[time-nuts] Advantages & Disadvantages of the TPLL Method
lists at rtty.us
Sat Jun 19 12:33:15 UTC 2010
Since ADEV is a measurement of noise, you want to be very careful about just which noise you keep and which noise you "throw away".
On Jun 19, 2010, at 7:06 AM, Magnus Danielson wrote:
> On 06/19/2010 12:49 AM, WarrenS wrote:
>> Say you have a nice logic gate with 1 ns delay
>> If you make it all nice and clean, and repeatable such as constant PS,
>> rise time etc.
>> Then one can get repeatable results say 100 times better from cycle to
>> cycle in the short term.
>> so down to 10ps repeatable.
>> Now make things even more clean with no variations and assuming random
>> Now if one is doing this at 10 MHz and only cares about the average over
>> 0.1 sec (10 Hz)
>> One can average 1,000,000 readings of the 10 ps jitter
>> If they are truly random, that can give you a 1e-3 improvement (square
>> root of number of samples averaged)
>> so now down to 10 fs of average jitter at 10 Hz for a 10 MHZ gate
>> starting with a 1ns initial delay.
> The square root improvement assumes white noise, and will work when white noise dominates. A logical gate or any other amplifier will also have flicker noise, which doesn't average out like that. The meaningful length of averaging thus depends on the cut-off frequency between white and flicker noise. To analyse it, Allan variance and friends needs to be applied. Thus, only for short-term stability may straight averaging work for estimation.
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