[time-nuts] Tbolt instability

[WarrenS]

John

I have had my TAPR Tbolt for a couple of months now so my sample size of one and experience is rather limited.
I have found MANY reasons why my unit did and still does sometimes behave like yours.

Sorry for the long answer to your long post, But
There is NO single one item that magically fixes everything.
In no special order of importance or max effect, 
I have listed many of  the items below that needed to be addressed 
in order to make mine the best that it could be.
Any and all of these items have caused me similar problems as you described, 
and they may need more than one line to fully explain.
Most if not all of these issues have been discussed in detail on the Nuts site before 
as will as others that I've missed or that have not yet applied to me.

Bottom line is that the Tbold is a good little unit for the money 
and it can be made into a great unit with some TLC.

1) Be sure it has the high resolution Temp sensor, If not CHANGE it.
This is a must so that it does not sometimes think it just had a 1 deg temp step.

2) Do not let the case temp change FAST, isolate it from the environment.

3) Keep the case temp from changing at all, I use a small light bulb in a box connected to a modified fish temp controller.

4) Make sure to have a good antenna and it is in a good OUTSIDE location with a good view.


5) make sure the survey location is correct

6) Don't let weak Sat signals jump in and out of range, (I change the default elevation and the sat level)

7) NOT let the unit  go into hold over.  I've seen that half the time the first holdover will pick a poor starting Dac value.
7b) which is not too bad, BUT when the unit returns from Holdover, it is going to have a giant phase jump.

8) Do not put any pressure on the case top or bottom, & put some rubber feet under it in the 4 corners.

9) Do not let it Tilt, move or vibrate it. (I put mine on its zero G axes)

10) Make sure the +12 volt PS does not change voltage or have ANY noise on it.

11) If you want to remove the line freq noise from the output, use a low noise PS on the +12 volts, &/or add a lot of cap. 

12)  Make sure  the Dac gain is correct, mine is 3.4, not the 5Hz/ volt default

13) The default TC is way too slow, I've found 500 sec with a damping of 1.5 NOW works good for me.
Without the TLC modes, It is safest to leave it at its default of 100 sec and 1.2.

14) Let it run in a long time before judging. It. needs > 1 month. 
The first two weeks, mine was terrible for use as a high precision Nuts quality type reference.

14a) Mine has been going constantly for 2 months, and getting better all the time, but still some times has a little weird behavior, 
That causes the phase to vary by 20 ns over a few hours,  but mostly it now stays within 5 to 10 ns

15) The Number one tool to make sure it is working and to see what it is doing, 
USE Lady Heater plots (not its ADEV) to verify all is working and to make adjustments.

In defense of the Tbolt, 
It's only real problem is we're asking it to do an order of magnitude better performance than it was ever designed for, 
Or than is really necessary. 
Given some TLC,  The fact it can even do the unreasonable demands expected by 'nuts' says a lot for it.

ws

********************
-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of John Green
Subject: [time-nuts] Tbolt instability

Tbolts seem to have a very good reputation for accuracy and stability. 
Most comments I have seen compare them favorably to the 
HP/Symmetricom Z3801.  Mine may be OK in the very long term 
but doesn't look too good in the short  term.

 <snip>