[time-nuts] Rb short-term noise (was RE: Is this a cesium...)

[Bob Camp]

Hi

Since you are dealing with two sources, both with random noise on them, there's no "easy" way to mathematically remove it. 

Bob

On Aug 25, 2012, at 1:02 PM, Tom Knox <actast at hotmail.com> wrote:

> 
> It seems by collecting data while changing the loop time that hump or 
> knee could be mathematically removed, which with good quartz could be the 
> beginning of a very serious standard.
> 
> Thomas Knox
> 
> 
> 
>> Date: Sat, 25 Aug 2012 14:10:41 +0200
>> From: azelio.boriani at screen.it
>> To: time-nuts at febo.com
>> Subject: Re: [time-nuts] Rb short-term noise (was RE: Is this a cesium...)
>> 
>> 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.
> 
> It seems by collecting data while changing the loop time that hump or knee could be mathematically removed, which with good quartz could be a beginning of a very serious standard.
>> 
>> 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.
>>> 
>>> Cheers,
>>> Magnus
>>> 
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