[time-nuts] More on SiLabs 5340

[Magnus Danielson]

Hi,

On 01/31/2018 04:27 AM, John Miles wrote:
> 
>> -----Original Message-----
>> There is a small detail that puzzles me: the ADEV for the internal
>> reference 10MHz and 30MHz bends upwards at tau greater than 0.1s.
>> Shouldn't that be visible by a change in slope in the phase noise
>> plot at ~10Hz as well?
>>
>> 			Attila Kinali
> 
> 
> Linear drift (which is what's happening here) has a bigger effect on the overall gestalt of an ADEV plot than it will on a phase noise plot.  This is because long-term drift looks similar to DC offset or noise at extremely low frequencies, both of which are attenuated by an HPF stage prior to each FFT segment.  But ADEV doesn't benefit from DC removal, and since it doesn't converge in the presence of drift, you get the usual upturn in the trace at increasing taus. 

As ADEV is defined, it is bound to be captured by linear, and higher
terms, drift. FFT is not directly, but on the other hand sensitive to DC
shifts that usually is handled by the window function, which also helps
to work on spectral purity. None of the measures is very perfect so they
need to be handled with care.

> When you see a highly-elevated noise trace at low offset frequencies, it's also a good idea to check for glitches in the 'f' measurement view, although in this case the L(f) spectrum is very typical of these sorts of parts. 

Glitches can totally ruin an ADEV.

Cheers,
Magnus