[time-nuts] Adjusting HP 5065A frequency

[WarrenS]

>Lowest hump you get from the high damping-factor (40) 

The plots and the Wenzel calculated damping factor and BW numbers, are for a 2nd order PLL per the schematic, and can be a bit misleading.
By making the Cap bigger (extreme case is to short it out) the loop becomes a 1st order freq tracking loop and the (B/W formula changes).
A first order loop lowers the freq noise, i.e the size of the ADEV hump,  but it really screws up the phase tracking and phase settling time.

Attached,  the Red plot shows the ADEV effect of just changing the Damping factor to 40.
As shown by the results, Too much damping is generally not a good idea in a 2nd order loop.
All depends on what trades-offs one is welling to accept.

ws

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

Magnus Danielson magnus at rubidium.dyndns.org said:

Warren,

On 10/23/2012 12:45 AM, WarrenS wrote:
> Tom said:
>
>> In general, when you discipline a OCXO you get that characteristic ADEV
>> "hump".
>> At some point there is a cross-over and you know/assume that at that
>> point
>> each must be contributing 1/sqrt(2) of the noise.
>
> Although putting the Hump at the cross over point is typically a good
> compromise, there are other choices which give different trade-offs.
>
> Attached is a excell plot showing the effect of different PID tunings
> on the Hump's size and location.
> This spread sheet (originally is from Wenzel's site) was for plotting the
> effect of the RC H/W values for a PLL when used as a clean up Osc,
> but can be used for setting the PID values of a disciplined GPS
> such as a Tbolt with a few changes and using different data.

As expected, the hömp (sorry for sudden Inspector Clouseau) depends 
greatly on the damping factor. Lowest hump you get from the high 
damping-factor (40) located at the cross-over point of the two curves.
The remaining hump is what you need to live with as the noises is 
spliced together by the high-pass/low-pass action of the PLL.

Good illustration. I haven't seen it before, but it is expected.

Cheers,
Magnus
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