[time-nuts] PLL question

[Magnus Danielson]

John Ackermann N8UR wrote:
> There hasn't been much discussion on the list about this idea.  I know 
> that Timing Solutions (now Symmetricom) used to make -- or at least 
> advertise -- a "clean-up oscillator" that was a very low noise, very 
> high short-term stability BVA oscillator that would lock to your Cesium 
> reference.  With a proper time constant (10s of kseconds?), that should 
> give the best of both worlds with superlative short term performance and 
> atomic long-term stability.

The math is simple and clear on this and it works well. A pure clean-up 
oscillator may infact have higher bandwidth/lower tau to reduce the 
effects of the loop itself, just as any other "step up" oscillator. When 
running the phase comparator at the output frequency or low division 
rate of it, a fairly high bandwidth is possible which allows a much 
simplified locking loop for a fairly good result. Using a mixer and 
active loop will work very well too.

If possible, keep the comparator frequency fairly high and avoid 
charge-pump detectors (according to my experience with cheaper-tronic 
onces where dead-band created low rate wanderings while simple designs 
excelled in stability).

Using a low phase-noise oscillator for cleanup is a wise idea, even if 
no frequency multiplication occurs. Active loop PI-regulation of 
sufficient bandwidth suppresses most of frequency trackings of the 
oscillator, so long-term stability is less of an issue where as 
phase-noise plots are.

Cheers,
Magnus