[time-nuts] Thunderbolt settings

Ulrich Bangert df6jb at ulrich-bangert.de
Tue Dec 2 09:59:05 UTC 2008


Dick,

to be honest, I did even some more things concerning the TBolt. Find
attached the results of an measurement where the oscillator disciplining
has been disabled and the internal PPS comparison has been logged over
some time. So it is free running OCXO vs. GPS. The falling slope in the
left is much due to GPS jitter averaging out with longer observation
times. The ascending slope is much due to the OCXO reacting to
environmental conditions. This measurement would indicate that 1000-2000
s is a good value for the time constant. However, I did not have the
heart yet to use such a big value. Neveretheless I use 1200 s TC in my
DIY GPSDO made from an M12+ and an FTS1200. 

I think that also an clarification is necessary what exactly different
manufactures mean by "time constant". I am using the term "time
constant" in the same sense as Stanford Research do in their PRS10
manual, where they talk about the pll's "natural time constant". When a
sudden step in OCXO frequency is noticed by the pll it will react to it
by changing the EFC voltage. It is exactly the "natural time constant"
that the pll needs to bring the OCXO frequency to its original value.
The pll reaction as a whole lasts much longer since it wants to bring
the phase difference back to zero.

Best regards
Ulrich   

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Richard Moore
> Gesendet: Montag, 1. Dezember 2008 22:00
> An: time-nuts at febo.com
> Betreff: Re: [time-nuts] Thunderbolt settings
> 
> 
> 
> On Dec 1, 2008, at 7:08 AM, Chuck and Ulrich  wrote:
> 
> > ------------------------------
> >
> > Message: 2
> > Date: Mon, 01 Dec 2008 09:02:33 -0500
> > From: Chuck Harris <cfharris at erols.com>
> > Subject: Re: [time-nuts] Thunderbolt settings -- one more time
> >
> > The T-Bolt is designed to be a turn key system, and is 
> already set up 
> > for the best operation the factory knew how to achieve. I 
> doubt that 
> > anyone has spent much time diddling the time constants.
> >
> > -Chuck Harris
> 
> Thx, Chuck for your reply.
> >
> > ------------------------------
> >
> > Message: 3
> > Date: Mon, 1 Dec 2008 16:08:44 +0100
> > From: "Ulrich Bangert" <df6jb at ulrich-bangert.de>
> > Subject: Re: [time-nuts] Thunderbolt settings -- one more time
> >
> > Chuck and Dick,
> >
> >> The T-Bolt is designed to be a turn key system, and is 
> already set up 
> >> for the best operation the factory knew how to achieve.
> >
> > Yes and no! Clearly the factory settings have to be so that 
> a lock of 
> > the pll is possible within a reasonable time, say minutes 
> to hours or 
> > so. With a freshly powered OCXO the initial aging effects 
> can be that 
> > large that really short time constants as the default 100 s of the 
> > Thunderbolt are necessary.
> >
> > I attach a measurement that I made on a freshly powered up HP10811
> > some
> > years ago. Note: This was by no means a new device but one that had
> > already thousands of hours lifetime. Clearly any regulation 
> loop will
> > need short time constants to follow this initial aging. 
> Note that the
> > aging effects are that big that frequency changes due to diurnal
> > temperature changes in my flat are not visible! Temperature has
> > otherwise the biggest impact of all environmental parameters on
> > oscillator stability.
> >
> > After some weeks of continuous (!) operation the time 
> constant of the 
> > Thunderbolt loop may be set to higher values. Mine 
> currently runs at 
> > 500.0 s. Some tests indicate that the time constant may 
> even be set a 
> > bit higher. I have made experiments on time constants with 
> a number of 
> > different gps receivers and different OCXOs (including RBs). As an 
> > overall result it turned out that TCs > 1500 s are impraticcal in a 
> > normal living environment. With some degrees Centigrade temperature 
> > changes in a typical center European environment along the 
> day that is 
> > the maximum time that allows the loop to follow the temperature
> > changes.
> > This includes RBs which have an measurable coefficient of 
> temperature
> > too.
> >
> > Best regards
> > Ulrich
> >
> >>
> >> Richard Moore wrote:
> >>> Dear nuts --
> >>>
> >>> Let me see if I can ask this in a way that will prompt 
> you to supply 
> >>> some input:
> >>>
> >>> What is the maximum Loop time constant the TBolt (or perhaps the 
> >>> TBolt monitor sw) will accept as input? What TCs are you TBolt 
> >>> owners using?
> >>>
> >>> Thx,
> >>> Dick Moore
> >>>
> >>> -------------- next part --------------
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> > f7077c5e/attachment.pdf
> >
> > ------------------------------
> 
> Thanks Ulrich. The default TC on my used TBolt was 100.0s. I ran it  
> for a couple of days and it was stable with a very low level 
> of total  
> DAC change -- about +10mV. After a few more days, and after 
> looking a  
> various ADEV plots, particularly yours, Ulrich, I set the TC to  
> 1000.0 sec. It refuses to take 10,000 sec, but I haven't 
> explored the  
> limits of what it will accept, and was hoping, since Trimble won't  
> tell me in the docs the acceptable ranges of anything you can 
> adjust,  
> that one of you would know.
> 
> Dick Moore
> 
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