[time-nuts] GPSDO Question

Tim Shoppa shoppa at trailing-edge.com
Mon Sep 3 07:14:34 EDT 2007

"Tom Van Baak" <tvb at LeapSecond.com> wrote:
> > Jerry
> > 
> > It is amusing/distressing to see that the myth that using an FLL to lock
> > an oscillator to the PPS output of a GPS receiver is a good approach
> > still persists.
> > The optimum solution is a phase lock loop.
> > Whilst building an FLL is instructive/educational, if you want the best
> > GPSDO performance you should really use a PLL.
> > 
> > Bruce
> It would seem for timekeeping applications, a PLL-based
> GPSDO will inherit the long-term accuracy of GPS with
> great fidelity.
> But for many frequency (e.g., transmitters) or time interval
> applications (e.g., frequency counters with finite gate times),
> I'd like to understand, in detail, what the difference between
> a PLL- and FLL-based GPSDO really is.
> Can someone point me to real data or even simulations
> with plots that show rms or adev differences between the
> two camps?

I can think of a couple applications that use FLL's rather than PLL's,
and it's never for good statistics. It's all about either:

1. Artificially speeding up lock
2. Power conservation (surprisingly common concern in consumer
   battery operated items)
3. Given a choice between a simplistic single-loop PLL with
   very low reference frequency or a FLL, they chose FLL because of
   better phase noise performance
#3 has become less and less an issue because the fraction of the
world confining themselves to single-loop synthesizers has become
smaller and smaller. Still, the latest ARRL Handbook shows a clever
example by N6NWP. (They call it "frequency stabilized" rather than
"frequency locked").

Now there are other some other applications that simply ignore
frequency and only lock phase through discrete jumps, that some of us
call "phase-jerk loops", that have incredibly bad phase noise
characteristics (meaning that if you listen to the result on a
speaker it sounds like a million screaming banshees!)
but work fine for clock and data recovery state machines.
Incidentally, the LCD WWVB-locked clocks at discount stores
fall into this category - they simply jerk their phase to lock
to WWVB, and do no frequency correction.

> Thanks,
> /tvb
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