Leo is right
Depends on the application. Phase lock for 1pps to trigger say,
simultaneous capture of many radio telescopes around the globe is a good
need for phase lock to a source. Frequency lock might suit many . change
of phase between two sources might indicate frequency change, or duty
cycle change.
For all my digital PLL and digital FLL implementations, the method of
capturing the data is identical... just the transfer function of the
filter is different. So they are nearly indistinguishable.
A choice might depend on the control loop bandwidth, and whether the
initial error (acquisition) is beyond the BW. In those cases depending
on the type of comparison, it might make sense to measure the frequency
error, and then move the (controlled) source within phase lock without
cycle slip range in one step. Useful for very long time constants and
large initial errors.
...and then what do you do when including relativistic effects ...
glen.
On 25/06/2019 3:42 AM, Leo Bodnar wrote:
Hi Dana,
I am just saying that, properly implemented, PLL and FLL are indistinguishable
as long as output signal is concerned while lock is present and that the phase
slew at regaining lock in PLL loop is counterproductive for one but necessary
evil for others. I have a feeling that FLL is looked down upon by general
public ever since PLL became a household term.
In a well designed PID loop "I" term makes sure that you don't have "permanent but
varying error."
All my messing about with loops, holdovers and recovery was pretty much with
your application in mind.
Cheers!
Leo
Are you saying that you want to abandon phase lock altogether in favor of freq
lock? Or just during the reacquisition following loss of and restoration of the
reference?
By me definition of pure freq lock, there will generally be some permanent
(but varying)
frequency error, so that phase error could accumulate without limit;
clearly an undesirable
thing in most applications.
My interest lies in having a stable LO for receiving, without accumulating
phase error (at least during times of missing reference). When the reference
goes away, I'll
accept some phase error accumulation. So for me, I think the best approach is
phase lock
under normal circumstances, but switch to freq lock during reacquisition of
phase lock.
Dana K8YUM
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