Said,
On 20/08/2014 15:42, [email protected] 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...
Tony
In a message dated 8/20/2014 11:07:59 Pacific Daylight Time,
[email protected] 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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