[time-nuts] Rb short-term noise (was RE: Is this a cesium...)
lists at rtty.us
Sat Aug 25 15:24:09 UTC 2012
…… and picking the cross over point based on noise plots of the sources on both sides of the detector will give you the optimum point.
On Aug 25, 2012, at 8:10 AM, Azelio Boriani <azelio.boriani at screen.it> wrote:
> OK, so speeding up the disciplining will shift the hump on the left and
> rise it, while slowing down will shift to the right and lower it.
> On Sat, Aug 25, 2012 at 1:31 PM, Magnus Danielson <
> magnus at rubidium.dyndns.org> wrote:
>> On 08/24/2012 11:35 AM, Azelio Boriani wrote:
>>> Magnus warned us about the use of very long time constant to act on
>>> disciplining. In my opinion it is better to use quieter data coming from a
>>> relatively fast sampling Kalman filter and correct as frequently as
>>> possible then train the filter with long time constants.
>> It really, really depends. I usually warn about using _too_ long time
>> constants. As John has correctly pointed out, the PRS-10 has a good crystal
>> oscillator in it, allowing for longer time-constants to be used.
>> There are many benefits of using an SC-cut crystal oven in a rubidium.
>> I also agree with John about the hump, its bound to be there due to the
>> PLL action. There are two sources of humpiness at the cross-over. The first
>> is that at the cross-over you transition from the low-pass filtered
>> reference noise and the high-pass filtered oscillator and loop noise. Since
>> the noises is uncorrelated, their powers will add. The cross-over filter
>> does not suppress one noise before the other kicks in to sufficient degree
>> of suppressing the additive effect. Another aspect is that the PLL Q-value
>> creates a gain at the cross-over point, and using too low Q values acts
>> like an equalizer to bring noise up.
>> This is to be expected and comes out of standard control system math.
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