[time-nuts] Problems and solution when using a Rigol DDS signal generator for counter testing

[Erik Kaashoek]

During the tests of a frequency counter using a Rigol DG812 signal 
generator locked to an external 10MHz clock it was observed that the 
output frequency of the DG812 was not stable
This can clearly be seen in [1], blue trace, where the counter was 
warming up and the frequency of the frequency variations changed over time.
To test if the external locking was related to the instability the 
external lock was disabled which lead to huge instability. [2] Pink trace
As can be seen in the pink trace the huge variations disappeared when 
the coax cable with the 10MHz reference clock was disconnected (at 48 
seconds) and appeared again when the cable was reconnected to the clock 
input (at 86 seconds), regardless of the Rigol running the whole time on 
its internal clock.
This seemed to hint at some interference between the external clock 
signal and the internal frequency reference happening when running both 
at internal or external clock.
To test this an  external clock signal was applied at the frequency (+/- 
0.01 Hz ) of the internal clock.
The instability of the output from the Rigol locked to an external clock 
close to is internal frequency disappeared completely ([1], red trace) 
and the remaining frequency variations where within the measurement 
uncertainty of the used frequency counter.

[1] http://athome.kaashoek.com/time-nuts/Rigol%20locking%202.png
[2] http://athome.kaashoek.com/time-nuts/Rigol%20pulling.png