Can an external GPSDO be used with a 6400? If so, what type of interface is required?

Hi Lon, There is a 10 MHz IN RCA jack on the back of the 6400. Any of the popular GPSDO units that provide a 10 MHz sine or square wave will work (check the manual to be sure that you are feeding a safe signal level to the Flex). Just plug the output of the GPSDO into the 10 MHz IN jack and you are good to go. No other interfacing is needed. I use the Leo Bodnar unit, and it works great.

You will want to set your 10 MHz reference to Auto or External 10 MHz. I leave it on Auto so that I don't have to switch it when I take my Flex camping. Make sure that the External 10 MHz Locked check mark comes on. If not, you may have to orient your GPS antenna differently, or make sure that you have a proper 10 MHz signal level.

If you use the GPSDO, either set the Offest to zero, or run the calibration routine (Start button). In my case, above, you can see that it wanted -2 ppb. There is no practical difference between zero and 2 ppb as it is a small fraction of a Hz. I ran this just to see what it would do. Since I live within 50 miles or so of WWV in Ft Collins, doppler effects of propagation are minor and that is what I attribute the 2 ppb to.

A GPSDO is not really required for a Flex 6000 radio, as the calibration routine is REALLY accurate (assuming that you can hear a good frequency source like WWV). I got mine for use with my spectrum analyzer, and just leave it hooked up to the Flex since it is in the shack anyway.