[LinuxPPS] Oncore Refclock interface & thermostat

[Hal V. Engel]

On Tuesday 06 January 2009 08:01:56 Udo van den Heuvel wrote:
> Remco dB wrote:
> > Again, but now a new subject ;-)
>
> Interesting ideas there.
> I was already contemplating teh use of a `gps-board` and a real (cone)
> antenna but was scared of a bit by the extra powering needed.
> (My Garmin GPS-18 is powered over USB)
> Mounting it in the PC is a good idea.
>
> Any recommendations for gps boards?
>
> BTW: how much power does the thermostat solution use?
>
> Kind regards,
> Udo

I am currently using a small patch type antenna.  But I have looked into some 
external antennas and plan to get one at some point.  Most of them consume 
20mA to 40mA.   This is not too much different in terms of total power load of 
the GPS and antenna from the power consumption of a patch antenna which is 
typically between 10mA and 25mA.  

Oncore modules like the UT+ units I am using have a power consumption of 155mA 
and can handle antenna loads of up to 80mA (most other Oncore models are 
lower).  So a typical unit with the antenna load will consume about 175mA to 
190mA.   With a higher power consumption antenna amp or if using additional in 
line amps this could be as high as 235mA (for the Oncore UT+) without 
exceeding what the module's antenna power circuit can handle.  These numbers 
are significantly higher than the 60mA power consumption of the Garmin 18LVC 
but this is in most cases less than 1 watt. 

The USB spec calls for a single power unit (IE. one connector) to have a max 
load of 100mA.  So strictly speaking the Oncore + antenna load exceeds that 
limit.  But my experience is that all of the USB ports that I have tried this 
with other than none powered hubs handle this load with no problems.   I do 
not see any voltage drop at the end of the USB "power cable" with the GPS 
powered up.  If this is a concern you could always use two USB cables to 
supply power which would raise the power "limit" on the USB side to 200mA.  
But again it appears that most of these USB connectors can handle loads well 
above the single connector spec without issues.  The only caution is that if 
you are using a USB hub to power one of these units make sure it is a powered 
hub.  Also at least with the Oncore modules I have played around with they 
will not power up if the voltage falls below about 4.5 volts.  I found this 
out when I tried to power one of these from a none powered hub.  So it appears 
that these can detect a low voltage condition.

If you can mount the GPS module in the computer chassis then there is lots of 
5V power available right from one of the internal PSU connectors and the 
limits of the USB interface become a non-issue.  The only concern I would have 
is that the inside of a PC chassis is a high RF noise environment and some GPS 
modules may not handle this very well.  In addition, there are processors that 
run at frequencies that are close to the L1 GPS frequency of 1.575GHz that 
could create significant problems.  One example is the Intel Atom operates at 
at a nominal speed of 1.6GHz and under these conditions you might need 
additional shielding around the GPS module. 

I have been very happy with my Oncore UT+ units.  They are simple to interface 
(although more difficult than the garmin 18lvc), widely available,  fairly 
cheap, well documented, highly accurate and well supported by NTP (thank you 
Reg).  In addition the UT+ modules are intended for use in high RF 
environments such as cell sites and radio tower sites and the RF sections have 
more shielding and higher levels of selectivity than other Oncore models.  The 
ones you see for sale on ebay and other places are for the most part working 
pulls from cell sites that are being replaced with newer units like the M12T 
and M12M Timing Oncores (higher precision and lower power consumption).   I 
expect that the high level of availability will continue for some time as 
these units were widely used for cell sites and were produced by the millions.   
These units are high quality hardware that was designed for a long service 
life so if the unit you get arrives in a working condition it will likely keep 
working forever unless something traumatic like a lightening strike happens.  

I got my latest units from here: 

http://cgi.ebay.com/Motorola-UT-Plus-Oncore-Timing-GPS-1PPS-50-
ns_W0QQitemZ300278266564QQihZ020QQcategoryZ25399QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

One of them arrived DOA but was promptly replaced by the vendor.  At the price 
he is selling them and with the free shipping on additional units it is worth 
while to get more than one unit IMO (IE. spares and for additional projects).   
These are a mix of firmware versions and one of the units I got was version 
2.2 and all of the others were version 3.2.  My understanding is that he also 
has some that are version 3.1.

Hal