[time-nuts] Ublox neo-7M GPS

Ed Palmer ed_palmer at sasktel.net
Thu Aug 21 00:40:00 EDT 2014


Thanks, Tony.  That's good info.

So now we've confirmed that the neo-7M has an NCO and it appears that 
it's resolution is 20 ns.  The data sheet shows the 'Accuracy of time 
pulse signal' is 30 ns RMS and 60 ns for 99%, but it isn't clear whether 
they're referring to jitter or error with respect to GPS seconds.

The original question was whether the neo-7M would make a good GPSDO.  
As we've seen, the answer is no.  Cheap, yes.  Good, no. Setting aside 
the NCO issue, the neo-7M isn't a timing receiver, it's a navigation 
receiver.  That limits it's performance in many ways.

Ublox sells timing receivers, but they're still NCO-based.  They're also 
significantly more expensive than the navigation receivers. One example 
is Synergy Systems' SSR-6Tr if it's still available.  It was announced, 
and discussed on this list, in 2012 but it still isn't listed on their 
web site so I don't know what it's status is. It's based on the LEA-6T 
timing receiver which has a spec for the 1 PPS is 'within 15 ns to 
GPS/UTC (1 sigma)'.  That can be further reduced with some extra work.

If the performance of an NCO-based unit isn't enough, you might want to 
consider Jackson Labs GPSTCXO which is a real GPSDO.  More expensive 
than the NCO-based units, but you get what you pay for.

No, I'm not associated with Synergy or Jackson labs.

So Graham, if you survived the firestorm started by your simple 
question, are you any wiser?

Ed

On 8/20/2014 7:56 PM, Tony wrote:
> On 19/08/2014 16:11, Ed Palmer wrote:
>> Does anyone have a neo-7M and an HP 5371A or a 5372A Analyzer?  Use 
>> the Histogram Time Interval function to measure a block of samples. 
>> That will show the length of the samples with a resolution of 200 
>> ps.  That's what I did a couple of years ago when I analyzed the 
>> Navsync CW-12 with the old and new firmware.
>
> FWIW, I just had a look at the timepulse on a NEO-7M. I configured it 
> to 10MHz, 50:50 duty cycle when locked, disabled when out of lock. I 
> don't have any of those Analyzers so I used an HP 54615B digital 
> scope. The period of the majority of cycles was 104ns with 'random' 
> cycles being 84ns. I did not observe any other cycle periods. I don't 
> know how accurate the time measurements are on the scope, but it looks 
> like the timing is derived from an approx 48MHz clock, and the timing 
> phase/frequency adjusted by periodically deleting 48MHz clock cycles.
>
> Although I said random, I couldn't make any observations as to the 
> statistics of the short and long cycles or their distribution - I 
> guess I'll have to write some software for my STM32F4 discovery board 
> for that.
>
> If I get time, I'll do the same with a Reyax RYN25AI receiver which 
> has a UBLOX MAX-7C module.
>
> Tony




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