Hi Dana,
Can you explain more what you use a GPSDO for? For most people a GPSDO
is merely a replacement for a stand-alone XO or TCXO or OCXO or even
rubidium. As such, the more stable and accurate the frequency the
better. Which is why a FLL-based GPSDO, as Leo describes, is a perfectly
fine solution.
> My interest lies in having a stable LO for receiving, without
accumulating
> phase error (at least during times of missing reference).
Ok, it sounds to me like you shouldn't be using a GPSDO at all. Instead
pick your best LO and use that for receiving (stable frequency). Then,
simultaneously, collect raw sawtooth-corrected GPS/1PPS data against the
LO (absolute phase). After you collect your data, apply any observed
phase drift to your frequency measurements via post-processing.
/tvb
On 6/23/2019 9:45 AM, Dana Whitlow 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 <[email protected]> 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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