Stromeko at nexgo.de
Mon Sep 19 18:09:04 UTC 2016
Gary E. Miller writes:
>> You 've made this argument before, but I think it's circular
> Really? I think the data is very clear, you can optimize for time
> or for frequency.
No, I still think you're misinterpreting it. The only reason you see
that apparent tradeoff is that once you steer the PLL fast enough
(higher loop bandwidth) all the measurement noise ends up in the
frequency variable, while if you steer it more slowly (lower loop
bandwidth), it will be seen predominantly in the offset variable. These
two variables are not independent and are not themselves an actual
measurement of what their names suggest.
>> You make the local clock follow external jitter faster.
> What external jiitter? Assumption #1 is that the PPS is way
> better than the internal XTAL. An assumption supported by the GPS
> data sheets.
The PPS may be (at time scales beyond 1s), but not the measurement that
NTP does and uses to steer the PLL. Looking at the PPS arrival times on
my rasPi (with ppswatch), there is around 10µs jitter when there is no
load and going up to 30µs with light activity. I'm running at poll=4,
so ntpd completely eliminates the 10µs jitter in the loopstats, as it
should. This jitter must be assumed to come from the measurement
itself since there's no way the GPS module produces that much and the
local clock doesn't have that large jitter at the 1s timescale either.
> And please notice I am optimizing for self-referencetial stable time,
> not some 'true' time. Until it is stable, no point playing with static
> offsets to make it 'true'.
You are in fact destabilizing it by making it follow spurious deviations
in the measurement. When disciplining an oscillator, you must not do
that for timescales lower than the Allan intercept with the discipline
clock, or you're actually making it worse than it already is.
>> BTW, both your plots showed relatively large swings in frequency
>> offset in a short period of time.
> Both plots? I'm up to dozens now Gotta be a bit more specific.
You've only posted two plots in the post previous to the one I responded
> Nope. My house has no A/C. It would likely be better if I did have
> A/C. If you look at the complete graphs (with CPU, Room, HR, etc.
> temps) you will see a temperature correlation.
Oh, I see it now. You're plotting days-hh:mm, not hh-mm:ss as I assumed
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