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

[Dana Whitlow]

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

On Sun, Jun 23, 2019 at 9:01 AM Leo Bodnar <leo at leobodnar.com> wrote:

> I have to draw your attention to practical aspects of why some designs use
> FLL rather than PLL.
>
> Consider a GPS locked OCXO outputting GPS synced 10MHz signal.
>
> Properly designed control loop will not produce much (if any) difference
> when the reference (GPS signal) is present.  In the end, integral of zero
> is zero.
>
> When reference (GPS lock) is lost the things are very similar too,
> holdover is just flying blind in the rough direction you were facing last.
> Accumulating frequency and phase offset on the way.
>
> However, when reference is restored the things are much different.
> After regaining the reference (which in case of GPS signal has unambiguous
> absolute time embedded into its phase) *proper* PLL loop will try to
> correct for slipped phase at the highest slew rate.  This can be huge.  If
> phase has drifted 1ms apart the loop will have to slew the phase all the
> way until it gets those 10,000 cycles out of the way.  This usually looks
> ugly in frequency domain and is very disrupting if you are using the device
> as frequency reference rather than an absolute time reference.
>
> Proper FLL loop will just gently (and reasonably quickly) get your
> frequency back and forget about all the lost phase.  Which is what a lot of
> users want.
>
> Initially, I have used PLL mode on GPS clocks that I am making, but
> switched over to FLL during the last few years.
>
> Cheers
> Leo
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