Well, I couldnt resist the tentation to make a quick check
of the absolute error of my unit.
Please consider that I'm still learning to deal with precise
time measurements, so feel free to check and correct my
procedures. Also the room temperature is far from ideal.
Measuring the output of a pair of FE5680A that I trimmed
against GPS (manually, no GPSDO still in my hands), the
counter was aroud 70-75 mHz high, so I was expecting
the internal oscillator a little on the low side, and it was low
by about 7.3x10^-9 :
The interesting part is top right, where I have data and linear
interpolation together. The line is the phase between
the GPS pps and internal 10MHz reference, so a positive
slope means that the 10MHz is lagging the pps -> slow.
It's not bad, the counter was never touched by me, and
probably not trimmed from a lot of time. According to the
manual it was still in spec, since the manual calls for recal
only past +-3x10^-8 deviation.
Ok time to try to trim the counter, I lightly touched the
fine adustment, after turning the pot the counter moves
back and forth slowly a pair of times, it takes several minutes, and
then stabilizes very slowly. Not an easy process, better to take
note of the screwdriver movements, and wait many minutes
between the adjustments. This is how the phase changed
from lagging to tracking the pps while I corrected the frequency:
This means that the two signals track each other, and
a quick check with the FE5680A confirms that I'm near,
somewhere around 2x10^-10 slow:
Now time to go to bed. As I have more data I will post them,
I think it will be difficult to show aging effects (unless I log the
unit for many days or the quartz is gone), since the unit
is aged and the aging should very slow now. Probably
it will be easier to try to see temperature effects.
Fabio.