[time-nuts] Progress on the FE-5680A serial commands!

[John Beale]

On 1/19/2012 9:52 PM, Scott Newell wrote:
> http://n5tnl.com/time/fe-5680a/graphs/fe5680_2_ch3.png
> http://n5tnl.com/time/fe-5680a/graphs/fe5680_2_ch4.png
> This assumes that the reply from the undoc command 0x5a is the raw counts
> from the 4 channel 12 bit ADC. I think channel 3 is a temp sensor. What
> would channel 4 be? Photodiode output?

Great detective work Scott! I'll bet Channel 4 is voltage to the VCXO. It 
is driven up and down at startup searching for the lock, then it settles 
down, but the oscillator components are also affected by temperature so the 
VCO voltage would logically track the temperature too.  At least it seems a 
reasonable theory.

For example, look at this plot of the 10 MHz output frequency after 
powerup. It has a similar look to your Channel 4, except for the post-lock 
drift.

https://plus.google.com/photos/109928236040342205185/albums/5680473650837554113/5681715799377076466?banner=pwa

-John Beale