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

Didier Juges didier at cox.net
Wed Dec 27 12:17:02 EST 2006


David I. Emery wrote:
> 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.
> 	
>   

I have found internal pictures of the 58540A receiver at 
http://www.realhamradio.com/58540A.htm

The spec/manual is available from tvb's web site (found it through 
google...)

It is interesting that the Thunderbolt and the 58540A have pretty much 
exactly the same dimensions and interface connectors, have both an 8 
channel receiver yet look completely different internally. Also, the 
58540A review linked above says the 58540A does not support some of the 
features that are available on the Thunderbolt, such as timing error 
which is available on the TB.

The locked spec is similar, with the 20nS 1 sigma. The holdover is 
specified quite differently though, +/-1 uS in 2 hours for the TB and 
100 uS/hour for the HP, that's worlds apart!!!

Of course, the software interface is totally incompatible, with the TB 
using T-SIP and HP using SCPI.

Didier


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