[time-nuts] Ublox neo-7M GPS

Thu Aug 21 06:34:01 EDT 2014

Sorry but I disagree. Having done extensive work with the M7 and M6 in  
connection with the with GPSDO work we are doing we have characterized the 
units  extensively.  First from what we can see the difference between a SSR-6T  
and a $ 16 M6 is that one has a TCXO and outputs sawtooth correction data 
but  uncorrected both are the same. 
Last year we did extensive work on Brooks GPSDO and it works well with  
uncorrected M12's and ublox $ 16 M6's.  With a Morion we got 1  E-12.
With a geometry shrink in the M7 silicon higher frequency is possible and  
also lower power. Ublox most likely wants lower power and higher  
performance but not necessarily lower sawtooth because those OEM's that  need it will 
get a version with sawtooth data. Basic engine is still the same.  Time nuts 
are not a big enough market.  Sawtooth is smaller compared to the  M6 doe 
to the higher clock frequency and it is safe to assume that when they  come 
out with a M8 it will even be less.
On the universal controller we have a GPS filter not correction on the  
input that does improve performance. 
I took a page out of Ulrich's work when I saw a picture of his GPSDO where  
he thermally isolated his M12. With the FE 5680 work I made the M12 part of 
the  Rb by mounting it with metal stand offs to the backplate of the Rb.It 
in turn is  temperature controlled.
In the case of my FE 405B work I actually placed the M6 inside the OCXO  
took the battery off. I think I have a picture if interested.
Not knowing that it can not be done I did what I call a GPS-PLL using a M7. 
 Attached  is the board layout on the right side is what we are presently  
using with the Morion, on the left is a version for 5 V OCXO's so Hams can 
use  12 Volt. The one on the right is driven by readily available parts for 
any Ham  and no adjustments. Total cost not counting GPS and OCXO below $ 10. 
We are  still fine tuning the filter but right out of the box we got 1 
E-10. This is for  Ham's not time nut standard. Data exceeds attachment 
limitations but any one can  contact me off list and I will send it. We destroyed 
the M7 have not figured out  how but a new one is on order and once testing is 
completed schematics will also  be available.  I have the bad habit layouts 
first documentation maybe  second. Frustrating for the team, but I am 
getting better. As I said before  mainly for Ham's and one of our Australian team 
member will roll it out to the  Ham community. But any body is free to use 
it I just think time nuts can do  better.
Bert Kehren

In a message dated 8/21/2014 1:30:50 A.M. Eastern Daylight Time,  
ed_palmer at sasktel.net writes:

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

_______________________________________________
time-nuts  mailing list -- time-nuts at febo.com
To unsubscribe, go to  
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the  instructions there.

-------------- next part --------------
A non-text attachment was scrubbed...
Name: Mini GPS PLL.pcb
Type: application/octet-stream
Size: 39621 bytes
Desc: not available
URL: <http://www.febo.com/pipermail/time-nuts/attachments/20140821/16f4f014/attachment-0001.obj>