[time-nuts] Lady Heather's Tbolt oscillator auto-tune function

ws at Yahoo warrensjmail-one at yahoo.com
Sun Sep 11 18:17:31 EDT 2016


If you have a good antenna setup and a constant temperature environment:

The way to get the lowest noise over short time periods on a TBolt is to set
the TC setting as high as you can (typically 500 to 1000+), and set the
Damping factor as fast as you can (typically 0.7 to 1).

The reason is that the Tbolt has a simple basic PI phase lock loop
controller.
The Proportional gain is a function of 1/TC and the Integrator gain is a
function of the damping.
The Problem is that the GPS phase error signal is a noisy thing with short
and long term peak variations of about 10ns. 

Any GPS phase noise is directly fed thru to the Dac output scaled by the "P"
Gain.
With a 10 ns phase noise step and a 100sec TC, the "P" gain will change the
oscillator freq by 1e-10, on the very next one sec Dac update.
This is not done to keep the time correct; it is needed to correct the
Frequency in the desired time constant period.

This then causes the continuous up to 1e-10 freq steps, with the factory
default setting, the effect of which can be seen by plotting the DAC
voltage.
If the TC is made twice as long then the Dac noise step size is 1/2, etc.
If you have a good setup and a low ADEV osc at long time constants, then the
longer the TC, the better the short term Peak freq noise will be.
If you want the "Dac P-gain freq noise" peak to be under 1e-11 then the TC
must be set >1000sec. (which then needs to use the extended TC method)

Where as the Damping setting controls the integrator gain and its effect is
filtered and non linear. A change in damping from say 2.0 to 0.7 will
increase the Dac noise by up to only 25% but reduce the setting time by over
10 to one.


>Bert posted:
>For our work frequency has to be better than 1xE-11 one second.
>Unless someone can help us with settings we will not revisit Tbolt.

The attached shows that a modified Tbolt can do 1 sec freq error of about
1e-11 peak over most of this 2hr run. The 1 sec ADEV of this run was about
1e-12.
Sounds like best to go back to your previous system, because you are not
going to get there by randomly changing the settings.


ws

********************

>> A damping setting in a Tbolt of greater than 2 is not a good idea.

>It is if one is only concerned about frequency stability and accuracy 
>and wants the best performance that can be obtained at tau < about 10 
>seconds.

>> And a TC setting of >1000 is not a good idea unless using the 'extended
>>TC' method.
>I did not suggest setting the TC >1000. 

>Best regards,
>Charles

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