[time-nuts] ADEV measurement question

[Matthias Jelen]

Hello,

I´ve got a question concerning ADEV-measurements.

I´m measuring the 15 MHz output of a KS-24361 with my SR-620 
with it´s internal (Wenzel) OCXO using Timelab. For the 
first shot I used the counters frequency mode with 1s 
gatetime. ADEV at tau=1s turned out to be arounf 2E-11, 
which fits the 20 ps single shot resolution of the SR-620 
nicely.

To overcome this limitation without setting up a DMTD 
system, I used the counter as TIC, feeding 1 kHz (derived 
from the counter´s reference) to the start channel, the 15 
MHz to the stop channel and put the counter into average 
mode / 1k samples. This gives me one averaged result per second.

The idea was that this shouldn´t change the measurement 
itself, because like in frequency mode with 1s gate time I 
get the averaged value over one second, but I expected 
trigger noise etc. to be averaged out to a certain amount. I 
have to watch out for phase wraps, but as the two frequencys 
are quite equal, this is not a big issue here.

As expected, ADEV at tau=1s got much better, it is now in 
the 4E-12 area, which sound reasonable.

What makes me wonder is the fact that result are 
significantly better now at longer taus (10..100s) also, 
despite of the fact that also in frequency mode these result 
were well aboce the noise floor (2E-12 @ 10s and so on...).

So, is it a good idea to use this kind of averaging, or am I 
overlooking something which turns the numbers better than 
they really are? I´m pretty sure I am not the first one to 
try this...

I´m looking forward to your comments.

Best regards,

Matthias