I purchased a used FE-5680A Rubidium Frequency Standard from E-Bay. These units are currently easy to obtain and are taken from cell phone tower equipment that is at end-of-life. However there is still quite a bit of life left in these. They can be reconditioned somewhat in your shop (using a heat gun) if you obtain a unit that doesn't lock. The unit I obtained achieves a frequency lock within a minute or two.
The FE-5680A requires a stable power supply of +15V/2A and +5V/100mA. I accomplished this by constructing a small linear supply based on an LM350T for the +15V and a standard LM7805T for the +5V. I used a laptop switching regulator supply to provide the +18V input as these supplies are inexpensive and can provide the required current.I made a custom 1/8" aluminum enclosure as the FE-5680A gets very hot and needs a good heatsink.
Once locked, the output is a very accurate 10MHz sine wave, accurate to 1*10^11. There is also a 1 pulse per second (PPS) output - handy for driving a digital clock.
