[time-nuts] New NTBW50AA

[Bob Camp]

HI

What can be said with reasonable confidence:

1) The unit isn't broken and it does not need to go back to RDR under their 30 day return policy.
2) The OCXO isn't broken and it does not need to be replaced.
3) The unit is functioning ok, but probably not as good as it could.
4) Without comparison to an independent standard, there's no way to know how well it is or isn't doing.

Bob

On Sep 18, 2013, at 10:44 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:

> 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
> 
> 
> 
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