considering how important temperature is to the frequency accuracy I would only consider fan cooling. For less than $ 10 you can have a system that will hold the temperature at 43 C. The fan is very quit. To get best performance I would cover all sides except the bottom plate with foam. I have run tests and with the fan running all the time at 25 C ambient the unit is held at 38 C figuring 5 C margin for change in ambient is plenty. After testing many fan solutions and evaluating performance I now use item 180780456923 for any thing that has to cool up to 13 W reliably. It is a copper heat sink/fan and all you need is a 1/16 aluminum plate, a top cover with the fan cut out to make sure the air moves over the fins (for testing I even used tape!) and a fan controller, all this can be done for less than $ 10. In my case it is more complex because I am installing it in a small case along with a Morion OCXO but the same fan is on the back. L shaped aluminum plates are on top and bottom bringing the heat out to the back, something I have already used successfully with a FRS Bert Kehren In a message dated 1/20/2012 10:34:54 P.M. Eastern Standard Time, paulswedb@gmail.com writes: I added a heat sink to the FE5680. Curious what temperatures have been measured after an hour warm up. Before heat sink 120 F top and bottom. With heat sink on the bottom quite a large one. Top of case free air 105 F Bottom 105 F Internal, (screw out of the end and touching the internal heat sink) 111-113F. Whats a good temperature to be shooting for? Thanks Regards Paul WB8TSL _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.