[time-nuts] Frequency Stability of Trimble Mini-T
mfeher at eozinc.com
Tue Oct 14 22:45:35 UTC 2008
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
18.104.22.168 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.
During the testing this terminal was in California, outside, so the
temperature extremes were just the day to night extremes and relatively slow
respect to time. I doubt if they knew the sats-in-view, but I will ask, and
I am certain that they have no idea about DOP, and I am not sure how that
would affect the frequency measured. I pointed out about 6 months ago that I
did not like where they had their GPS antenna mounted as it was, at times,
blocked by the 2.4 meter reflector. They do believe that at times they lost
some birds and since re-located the GPS antenna.
I did contact John Vig about this, however at this time he is too busy with
his activities as President elect of the IEEE, a post which he will take in
2009. He also retired from the Army in 2006. He did give me another contact
within the Army, Ray Filler, and Ray is in agreement with all of my
assumptions and conclusions, and is in the process of reading a white paper
on the subject that I wrote. I was just hoping to get some ideas from this
august group, hence the reason for my post. I will let you know if I find
out anything else. BTW, loopback testing on the terminal at threshold Eb/No
yielded the desired BER without any synch losses over a 24 hour period.
Thanks & Regards - Mike
Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Tom Van Baak
Sent: Tuesday, October 14, 2008 3:38 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Frequency Stability of Trimble Mini-T
I keep hoping to find a Mini-T on eBay but no luck yet and so I
don't have plots on my web site for that unit. Does anyone have
Question on the spec and on your plot, though. What is the gate
time of the frequency measurements? If you average for an hour
or a day many of those peaks would melt.
Also most look like phase jumps not frequency jumps. When you
see frequency jump up and then down by the same amount as a
pair it's usually the result of a single phase jump.
Did you log sats-in-view, or DOP, or temp during your run to see
if the jumps correlated to that?
Finally, "frequency drift" is not at all the same thing as "frequency
excursions", so what did they really mean?
> The attached URL is a 11 day plot of a 30.5 GHz carrier taken every minute
> by an Agilent 53152A microwave counter. The Mini-T has an ovenized
> oscillator. It appears that the GPS attempts to over correct the
> hence the large frequency excursions. The real large excursions, around
> 96 hour range, were due to a loose connector. The requirements are such
> over a 90 day period the total frequency drift shall be no more than 1
> Someone in their ultimate wisdom, decided that since they do not have the
> time to take measurements for 90 days, they will take it for 24 hours and
> not allow any frequency excursions over 11 Hz at 30.5 GHz. Obviously, this
> plot does not pass their requirement. When the oscillator is allowed to
> run, the frequency jumps are not there. Has any one else made any
> measurements on the Trimble Mini-T. Any observations or suggestions are
> appreciated. Regards - Mike
> Mike B. Feher, N4FS
> 89 Arnold Blvd.
> Howell, NJ, 07731
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