[time-nuts] Things to look for when buying a few FE-5680A's?

Rex rexa at sonic.net
Thu Jan 26 02:21:28 EST 2012

On 1/22/2012 9:49 AM, Chris Albertson wrote:
> The current batch of (about) $40 units are different from what was
> available a year ago.  These new ones require 5V DC input in addition
> to 15V and can only be programmed via RS232 a few Hz away from 10MHz.
> So they are only good for use as a 10MHz reference

For historical accuracy, in Jan 2005, I bought an FE-5680A which didn't 
work with just a 15 V supply. After some hacking, I think I was the 
first (at least on this list) to figure out that it also needed 5V on 
one of the pins to operate. I was not able to program that one. Though 
reminded by these many recent emails, I haven't yet pulled it out of my 
stash to see if I missed something when I tried to communicate on RS-232 
back then.

So the *new* ones are not that new, but they are newer than the ones, 
more commonly known before 2005, that worked on only 15 V and could be 
programmed over a wide frequency output range.

Over the years, one thing is quite obvious -- Frequency Electronics Inc. 
(FEI) made many similar but different versions of the FE-5680A. There 
seems to be no definitive way to  distinguish versions that have no 
external physical differences (like an sma output). Also, FEI has not 
been responsive, to my knowledge, to any questions from us surplus 

It is fortunate that this latest round of FE-5680A's seem to be 
consistent in characteristics and many clever people have derived a lot 
of details about their internals and programmatic interface. It is great 
that the current available surplus product seems consistent, but be 
aware that there were many indistinguishable iterations of the 5680A on 
the surplus market over the last decade or so.

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