[time-nuts] Frequency Stability of Trimble Mini-T

SAIDJACK at aol.com SAIDJACK at aol.com
Thu Oct 16 18:18:29 UTC 2008


Hello Mike,
 
the problem is one of statistics. Not all crystals exhibit this, one may  
find only about 20% or so of a typical manufacturers OCXO will exhibit these  
jumps, and with varying intensity. So unless these are directly addressed at the  
factory (which does not seem to be the case for the Mini-T), then some units 
may  meet spec, while others won't, and there is no way to tell the 
difference. As  far as I know, there is no OCXO vendor that by default tests their 
product for  these jumps during production.
 
I think you may be fighting an uphill battle to try to have the Army change  
their specs :) We don't understand some of these specs either, but decided  to 
just meet them rather than trying to get them to change their ways. In the  
process we were able to find and design IP that gives us a slight edge.
 
 
Due to confidentiality, I can't comment much online on the GPSDO we  did for 
this particular application other than that it is based on our  FireFly-II 
GPSDO; but will contact you offline.
 

On the EFC drift you had seen in holdover on the Mini-t: there would be no  
correlation between EFC voltage and jumps without having GPS active, since the  
crystal will just change frequency without the firmware ever knowing anything 
 about this change. So the EFC voltage would stay constant while in holdover, 
 while the output frequency may vary.
 
But with GPS enabled, the unit would immediately see the phase shift  
(frequency error), and start correcting for it. This correction is what we can  see. 
The crystal may require a slightly different EFC voltage after the  jump to 
generate 10.0MHz because something physically changed in the crystal  (such as a 
mechanical stress relief, particles leaving/reaching the quartz,  etc). This 
voltage difference can be between 50 microvolts, all the way up to  several 
millivolts.
 
bye,
Said
 
 
In a message dated 10/16/2008 10:49:45 Pacific Daylight Time,  
mfeher at eozinc.com writes:

Said  -

To date there has been no data taken just on the EFC to see if there  is any
correlation. However, late last year a test was run for about 11  days on the
Mini-T, without the antenna connected, and a slow downward  drift was
evident. In fact, the drift was such that the 11 Hz requirement,  at 30 GHz,
was easily met even though there may have been noise on the EFC.  Like I said
previously, I am in the process of fighting the 11 Hz  requirement as I
believe it is totally unrealistic. Even the spec of 1 KHz  is unrealistic at
the high data rates being used. But, since that can be  easily met, at least
for the time being, I am not ready to tackle that  issue. I did receive one
of those 1938A units from Rick, but have not had a  chance to play with it
yet. 

I think I mentioned in a previous post  that John did contact me over the
weekend and told me he retired 2 years  ago. Now he is too involved with his
new post as President of the IEEE  starting in 2009. I was hoping he could
help me fight this battle. We  worked on some of the same issues together on
and off in the last 25 or so  years. 

I would definitely be interested in your unit. I assume then  that it was
also for a terminal to be used with the AF WGS satellite.  Regards - Mike



Mike B. Feher, N4FS
89 Arnold  Blvd.
Howell, NJ,  07731
732-886-5960




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