measurement of GNSS timepulse vs. OCXO
> Â * STM32 32-bit timer capture (~6ns resolution) to resolve
> Â Â aliasing/ambiguity of TDC measurement
> Â * <10ns relative accuracy within 10 km radius while on the move
> Â Â (preferably <1ns)
> Â * stabilized pulse output derived from OCXO (not directly from GNSS
> Â Â receiver)
> Â * configurable pulse properties (not only 1PPS) with fine-tunable
> Â Â delay (~200ps resolution via STM32G4x4 high resolution timers)
> Â * integrated distribution amplifier for 10 MHz and 1PPS outputs
> Â Â (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:
>
> Â * Minimize digital processing of OCXO output. Feed 10 MHz directly to
> Â Â TDC7200 without division via 74HC390 to minimize temperature
> Â Â dependence [10].
> Â * Only use low-jitter clock ICs (e.g., LMK1C110x for fanout). No
> Â Â 74-type components, which AFAIK do not have jitter or phase noise
> specs.
> Â * Active 1 Hz low-pass filter after tuning DAC to a) minimize noise
> Â Â (from supply, voltage reference, and DAC) fed to OCXO control
> Â Â voltage input and to b) enable finer than 16-bit tuning resolution
> Â Â via PWM. Assuming a 2 ppm tuning range, 16 bit yields 30 ppt
> Â Â frequency tuning resolution. At 10 seconds, that's already 300 ps,
> Â Â which is higher than the TDC7200's ~50 ps resolution. Hence, a finer
> Â Â tuning resolution may come in handy, of course depending on the
> Â Â 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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