[time-nuts] Motorola Oncore M12+T Sawtooth

[Graham / KE9H]

The source of the sawtooth is not the GPS receiver L.O., it is the
frequency of the master clock in the CPU.

The 1PPS line is a GPIO line from the CPU.
This means that the 1PPS line is constrained to only change state
on the edge of a CPU clock.  The CPU can calculate exactly when the
the 1PPS line is supposed to change, but if it is a 20 MHz CPU clock,
it can only change every 50 ns.  The sawtooth correction is the difference
between the constrained transition and the ideal transition.

--- Graham / KE9H

==

On Thu, Jan 29, 2015 at 4:27 PM, Tom Van Baak <tvb at leapsecond.com> wrote:

> google for: m12 gps sawtooth
>
> The +/- 127 comes from the range of 8 bits.
>
> /tvb (i5s)
>
> > On Jan 29, 2015, at 12:44 PM, Tom Wimmenhove <tom.wimmenhove at gmail.com>
> wrote:
> >
> > I've built a bunch of carrier board for the M12+T timing receiver. One
> > thing I could not find much information about is the maximum sawtooth
> > (which, I assume, related directly to the frequency of the receiver's
> > internal LO). I know my particular module outputs a correction values
> > between -15 and +15, in which case I could use a DS1023-25 delay-line
> chip
> > which uses .25ns increments instead of the DS1023-100, which uses 1ns.
> This
> > would give the overall circuit slightly better linearity. Problem is that
> > the datasheet doesn't say anything about this, it just says -128..+127ns.
> > Does anyone know if there are receiver units out there that have
> different
> > LOs ?
> >
> > Regards,
> > Tom
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