[time-nuts] Interesting paper: Don't GPSD' your Rb...
Magnus Danielson
magnus at rubidium.dyndns.org
Mon May 7 16:52:14 UTC 2012
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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