[time-nuts] Tbolt Damping setting

WarrenS warrensjmail-one at yahoo.com
Wed Dec 16 17:43:49 UTC 2009


Peter

>PV) Ha ha - thank you teacher!

    Yea, Sorry about that, I could not help myself.
I was  just showing some new Information that had not been presented before.

***
>PV) I appreciate that everyone's will be different, depending on individual characteristics
> of the Thunderbolts, and also (perhaps more importantly) the aerial and its position. 

    Give yourself a double star for those SO TRUE statements

***
>PV) However, I think it would be interesting to at least see the relative differences for one location.

    I've got Day & Days of that, but not something that can be presented in a posting, and it all depends are so many other things.

***
>PV) I would like to know what the best time-constant is to use.
> I also plan to try reducing the signal level threshold (from the current 4AMU) 
> as recently suggested, and try to see some quantifiable results.

    THE Problem is a little bit of knowledge is a dangerous thing.
Lowering your AMU can either make things a whole lot better, if your noise is now very high, 
OR a little worse if your noise is already good.
Much also depends on the Elevation setting you are using.
and even more depends on the Damping setting AND DAC_GAIN
***

What helps noise is to get rid of or lock out weak signals.  
The way that the Tbolt does its PID Derivative control, 
noisy Weak satellite signals that are coming and going tend to go around the TC filter and are not filtered enough by it.
That causes extra Osc noise. This is why raising the elevation and AMU mask can help Noise a little. 
BUT it is a compromise,
What you do NOT want to happen is to ever loose all satellites, (Noisy ones are much better than none)
Because If  the Tbolt does go into holdover, then all 'noise hell' breaks loose (compared to ns & 1e-12).

So IF you have a good strong antenna & sky view, HIGHER AMU and/or ELV values will help a little, 
IF you have a marginal antenna and view, then Lower AMU values can HELP a WHOLE lot.

To complicate things more, all Tbolts are not the same.
I have two units, Both are setup the same way with High TC and low damping.  
I ran a temp sensitivity test on them both together with a 3 deg controlled temp rise in 10 min. 
One changed under 20 ns and  2e-11 Hz before returning to the correct value 20 minutes later
The other 'lost it mind' and change >250 ns, > 5e-10 Hz  before I forced it to do a fast resync.

The interesting thing is the one that 'took off' like a rocket when the temp changed, 
Is my GOOD unit that has 1/3 the noise output and is much more stable long term,
But turns out that is only as long as I continue to keep its temp, controlled to 1/100 of a deg C or so.

The new Lady heather has a active Temperature control mode that REALLY helps  
to get good performance with one of my units.  My other Tbolt could care less if it's temp is held constant.

This explains why different sensitivities are seen to changing temperature, 
and why some units can run well with a SLOW TC setting and some can't.


What I have found to be a better set of Tbolt default values for the typical well setup unit are: 

1) Dac_GAIN -3.5Hz / Volt         (factory Default -5, range -2.5 to -4.0)   will vary with each unit depending on its OXCO

2) TC = 350 sec                         (factory default 100, range 250 to 1250)

3) AMU   => 4 for a good antenna,  << 4 for a poor antenna 

4) ELV = As high as possible (20 works good for me) without EVER loosing all satellites, 
The goal is to lock out any satellites with a low or poor signal.

5) Damping = 0.85                    (factory default 1.2, range 0.5 to 1.5, depending on users goals

5a) If your concerns are freq accuracy and you don't care about Phase accuracy, 
Set the damping from 1 to 1.5  with the Factory default of 1.2 being a good Nominal Value.

5b) If your concerns are Phase accuracy and are willing to accept some increase in freq noise, 
Set the damping from 0.5 to 0.7  with  0.6 a good nominal value.

5c) And for a compromise between both Phase and Freq 
Set the Damping from 0.7 to 1.0, with 0.85 a good nominal value. 

Lower damping settings < 1.0  are best used only with longer TC settings > 100  

ws

*************************

> Good points, and You got 4 1/2 out of five correct, Not bad at all.

Ha ha - thank you teacher!

Fairy nuff, yes, your plots show the long-term effects.  I would like
to know what the best time-constant is to use.  I appreciate that
everyone's will be different, depending on individual characteristics
of the Thunderbolts, and also (perhaps more importantly) the aerial
and its position.  However, I think it would be interesting to at
least see the relative differences for one location.  I recently
learnt that our national mapping organisation (The Ordnance Survey)
average the results from their L1/L2 Leica 1200 system receivers, for
two hours.  Is the oscillator in the Leica significantly worse than
that in out Thunderbolts, or could we also benefit from a
time-constant of longer than 1000 seconds?

I will try to find a quiet rubidium, and do some comparisons against
that - the results should, at least, be valid out to a few thousand
seconds.  I also plan to try reducing the signal level threshold (from
the current 4AMU) as recently suggested, and try to see some
quantifiable results.

     TTFN,

          Peter Vince  (G8ZZR, London, England)




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