[time-nuts] 5680A what is a reasonable case temp ?

beale beale at bealecorner.com
Tue Feb 28 20:06:03 UTC 2012

>  From: Chris Albertson <albertson.chris at gmail.com>
> I'm ready to mount on of my 5680A in a case and looking for suggestions on
> what one would consider a reasonable maximum case temp.

With the FE-5680A mounted to a 4x6x1" finned aluminum heatsink as shown here:

the opposite side of the case reaches 55 C in a 20 C ambient, with no fan whatsoever. Running a 40mm 12V fan at 5V aimed at the heatsink drops that to 45 C. At 5V the fan is nearly silent, and the airflow is quite weak.  Despite the internal temperature compensation via DDS offset updates that the unit apparently does, I measured one of my units to have a 7E-12 per degree C temperature sensitivity, larger than I expected. So I do want to use a thermostat on the fan.

The Symmetricom XPRO manual http://www.symmetricom.com/link.cfm?lid=7082  (which I believe is a somewhat similar Rb design) estimates that the lifetime of that unit doubles when baseplate temperature is reduced from 60 C to 40 C (MTBF: 243k hours => 591k hours).  Note that 243k hours is about 28 years. 

On the other hand, my assumption is these surplus FE-5680A units were cheap because they were considered end-of-life already.  Maybe for something somewhat fixable, like the 60 MHz VCXO center frequency having drifted out of a spec window.

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