[hpsdr] GPS 1PPS board

[Chris Albertson]

On Mon, Apr 6, 2009 at 9:11 AM, Graham / KE9H <KE9H at austin.rr.com> wrote:

> Although a cheap way to get started, this is a navigation receiver, not a
> timing receiver,
> and it will not be a good choice for frequency control.  This one's spec is
> that the 1 pps
> signal has a jitter spec of +/- 1 microsecond.  Timing receivers, such as
> the Motorola
> Oncore UT+ have a timing mode, that provides jitter around +/- 50
> nanoseconds,
> without sawtooth correction, and sub 10 nanoseconds with sawtooth
> correction.

It's not that bad, the purpose of the 1PPS signal is to correct for
long term drift by providing a direct real time link to a national
standard frequency reference.
What the controller might do is count how many times the signal from
your 10Mhz oven-ized
XTAL oscillator crosses zero in a one second period.   Subtract this count from
the expected number.  You will have a small positive or negitive
number.  The idea is that you very slowly adjust the oscillator so
that tens of thousands of these small numbers add up to exactly zero.
This corrects for effects like the age of the crystal or the
temperature sensor on the crystal oven slowly drifting over time.
These effects are slow enough that you can average over many thousands
seconds.

Basically what you are doing is never "jerking" your 10Mhz OSC.  You
are instead using the 1PPS signal to slowly adjust the "faster or
slower" knob that we used to see on the back of clocks.

Yes a better GPS unit will be better because your 10Mhz oscillator
will converge faster because it needs to average few 1 second periods.
-- 
=====
Chris Albertson
Redondo Beach, California

 1239054569.0