Noctua Fan Swap on a Calnex Rb/GPS Frequency Reference

A Calnex Rb/GPS frequency reference combines a SpectraTime rubidium clock with timing-grade GPS to provide a stable 10 MHz output; its circuit board also derives a 1 pulse-per-second signal for the front panel and powers the unit. Because the clock needs to remain on continuously and can take hours or longer to warm up, the author wanted to reduce the noise from its original Delta fan, which was irritating despite not being especially loud. The device’s cooling assembly uses a pin-fin heatsink attached to a heat spreader, with the clock module mounted to it.

The fix was to replace the 12-volt Delta fan with a Noctua NF-A4x20 FLX. Since the Calnex board’s small, unusual four-pin fan connector could not be used with the available wiring, the author cut the fan cable and joined its wires to the Noctua fan using the supplied OmniJoin connectors. The heatsink was removed through the bottom of the unit to make the work safer and easier, then reinstalled along with the covers. The author notes that the equipment is straightforward to service: its internal parts are accessible, and a single Torx T10 screwdriver fits all the screws. After reassembly, the reference was returned to service; it typically needs 20-30 minutes to acquire rubidium timing lock and synchronize with GNSS.