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

Thu Oct 16 17:38:12 UTC 2008

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

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of SAIDJACK at aol.com
Sent: Thursday, October 16, 2008 12:40 PM
To: time-nuts at febo.com; tvb at leapsecond.com
Subject: Re: [time-nuts] Frequency Stability of Trimble Mini-T

Hello Mike, Tom,

just came across your post. This looks to me like a classical crystal  
frequency jump that the unit is compensating for. Looks like the Mini-t is a
bit  
under-damped in it's response.

John Vig talks about these jumps in his paper, and he mentions that  the
root 
causes are not well understood. One way to verify this is to plot the  EFC 
voltage (not sure if this can be done on the Mini-t), and see if it "jumps",

similar to the plots in the Vig paper.

Co-incidentally we have developed a unit for this same Army requirement  
(11Hz requirement), and early on had seen a similar issue which we solved,
and  we 
can now easily meet this requirement. Let me know if you would like more  
details, we can provide these offline.

Typically crystals jump <1ppb, but I have seen worse. That busts the  11Hz 
requirement of course. With GPS disciplining, these jumps can now  easily be

seen. "Curing" the jumps is a combination of crystal blank processing,
testing, 
burn-in, and other factors. There is no easy solution.

You actually have stumbled upon an area where, as well documented as  Quartz

oscillators are, there is quite a lack of documentation and research in  my 
opinion.

One note: some of the time nuts had received E1938A' units that  had tags on

them saying "crystal jumps", so this problem happens in the best  families
:)

bye,
Said

In a message dated 10/14/2008 17:13:04 Pacific Daylight Time,  
mfeher at eozinc.com writes:

Tom  -

Thanks for your reply. I was not present during the testing.  In my opinion,
and now I have been able to convince others, the contractor  passed the
test,
which is derived from MIL-STD-118-164ACN2. The main  specification states
the
following:

5.1.2.5 Carrier  frequency accuracy. The carrier frequency accuracy at the
antenna feed  shall be within 1 KHz of the intended value for all RF
carriers.  Recalibration intervals to maintain this accuracy shall not be
less than 90  days. 

This is on an Army satellite terminal. Another branch of  the Army came up
with the 11 Hz requirement over 24 hours (for testing  purposes), which is
just 1KHz/90, and I feel it is total rubbish.  I  myself have been trying to
find out the gate time used on the counter, but  have not been abler to get
it yet and not sure I will. Needles to say,  using the stated counter, and
logging a measurement at one minute intervals  with a resulting readout down
to 1 Hz at 30 GHz, probably does not leave  too many options for gate time.
The jumps do not exactly come in pairs,  although they seem to, but,
remember
there are close to 16,000 points on  that graph. I agree with you that
averaging would make the plot into a  straight line. 

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