atomic clock

Last update: Thu Jun 5 22:25:40 2025

Presentation

Principle

Use a Rubidium Frequency Standard (model FE5650a Option 58) as a counter: you start at a point of time, you use the RFS to measure time passing by ans since it's ultra precise, you're in for an extra precise time keeping.

Points of interrest

Schematics

In my case the wiring will be:

Red: +15V

Black: 15V Ground

Green: +5V

White: 5V Ground

Blue: Test 8Mhz output

Gray: Lock

Orange: 1 PPS TTL

Bill of materials

Connector is this one and too expensive for me (honnestly 6.5 USD + shipping for a piece of plastic, no thank you)

Implementation

Serial communications

All the pictures found on the net mention a Sipex SP233ACT chip as the U404 on the silkscreen, but on my RFS the trace is unpopulated and instead a tiny-tiny 8 pin TSOP chip with "FDM" written on is here. And the serial 4 pin connector is gone, replaced by a nice molex (?) connector at the top with 15 pins. So conclusion, I have a more recent version.
Checking the pinout of the original SP233ACT and my tiny chip, I can see that in fact the Serial1 is implemented, and I can wire them easily (^_^)v

Left: what I have. Notice in the bottom-right corner the big printout free and the small chip.
Right: what I found on the net, looks pretty older compared to my version (not necessarily a good thing).
The bottom right pinout should be (left to right): Gnd - Tx - Rx - 5v - 5v

4 pins are mapped easily to the original circuit, let's use them. The rest must be power.

Indeed, that smells like it. We need to use the pins T1IN and R1OUT because we'll be TTL side.

Source code

Pictures

Links

Helpful sources

http://www.dty.sakura.ne.jp/keisoku/rubidium/fe5650.htm (Japanese)

http://www.leapsecond.com/museum/fei5650a/

http://www.qsl.net/zl1bpu/PROJ/Ruby.htm

EEVblog #236 https://www.youtube.com/watch?v=FRdGsSu5Nec

EEVblog #235 https://www.youtube.com/watch?v=I55uLRRvLCU

Mike electronic stuffs takeof https://www.youtube.com/watch?v=ymV9LwhD0W0

FAQ http://www.ko4bb.com/doku2015/doku.php?id=precision_timing:fe5680a_faq

Misc http://www.ko4bb.com/doku2015/doku.php?id=precision_timing:fe5680a_pin-out

Misc http://www.ko4bb.com/doku2015/doku.php?id=precision_timing:fe_series_oscillators

Inspiration

The main inspiration: http://hackaday.com/2014/11/27/jaw-dropping-atomic-clock-build/

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