[time-nuts] Interesting paper: Don't GPSD' your Rb...

[Azelio Boriani]

OK, got it. Yes, something like the dithering with a DAC to increase the
resolution.

On Mon, May 7, 2012 at 6:52 PM, Magnus Danielson <magnus at rubidium.dyndns.org
> wrote:

> Azelio,
>
> On 05/07/2012 09:56 AM, Azelio Boriani wrote:
>
>> Magnus,
>>
>>  If you flip back and forth, then it makes sense because your phase
>>>
>> deviations>will be less.
>>
>> Can you further explain this? Thanks.
>>
>
> Certainly!
>
> Consider that you flip back and forth between two levels, let's just say
> 50% "high" and 50% "low", the rate of flips considering the time-constant
> of the adjustment will matter, since if you let the time be so long that
> the alignment "ring out" you will get maxium frequency and phase deviation,
> where as if the rate of flipping is higher, then it drift just a little
> towards "high" when it gets a "low" and drift towards that... which gives
> you much smaller phase and frequency deviations... and you stay very close
> to the 50% level between "high" and "low". Essentially, the time-constant
> of the low-pass filtering will for a higher rate fairly well dampen the
> changes where as slow changes is in the pass-band with almost no low-pass
> filtering effects.
>
> It is thus nothing stranger than a low-pass filter and a square-wave of
> different frequencies.
>
> Also, if you update to slowly, you will build up larger errors before you
> have to steer back since you measured an error. Too low comparator rate in
> a PLL forces heavy filtering just to lower that comparator sub-frequency.
> Using high enough rate, the filtering process will be more continuous
> rather than very obviously sampled oriented.
>
> I've been bitten by this before. I've seen what too low comparator
> frequency does to create instability and modulations.
>
> Cheers,
> Magnus
>
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