[time-nuts] TIC resolution impact on GPSDO's performance

[David I. Emery]

On Tue, Dec 26, 2006 at 04:14:45PM -0600, Didier Juges wrote:

> When reading the data sheet for the Thunderbolt, and reading all the 
> pitfalls associated with non-integrated GPSDO designs using stand alone 
> GPS receivers, such as sawtooth correction and quantization noise, it 
> seems like the integrated approach used in the Thunderbolt is the best 
> way to go. Not only it is cheaper and simpler, and therefore should be 
> more reliable, but it avoids an entire class of problems and should 
> perform better, everything else being equal (receiver sensitivity and 
> internal noise, OCXO quality). In this case, simplicity goes with better 
> performance, which is not always the case.

	It is my understanding that the Motorola Oncore timing receiver
modules (UT,VP and onwards to the M12+ family) can be ordered in a
version that accepts an EXTERNAL 10 mhz rather than using an internal
crystal for timing.   At least I think it is a 10 mhz input, but I am
quite sure they can be ordered in a version that does not generate
timing or frequency from an internal xtal oscillator.

	And if my presumption is correct that the Trimble Thunderbolt
hardware is either identical to or very similar to the HP/Symmetricom 
58540A hardware (and both OEMed from a company in Japan) I think you
will find that they do use a Motorola GPS timing receiver in external
clocked mode using a clock derived from the 10 mhz OXCO.   I beleive my
58540As do anyway.
	
> Yet, I am surprised that so many of the OEM timing receiver solutions 
> use the *conventional* approach. For instance, the Lucent receiver John 
> just bought seems to have a discrete, independent GPS receiver (the 
> darker board on top), and many companies still build stand alone GPS 
> receivers specifically for timing application without embedding the 
> GPSDO logic and an OCXO. That does not seem to make much sense to me.

	I have ordered a Lucent RFTG-M-XO box but it has yet to arrive -
however, looking at the photos published today, it does not seem clear
they don't use a GPS receiver with an external clock.  Not easy to tell
since the guts of the Motorola module are under a shield can.

	There is certainly no reason to integrate a GPS receiver with
the OXCO PLL and status electronics if one can buy an OEM one that takes
an external clock for less money (and hastle over firmware development
and licensing for the GPS side).   GPSDOs are a small market compared to
the overall market for OEM GPS solutions and there are economies of
scale involved.

	And as a final note - a Datum 9390 box I have that dates to
beginning of time (GPS time that is) used a Rb derived clock for the
antique OEM Trimble GPS receiver board it uses so that approach has been
around from the early days...

-- 
   Dave Emery N1PRE,  die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."