[time-nuts] Subject: Re: GPS 1PPS, phase lock vs frequency lock, design

[Bob kb8tq]

Hi

In a GPSDO, an FLL can be done with no “cycle slips” between readings. In that case, the I term will indeed 
correct for long term errors. The net result will be effectively the same as a PLL for long term error. That is by
no means to say that *all* FLL’s are done this way. Only that it is one possible implementation. 

Bob

> On Jun 24, 2019, at 9:16 PM, Glen English VK1XX <glenlist at pacificmedia.com.au> wrote:
> 
> 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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> 
> 
> 
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