Re: GPS versus source dependency / ballistic theory

From: Myxococcus xanthus (mold-guardian_NO_SPAM_at_comcast.net)
Date: 09/20/04 

Date: Mon, 20 Sep 2004 00:32:16 GMT

"Henri Wilson" <H@..> wrote in message
news:qe0sk0djk9t9uba88fv3ad6a5pivmci0on@4ax.com...
> On Sun, 19 Sep 2004 03:52:25 GMT, "Myxococcus xanthus"
> <mold-guardian_NO_SPAM@comcast.net> wrote:
>
> >"Henri Wilson" <H@..> wrote in message
> >news:smgpk0dbuvsufvubo60hqvke0mhgaib7f9@4ax.com...
> >> On Fri, 17 Sep 2004 04:01:13 GMT, The Ghost In The Machine
> >> <ewill@aurigae.athghost7038suus.net> wrote:
> >
> >> >I'd be somewhat interested in the details, but presumably
> >> >the clock is actually a very stable quartz oscillator
> >> >hooked into a PLL and a resonant cavity. The resonant
> >> >cavity points to a fountain of atoms on Earth; I don't know
> >> >how it's set up in space but presumably they're moving
> >> >rather slowly across the cavity. As a beam goes through
> >> >the atoms the atoms resonate/resonate/reradiate, and the
> >> >resulting signal can be used to steer the oscillator. The
> >> >precise frequency is 9192631770 Hz on Earth, for cesium atoms.
> >> >Since that's a ~ 9.2 GHz clock a quartz oscillator can easily
> >> >do it, methinks -- but there's oscillators and there's
> >> >oscillators, as jitter would be a Very Bad Thing here.
> >>
> >> The way I read it, a quartz oscillator is synched at an
> >> intermediate frequency
> >> by an extremely stable electron transition rate of cesium
> >> atoms.
> >> It is a simple procedure to correct the quartz oscillator if
> >> the cesium
> >> frequency changes slightly when in free fall and cutting the
> >> Earth's fields.
> >
> >Uh, no, Henry. It is most decidedly NOT simple. In the feedback
> >loop of the
> >phase locked loop are two hardwired frequency divider chains that
> >step down
> >the outputs from (a) the voltage controlled crystal oscillator
> >and (b) the
> >cesium or rubidium atomic frequency standard to a common
> >comparison
> >frequency, which is monitored for phase error. Through careful
> >choice of the
> >two divisors, the VCXO output frequency can be almost exactly
> >matched to the
> >frequency dictated by GR requirements.
> >
> >The frequency dividers are at the heart of the PLL, and it is
> >obviously quite impossible to fine-tune them.
>
> Why do you say that? It is simple.
>
> It isn't hard to drop or add a 'tick' every few thousand pulses.

That is NOT how frequency synthesizers and phase-locked loops work.

Scenario 1:
Let us assume that the cesium atomic reference frequency (CARF)is
9.192631770 GHz and we want the voltage controlled crystal
oscillator (VCXO) to be locked to a frequency of 10.230000000000
MHz.

A -hardcoded- custom frequency divider would divide the CARF
frequency by 306421059, and a second -hardcoded- custom frequency
divider would divide the VCXO frequency by 341000 to achieve a
common frequency of 30.00000000000 Hz. A phase comparison circuit
monitors any phase differences between the 30 Hz signal from the
CARF and the 30 Hz signal from the VCXO and adjusts the VCXO to keep
the phase difference to zero.

Scenario 2:
We find that we need to apply a GR correction of of deltaF/F =
-4.4647e-10, so we want the VCXO frequency to be locked to
10.229999995432 MHz.

A MAJOR CIRCUIT REDESIGN IS NECESSARY.
A -hardcoded- custom frequency divider would divide the CARF
frequency by 530646691, and a second -hardcoded- custom frequency
divider would divide the VCXO frequency by 590529 to achive a common
frequency of 17.32345066107 Hz. A phase comparison circuit monitors
any phase differences between the 17.32345066107 Hz signal from the
CARF and the 17.32345066107 Hz signal from the VCXO and adjusts the
VCXO to keep the phase difference to zero.

Note 1:
Adding or dropping a 'tick' every "few thousand" pulses is NOT an
option, since that introduces jitter.

Note 2. This example used my made-up divisors, good only to 1e-13
since I calculated them with 32-bit integers and 64-bit doubles;
however they should serve to illustrate my point.

Summary: It is -not- simple to fine tune the frequency synthesizers
used in the GPS, because PLLs do -not- work the way Henry seems to
think they do.

Myxococcus xanthus

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