[time-nuts] New NTBW50AA

[Charles Steinmetz]

Dave wrote:

>You didn't say, but I get the feeling maybe I should be getting 
>smaller ppt numbers?  When you say  noisy, is high ppt generally a 
>result of excess noise (front end NF)?

Hard to say about the ppt numbers -- my TBolts give somewhat lower 
numbers than you are getting, but they use different OCXOs with 
different phaselocking circuitry, so I can't say what would be 
considered normal for the model you have.  Also, as several have said 
already, the GPSDO is measuring itself with itself (no independent 
reference), so its proclamations need to be taken with more than some 
caution.

Excess noise on the GPS signals (from whatever cause) causes timing 
errors in the GPS, which makes the GPS think the oscillator needs 
correction, which leads to DAC adjustments that aren't 
necessary.  But if you are seeing at least some numbers in the 50 dBc 
and higer range, there is nothing further you can do in this 
regard.  (Well, if the model you have supports it, you can set a 
"signal level" mask that will exclude satellites with signals below a 
dBc threshold of your choice.  As with the elevation mask, if you get 
too aggressive you may force the unit into holdover when there are a 
few OK but not great signals.)

As we all have said repeatedly, you need a good, accurate 
survey.  Also, it is the nature of crystal oscillators that when they 
are disturbed (turned off and back on, frequency adjusted, 
handling/moving, fast temperature changes, etc., etc.) that they take 
time to settle back down -- often months.  Now that you have a 
permanent antenna location, do a precision survey, SAVE IT, then let 
the unit sit undisturbed for several months and see where you are.

You are certainly in the ballpark -- there are clearly no gross problems.

Bob presumably knows more than I do about non-TBolt Trimbles (since I 
know nothing about them).  If he says you can't adjust the TC and 
"gain" I'd be inclined to believe him.  (And if you can't adjust the 
"gain," there is no point in measuring the EFC characteristics of the 
OCXO, except for curiosity.)

>The antenna is a Lucent PCTEL "GPS-TMG-HR-26N, High Rejection 26dB 
>With Enhanced Narrow Band Filtering", apparently for high RF 
>environments, only one I've got or tried

You may have read on another thread about narrow bandpass filters 
tending to have rising group delay at the passband edges.  An antenna 
with "Enhanced Narrow Band Filtering" very likely has more group 
delay error than one with more gentle filtering, so it is possible 
that satellites with high doppler shifts are causing the GPS timing 
solution to shift around more than it would with a different 
antenna.  The one you have is presumably good enough for cell timing, 
since that is what it was designed for.  The question is whether the 
lower group delay of an antenna without "Enhanced Narrow Band 
Filtering" would be enough better that you could tell.  Without 
measuring the filters, we can't really predict what to expect.  You 
might ask the seller if he would exchange your antenna for one that 
doesn't have the extra filtering.  You almost certainly don't need it 
in your rural location, and an antenna without it would at least not 
be any worse, all else equal.

Best regards,

Charles