[time-nuts] Systematic noise

Magnus Danielson magnus at rubidium.dyndns.org
Tue Feb 12 19:09:11 EST 2013

Fellow time-nuts,

Again I feel that it is good that I do not trust the samples I have been 
given, but start by measuring them. Again I have found a sample where I 
have systematic noise from the oven controller. Looking at the frequency 
plot, I see triangle shaped frequency modulation and the period gives a 
bump just above 1 s (guestimate around 1.6 s). The ADEV has a flat floor 
at about 2.5E-11 except for the bump with a few multiples which is 
expected. Otherwise fairly clean and the phase noise seems to pan out at 
around -157 dB, but I haven't looked at the 0.01 to 1 Hz spectrum where 
I now expect the "fun" spike(s) to be.

Will be fun to see how the all-nighter measurement looks, as the house 
heating cycles should crunch in if it is too sensitive.

Will swap oscillator and make a reference measurement tomorrow.

I use a pair of BVAs as reference, and they give me about 2 decades of 
margin for the ADEV, but since I measure them against each other, it's 
nice to get a floor measurement (actually, since it is the sum of the 
two, it will be about twice any of them).

Anyway, the interesting part was that I found this systematic noise from 
the oven controller. I've found that once before. That part didn't get 
selected, as it was clearly non-stable linear oscillation mode, where as 
this is clearly controller steering having this systematic effect. 
Expecting a digital controller to be found in there. Will be interesting 
to see of there is long term systematic effects from the house 
environment. The warm-up was almost entirely classic crystal warm-up drift.

The vendor of the part is not mentioned out of discretion.

The lesson to be learned is that oven controllers can create systematic 
noise, and you need to figure out if you can live with it or not. Some 
modern parts can expose such behaviours, but have a perfectly working 
oven controller otherwise.


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