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

[Bob kb8tq]

Hi

A lot depends on just how long your “frequency” samples are. If the GPS is good to 
a nanosecond, you are at 1x10^-11 at 100 seconds. It’s a rare crystal based GPSDO
that will hold closer than that frequency wise.

The FLL is non peaking so that’s going to help things a bit as well. 

Bob

> On Jun 23, 2019, at 12:47 PM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> 
> Correction from Dana:
> 
> I meant "... without accumulation of phase error during normal times ...".
> 
> Sorry about that.
> 
> Dana
> 
> 
> On Sun, Jun 23, 2019 at 11:45 AM Dana Whitlow <k8yumdoober at gmail.com> wrote:
> 
>> 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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>> 
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