[time-nuts] RE: phase locking Rb to GPS

Magnus Danielson cfmd at bredband.net
Mon Oct 24 14:58:21 EDT 2005

From: "Tom Van Baak" <tvb at leapsecond.com>
Subject: Re: [time-nuts] RE: phase locking Rb to GPS
Date: Sun, 23 Oct 2005 14:42:39 -0700
Message-ID: <001201c5d81a$b1b1dc60$c621f304 at computer>


> > Indeed, but you can also handle that by using two oscillators, one for
> long-
> > term stability and one for short-term and lock the short-term to the
> long-term
> > oscillator with a suitable PLL bandwidth. Comes at some additional cost,
> but
> > then you can optimize each end better rather than compromizing on one
> crystal
> > oscillator. That ain't a free lunch either. In the end, you should not
> stare at
> > the numbers, one must recall the intended application and make that work.
> You've hinted at the scenario that I often use here
> in my lab. I do use two oscillators. For short-term
> I use a nice quartz OCXO -- free running. And for
> long-term I use daily averages of 1PPS pulses from
> a GPS receiver.
> But they are not disciplined; it's just two completely
> separate references. Neither helps the other but also
> neither pollutes the other.
> This works because, in most cases, for me at least,
> absolute accuracy is not critical during short-term,
> low-noise measurements. And similarly, undisciplined
> averaged sawtooth corrected raw GPS 1 PPS timing
> provides a slightly better long-term reference than a
> GPSDO provides. So you get the best of both worlds.
> Granted a GPSDO sort of gives you both in the same
> box but do you lose a bit of performance on either end
> as a result.

Indeed. In what I described the short-term oscillator cleans up the short-term
noise of the long-term oscillator while the long-term oscillator takes over the
long-term part from the short-term oscillator and you get pieces of the best
in one signal. The type of PLL loop does makes a difference at the cross-point
between the oscillator responces. Also, a Kalman filtered version would be able
to stabilize at a point which is optimum (for some measure of optimum that is)
for those two oscillators at that point in time.

If you want GPSDO, you can build a line of oscillators, each cleaning up their
region of the phase-noise spectrum / tau-region.

Doing it the way you do in your lab will work for some work, but for other
things you do want synchronous clocks and performance, so you have to lock them
up one way or another.


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