Hamref proto v0 1

HamREF

A small GNSS-disciplined 10 MHz frequency reference and precision time server for the amateur radio shack — built around the u-blox LEA-M8S GNSS module.

Status: early architecture / pre-prototype. Proto-1 schematic and PCB (v0.1) are generated and routed, not yet built or tested; a firmware proof of concept (PIO counter + disciplining loop, host-simulated, not yet run on hardware) is in firmware/. This README documents the design as currently planned so the repo has a real starting point; sections below are marked TODO where work hasn't started.

Proto-1 bring-up board (v0.1): 4-layer, 100 × 80 mm (fits the Fischer AKG 105.26 case grooves), 3D view from KiCad. Design files in hardware/proto-v0.1/.

The proto-1 board has four coaxial connectors along the rear edge: SMA (GNSS antenna input), BNC REF OUT (10 MHz from the OCXO), BNC 1PPS OUT and BNC AUX OUT. AUX OUT outputs a programmable frequency (3.3 V CMOS) generated by the Si5351C synthesizer, which is clocked from the OCXO, so it is locked to the same reference as the 10 MHz output.

What this is

HamREF provides two things a well-equipped station needs and usually has to buy separately:

• A 10 MHz reference output (BNC, 50 Ω) to lock transceivers, synthesizers, and test equipment, plus a programmable AUX output for other frequencies (e.g. 25 MHz, 27 MHz for satellite LNBs, 40 MHz for a Pluto SDR, or an arbitrary frequency).
• A 1 PPS timing output which can be used to lock and synck devices to the world-standard one pulse per second.
• A precision time source for the shack computer, disciplined by GNSS, accurate enough for FT8 and other digital modes.

Status (GNSS lock, satellite count, PPS accuracy, OCXO discipline state) is shown on a small monochrome OLED plus two multicolor status LEDs (GPSDO / NTP) on the front panel — color and blink/steady pattern indicate different states. A front-panel rotary encoder drives the menu; full parameter configuration is also available over USB (see Enclosure below).

Why not just use the GNSS module's own 10 MHz output?

The LEA-M8S can output a configurable timepulse from 0.25 Hz to 10 MHz directly, but that signal is a digitally synthesized pulse train (derived from the module's internal TCXO + fractional-N synthesizer), not a clean analog oscillator. At 10 MHz its jitter (the module's spec is 30 ns RMS / 60 ns 99% at 1 Hz) is significant relative to the 100 ns period, it can show small phase steps at each second boundary, and it provides no holdover if GNSS lock is lost.

HamREF instead uses the classic GPSDO approach: the LEA-M8S's single TIMEPULSE output is configured as 1PPS, and that one pulse-per-second reference is used by the MCU to discipline a separate free-running OCXO via a DAC-driven EFC loop with a long time constant. The 10 MHz output comes from the OCXO itself — far lower phase noise, plus holdover if satellite lock drops. The same 1PPS/NMEA data feeds the time service to the host computer.

Full rationale and the architecture decision log: docs/architecture-and-plan.md (in Finnish).

Enclosure

Fischer Elektronik AKG 105.26 (part AKG1052680ME) — an extruded aluminum mini case for 100 mm eurocards, 105 × 100.3 × 26 mm, natural anodized, with integrated cooling-fin channels. Front and rear panels are separate 2 mm cover plates.

The 26 mm case height (≈22 mm usable panel height) ruled out the originally planned 240x240 color TFT. An EC11-class rotary encoder does fit, though (body ≈11.7×12–13×4.5–8.5 mm, ø7 mm panel bushing — verified against the Farnell EC11 datasheet), so the front panel keeps a physical control after all:

The 22 mm limit also constrains component height on the PCB (OCXO included) — still an open item, see the architecture doc.

Architecture overview

Block diagram (editable source: HamREF-architecture.svg).

The GNSS module's 1PPS (TIMEPULSE) disciplines the OCXO through the MCU: the RP2350 measures the 10 MHz against the PPS with PIO and steers the EFC through the DAC. REF OUT (10 MHz) comes from the OCXO itself, 1PPS OUT from the GNSS TIMEPULSE, and AUX OUT is a programmable output (Si5351C clocked from the OCXO).

What is the USB-C port, in plain words? It is a normal USB-C connector (USB 2.0 only, no Power Delivery, so always 5 V) wired straight to the RP2350's own USB controller. One cable does two jobs: it powers the whole device and it carries data. The data side is a virtual serial port over USB (class CDC-ACM): the computer sees an ordinary serial port (COMx on Windows, /dev/ttyACM0 on Linux, /dev/cu.usbmodem… on macOS) and needs no special driver. Over that port the device sends the GNSS text sentences and PPS timing data to a small helper program on the computer, which feeds chrony; the computer can also send configuration commands back.

Key hardware decisions so far:

See the architecture doc for the full reasoning, open questions, and the phased project plan.

Status / roadmap

• Requirements and architecture defined
• Enclosure selected (Fischer AKG 105.26) and display/panel layout locked to it
• MCU selected (RP2350 / Pico 2)
• Proto-PCB v0.1 (bring-up board, see architecture doc)
• Breadboard proof of concept (PIO PPS capture + frequency counting, disciplining loop, USB time delivery)
• KiCad schematic
• PCB layout
• Enclosure machining (panel cutouts)
• Firmware proof of concept (PIO gate counter, disciplining loop, host simulation) - see firmware/
• Firmware on real hardware (calibration, OLED/menu, GNSS configuration, host helper)
• Calibration and verification against a reference

Repository structure

hardware/proto-v0.1/   KiCad project (schematic, PCB) for the proto-1 board
hardware/tools/        Python scripts that generate the schematic/PCB
firmware/              RP2350 firmware (Pico SDK) and host simulation
docs/                  Architecture notes, hardware plans, images

Building it yourself

Firmware: see firmware/README.md (Pico SDK, CMake, UF2). The disciplining loop can be tried without hardware with the host simulation described there. Hardware: open hardware/proto-v0.1/ in KiCad; the pre-order checklist is in docs/hardware/board-finishing-checklist.md. A bill of materials and bring-up guide will follow once the board is built.

Hardware reference documents

Design decisions are based on:

• u-blox LEA-M8S Data Sheet (UBX-16010205)
• u-blox LEA-M8S/M8T Hardware Integration Manual (UBX-15030060)
• Fischer Elektronik AKG1052680ME data sheet (enclosure)
• Raspberry Pi RP2350 datasheet and Waveshare RP2350-Plus schematic / STEP model (docs/datasheets/)

License

Not chosen yet (candidates: CERN-OHL-S for hardware, MIT or GPL for firmware, CC BY-SA for docs). Until a license is added, all rights are reserved.

Authors

Erik, OH2LAK Others will be too, when I find them and convince them this is a good project :)