Sunday, September 20, 2015

FE-5680A Breakout board users guide

This is the permanent home for the user's guide for the FE-5680A breakout board.

The FE-5680A is a Rubidium frequency standard widely available used on eBay. It has a DB-9M receptacle as its interface to the outside world and requires +15 VDC @ 1A and +5 VDC at ~100 mA to operate. This breakout board supplies both of those from a 20+ watt 16-24 VDC input - easily obtainable from a surplus laptop power adapter.

In addition to the 2.1mm barrel connector for input power, there is a two pin SIP header footprint on the bottom of the board which can be used as an alternative. There's also a 2 pin SIP header on the top of the board with the RF output, and a 4 pin SIP header that carries the PPS and RS-232 signals. Lastly, there is an LED in the corner of the board that indicates that the module has obtained a physics lock.

The 2 pin and 4 pin SIP headers bring the respective interface pins from the module directly out with no conditioning or other circuitry. The LOCK LED is buffered by a transistor so as to not unduly load the respective module pin (doing so can prevent the PPS signal from working). The pinouts of the various models of FE-5680A vary quite significantly, and the pin labels are nominal ones for the most frequent case.

Some 5680As do not require an external 5 volt power supply. If yours is one, then take an Xacto knife and cut the trace running between the two pads of the solder jumper immediately adjacent to the interface connector. Be careful to not cut any adjacent traces on the board. Keep the knife blade only between the jumper's pads.  If you need to re-enable the +5 supply you can solder the jumper closed again.

Note that both the 5680A and portions of the breakout board (particularly around the +5 LDO) will get quite hot during operation. This is normal. In particular, the module is internally heated to insure stability of its internal oscillator. Attempting to dissipate this heat will simply make more work for the oven.


Not all FE-5680As have the same pinout (the one indicated is the most common, so far as we are aware). Please carefully check your particular model against the pinout below to insure your unit is not damaged by improper connections. Before connecting your breakout board to your FE-5680A, please power it up and verify that the correct voltages appear on pins 1 (15V) and pin 4 (5V) relative to pins 2 and 5 (ground).

DB-9 pinout

  1. +15 V supply
  2. ground
  3. !LOCK (low means lock achieved)
  4. +5 V supply
  5. ground
  6. 1 PPS
  7. RF out
  8. RS-232 serial in
  9. RS-232 serial out


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