[time-nuts] GPSDO Question

[SAIDJACK at aol.com]

In a message dated 9/2/2007 13:16:32 Pacific Daylight Time,  
tvb at LeapSecond.com writes:

>But  for many frequency (e.g., transmitters) or time interval
>applications  (e.g., frequency counters with finite gate times),
>I'd like to  understand, in detail, what the difference between
>a PLL- and FLL-based  GPSDO really is.



Hi Tom,
 
an FLL has some distinct advantages:
 
  * Sometimes quartz crystals exhibit well-documented  random jumps in 
frequency/phase. Causes for this are speculative, but the  effect on some crystals 
can be rather significant. In an FLL, the recovery is  only 1/2 as long since 
only the original frequency has to be attained. In fact  if it is just a phase 
jump, then there won't be any correction needed in an FLL,  but the PLL will 
have to fully recover by pulling the frequency off it's  optimum.
 
In a PLL, the original frequency, plus an additional negative  frequency 
offset has to be attained to push-back the phase, so the frequency is  "off" for 
much longer.
 
  * An FLL can exhibit much less frequency error than a PLL. The  amount is 
related to the temperature sensitivity of the OCXO. A PLL has to  modify the 
frequency much more to attain an overall zero phase difference. This  is more 
pronounced the more sensitive the OCXO is to temperature changes.
 
 * An FLL can have very low deviation on the crystal and still work  
correctly, say a maximum of +-1E-014 change per second, as long as the total  
available frequency change is greater than the total expected frequency  error due to 
temp, aging, motion etc. Thus an FLL will exhibit higher  frequency accuracy 
over time than a PLL.
 
A PLL will have to be much more aggressive on it's control response to  
maintain phase lock since it has to correct the accrued phase error over  time, 
while the FLL just has to correct for instantaneous error, not integrated  error.
 
Of course most of the instruments found in a lab are more sensitive to  
frequency errors than phase errors, such as a frequency counter (used in  frequency 
mode), Spectrum analyzer, Jitter analyzers, RF signal generators etc  so a 
PLL can actually degrade their performance versus an FLL.
 
Lastly, the phase-error in an FLL is related to the sensitivity (gain)  of 
the loop, and is smaller the better the OCXO is.
 
An FLL can be simply seen as a PID controller where the I and D terms are  
set to zero. The literature has lot's of mathematical information about why the  
noise is much less in a proportional-only loop then when adding  the integral 
(phase) part to it.
 
This brings up another question: how good are true PID loops that also make  
use of the differential term (e.g. correcting for rate of change of the 
phase)?  The literature talks about the differential term being hardly used in the  
industry because it can add noise and instability to a system...
 
bye,
Said
 
 
 
 



************************************** Get a sneak peek of the all-new AOL at 
http://discover.aol.com/memed/aolcom30tour