[time-nuts] test mailing from a new member

Hal Murray hmurray at megapathdsl.net
Wed Jun 17 17:49:43 EDT 2009


david.bengtson at gmail.com said:
> Specs.. Hmmmm. Good question. The application is for a frequency
> reference, so frequency stability vs. PPS accuracy is important. Cost
> is perhaps less important than size. The OCXO under control is a
> small, surface mount part with +/- 0.5 PPM over temp, but flexibility
> to use higher accuracy parts would be good. As this is for an internal
> application, I'm not sure how much else I can disclose. 

0.5 ppm is pretty sloppy for an OCXO, at least relative to what this group 
normally talks about.

The other major parameter to consider is the location of your GPS antenna.  
Don't forget things like snow and birds building nests.

For a given quality output, there is a trade-off between the quality of the 
crystal and the quality of the GPS signal (antenna location and 
antenna/receiver sensitivity, noise rejection etc).

Begin core-dump mode:

GPS units come in several flavors:
  Navigation
  Timing
  Surveying

The typical consumer GPS gear is setup for Navigation.  Most of the low cost 
units use NMEA (National Marine Electronics Association) protocol on the 
serial port.  Some of them have PPS signals.  The Garmin 18x-LVC is popular 
for running NTP on a PC.  Most of the newer/cheaper ones use USB which 
obviously doesn't support a PPS signal.  They generally need to see at least 
3 satellites in order to get a lock.  (4 is the right number, but they fake 
it with 3 by pretending to be at sea-level.  Maybe that's ground level, with 
elevation from either a map or recent history.)

The timing units can get away with only 1 satellite because they know where 
they are located.  You either tell it the location or tell it to run a survey 
which averages the location over many samples.  Motorola used to make a line 
of good timing receivers, but they got out of that business and sold it to 
iLotus available through Synergy:
   http://www.synergy-gps.com/


GPS units typically come in 2 packaging styles.  Some have the antenna and 
receiver packaged separately.  Some come packaged together.  Some of the 
antennas and packaged-together units are setup for serious outdoor mounting.  
The packaged-together units often use RS-422 (rather than 232) so they work 
with longer cable runs.


You said size is important.  Can you move all the GPSDO stuff outside your 
box?  You have to bring in one cable.  It might as well be 10 MHz rather than 
the GPS antenna feed.

As a straw man, consider something like a T-Bolt.  Even if you don't use it 
in your product, consider getting one for your lab so you will have something 
to compare with and/or a handy lab reference.


-- 
These are my opinions, not necessarily my employer's.  I hate spam.







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