[time-nuts] Frequency Electronis 5608A standard

[John Ackermann N8UR]

I've been playing with a Frequency Electronics 5608A "Frequency 
Standard."  It's a 2U rack mount beast with battery pack and a module 
labeled as a 10 MHz Rb standard, and another as a 5 MHz VCXO.

I originally thought that the VCXO was a cleanup oscillator for the Rb, 
but that doesn't seem to be the case.  As best I can tell, the Rb module 
accepts a 10 MHz *input* and its output is a control voltage that goes 
to the VCXO.  The 5 MHz VCXO goes through a doubler to send 10 MHz to 
the Rb, and has distribution for a bunch of 5 MHz and 1 MHz outputs on 
the rear panel.  There are no 10 MHz outputs.

The "LOCK" indicator on the front panel lights up.  Early tests indicate 
that the short term stability is pretty good, but it has a bathtub 
shaped aging curve that's more like a crystal than an Rb.

Google doesn't turn up any hits on this model number.  Has anyone here 
encountered one of these guys and know what it was used for, or have 
info on the design?

Thanks,
John