[time-nuts] Low cost GPS module for < 100ns timestamping error

Tony tnuts at toneh.demon.co.uk
Mon May 12 06:03:24 EDT 2014


On 10/05/2014 04:19, Chris Albertson wrote:
> On Fri, May 9, 2014 at 10:46 AM, Tony <tnuts at toneh.demon.co.uk> wrote:
>
>> But isn't that only supported by 'timing' GPS modules that allow you to
>> specify the location? But they are rather more expensive than the common
>> navigation type modules - are there sub $15 modules that support that
>> single-satellite timing feature?
> With a $15 budget.  I think you are limited to the old Motorola Oncore
> type.  The UT+ sells for about $15 on eBay.  There are two versions of the
> UT.  Get the timing one.   These aren't to bad.  The PPS one sigma error is
> about 50ns and the UT runs on 5 volts.   The newer MT+ version is better
> but at least $30.

Unfortunately they use way too much power - 800mW maximum compared to 
50mW for a UBLOX MAX-7c which are around $15. It also is specified at 
50ns rms, 99% < 100ns.  It appears that most, if not all, the timing 
type modules are higher power as well as more expensive; unless anyone 
has any better suggestions it looks like I'll have to stick to 
navigation type modules.

> Does it make sense to place thermal insolation on a TCXO?  It does on an
> OCXO because there is a thermostat inside the device and the thermal
> insulation will help the OCXO maintain a constant temperature.   But a TCXO
> will just run hotter.  I think all you need is a box to keep drafts and
> direct sunlight off the TCXO.
The insulation won't make much difference to the overall temperature 
excursions of the TCXO but that doesn't matter because the timing is 
continually corrected by the GPS; it will however significantly reduce 
the rate of change of temperature thus reducing the timing errors during 
holdovers when GPS is temporarily unavailable.

I might need to allocate some of the budget for spikes on the antenna to 
prevent pidgeons perching on it!

Tony H




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