[time-nuts] Digital Phasae Lock Loops

Jim Lux jimlux at earthlink.net
Sat Oct 17 10:48:08 EDT 2015

On 10/17/15 6:17 AM, Alex Pummer wrote:
> actually, that is a ketch 22, if the loop bandwidth is to low, you will
> have low noise , but it may will not lock at all, an other way to try to
> filter out the noise, also you may make the loop filter digital, but
> leave the the PLL analog, that could have  the possibility to have the
> advantage to be able to change the loop bandwidth  increase for locking,
> and reduce after the detected locking

changing  loop bandwidth between acquisition and tracking, or, 
similarly, (effective) loop bandwidth that changes with SNR are pretty 
common strategies.

In the deep space transponder world (where you are acquiring and 
tracking a very narrow carrier at -155 or -160 dBm against noise of 
-170dBm/Hz) you also want to know what order filter you should be using 
in the tracking loop.  If there's an expectation that the frequency 
being tracked is changing (e.g. Doppler), then a low order loop may not 
be the best choice.

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