[time-nuts] UBlox GPSDO

[Attila Kinali]

On Thu, 16 Apr 2015 09:24:02 -0700
Wayne Holder <wayne.holder at gmail.com> wrote:

> According to page 51 of the protocol manual for the UBlox NEO-M* series GPS
> modules, the timepulse can be programmed to output a 10 MHz signal rather
> than a 1 PPS signal:
> 
> 
> http://www.u-blox.com/images/downloads/Product_Docs/u-bloxM8_ReceiverDescriptionProtocolSpec_(UBX-13003221)_Public.pdf
> 
> I'm curious if anyone has tried this.  Seems like a possible way to get an
> inexpensive GPSDO, as these modules are very inexpensive.

Yes, this feature has been introduced with the LEA-6T and AFAIK
a couple of people were using this output for building an GPSDO.
For the main user of this, they introduced a special variant
that could directly produce the needed "odd" frequency: the LEA-M8F
(you have one guess who the main user of this is ;-)

Hint: Set the clock ouput to 8MHz instead, as the internal clock
is a 48MHz clock and the timepulse is generated from it by division
(or rather a counter). Having an interger divisior avoids quite
a bit of phase noise. Additionally, there are two modi at which the
time pulse can run: constant frequency or constant phase. The latter
will have phase jumps (and thus phase noise) with new fixes (or was
it once a second?) to adjust for internal oscillator frequency/phase
offset. While the former mode will try to keep the phase continous
by keeping the frequency approximately constant (and thus introduce
a phase error)

(Dont ask me where this is documented, i've found it somewhere
in one of the many u-blox documents, many many moons ago)

			Attila Kinali


-- 
< _av500_> phd is easy
< _av500_> getting dsl is hard