[time-nuts] Tbolt issues
csteinmetz at yandex.com
Sat Sep 10 20:42:34 EDT 2016
> The most important thing to get good Tbolt frequency performance is the
> antenna, with good sky view and correct location setting.
Agreed. Nothing will go right if the antenna, sky view, and surveyed
location are not as good as you can get them.
> The Tbolts damping setting is what controls how much 'freq noise' is
> added to correct for time error (i.e. Phase error). You can set it to
> optimize whatever you want.A damping of 0.7 adds ~25% freq overshoot
> noise and gives you the lowest phase error.
Meaning, it corrects the PPS position as quickly as practicable but
horses the frequency around quite a bit to do it and overshoots before
> A damping of 1.0 adds ~10% freq overshoot noise and any time/phase error
> takes about 3 times longer to correct.
> A damping of 1.25 adds <5% freq overshoot noise and the time error takes ~6
> times longer to correct.
> With a damping setting of 2, less that 1% of freq noise is added to correct
> for time errors and phase errors takes >>10 longer to correct.
> With a damping of >=10, the time correcting is so slow that time/phase
> correction can take days, and there is *no* added freq noise.
These are good rules of thumb. Someone who is interested solely in a
frequency reference is well advised to set the damping >>2. I think I
ended up around 6-12. There was a minor improvement if I increased it
to 50, but then recovery from holdover took longer than I was prepared
Speaking of which -- recovery from holdover will be slow when a Tbolt is
tuned this way, so do everything you can to speed it up. Allow "jam"
setting of the PPS when the error reaches, say, 65-75nS during recovery,
and also allow quite a lot of frequency error during recovery (IIRC, you
can allow the Tbolt to vary the oscillator up to parts in 10e-9 during
recovery). Then, DO NOT USE THE 10MHz OUTPUT WHEN THE TBOLT IS IN
RECOVERY. Wait until it is fully recovered.
And again (back to the first item), make sure everything about your
antenna system and location is in perfect order, so it will only very
rarely go into holdover. ALSO, make sure that the Tbolt sees a nice
thermal environment, either by actively controlling the temperature
surrounding the Tbolt itself, or by isolating it from ambient so the
oven can always keep up with any changes. The latter is my preferred
technique -- see old list messages discussing "cast aluminum boxes",
"thermal mass," "thermal inertia," and "thermal capacitance." But there
is nothing wrong with active control, either, and LH can do that for you
with a few external parts.
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