[time-nuts] Ublox neo-7M GPS

Tony tnuts at toneh.demon.co.uk
Thu Aug 21 05:48:58 EDT 2014


On 20/08/2014 15:42, SAIDJACK at aol.com wrote:
> Hi Tony,
> that's consistent with what I remember. Do you have the capability to count
>   the number of 10MHz pulses per second to see if it is phase-coherent with
> the  UTC 1PPS pulse?
I don't have any means to do that at the moment - when I get time I'll 
try programming my STM32F4 discovery board (168MHz ARM Cortex M4) to 
take some measurements. However as I haven't used the timers on that yet 
it'll take a bit of time to get it right!

> I am thinking that the software may be using statistics to approximate 10
> million cycles per second, which would mean they may or may not be exactly
> 10  million cycles..
> thanks,
> Said

As I said, I measured the pulses at either 104ns (9.6MHz), with some 
being shorter at 84ns. I've just repeated the measurements on a Reyax 
RYN25AI (UBLOX MAX-7C) with the same results. Actually the scope 
measures the longer pulses at either 104.3ns or 105.2ns and the shorter 
pulses at 82.89ns or 83.82ns. Since it is a 1GS/s scope, the differences 
are almost certainly just the scope sampling uncertainties rather than 
actual jitter in the clock.

Note that these measurements were with the clock free running as I can't 
get a GPS lock indoors.

This time I had a closer look at the clocks using a slower sweep to show 
approx 19 cycles. It was then clear that there was always one short 
pulse followed by 4 long pulses. This makes sense - the xtal oscillator 
must be 48MHz (or more likely 24MHz multiplied up) so that the short 
pulses are 4 clocks (83.3ns) and the long pulses are 5 clocks 
(104.17ns), so 4 long plus 1 short = 24 of the 48MHz clocks. So 5 clocks 
out = 500ns, averaging 10MHz.

How it distributes the short clock pulses (or if it makes phase changes 
larger than 21ns) when it has to lock to the GPS clock remains to be 
seen but is harder to measure and will require some sort of 
analyzer/timing capture tool. Perhaps if I get the time to program that 
ARM chip...


> In a message dated 8/20/2014 11:07:59 Pacific Daylight Time,
> tnuts at toneh.demon.co.uk writes:
> 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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