From Milliseconds to 26 Nanoseconds: How a $20 eBay SFP Module Beat My Entire NTP Setup
Summary
The author demonstrates a sub-30‑ns timing chain using a $20 Oscilloquartz OSA-5401 SFP module, a BH3SAP GPSDO, and a Raspberry Pi 5, synchronized with ptp4l and chrony. The piece chronicles hardware installation, debugging of chrony refclock selection, and a bias introduced by the GPSDO MCU path, then shows how removing a trust flag and tuning parameters yields end-to-end timing around ±26 ns, far surpassing prior NTP-focused setups. It also provides concrete configs and performance metrics to replicate the result on similar SMB networks.