[time-nuts] FEI 5680a status update

Dan Sawyer dansawyer at earthlink.net
Sun Mar 15 22:56:49 UTC 2009

After several weeks of slow evening and hour here and hour there the FEI 
5680a is performing on the bench. I was able to get the serial interface 
to operate for a brief period. The 'S' and 'F' commands worked long 
enough to change the divisor to produce close to a 10 MHx output, it was 
7 Hz low. After pressing the 'up' button several hundred times the unit 
was very close to 10 MHz. I am not sure why the serial interface stopped 
responding, I never tried the second documented 'packet' command set. 
There is also a reset pin on the PIC, it is possible this is not 
resetting on power up.

Now the search for a means to measure the signal error. I tried the 
oscilloscope method of syncing of a 10 MHz signal produced from a GPS 
locked oscillator, however it had enough jitter and back and forth to be 
impractical use. This is where the thought to divide the 10 MHz from the 
5680a by 1000 to get a 10kc signal and trigger that to compare with a 
GPS produced 10kc signal. This produced good results, the FEI unit 
drifts about 450ns per hour which calculates to be about 1.2**10-10. 
This measurement is consistent half a dozen periods ranging for 1 to 4 
hours and document range of results for using the F software command.

I have not measured the effect of a button up or down, however it was 
significant enough to be obvious that either one up or one down was not 
correct. There is supposedly a software command set to change the C 
field. It is documented to have a narrow range; I am not sure of the 
algorithm to calculate the 'check sum' for the data segment. I would be 
interested in others experience with these units.

Thanks - Dan kb0qil

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