[time-nuts] U-blox teaser

[John Ackermann N8UR]

You have to be careful comparing the LEA-M8F with other GPS units.  It 
does have low jitter because the "TIMEPULSE" signal is derived from the 
TCXO which is locked to the GPS time mark.

But their claim of "essentially jitter free" depends on your definition 
of "essentially" -- attached is a comparison of LEA-M8F PPS jitter vs. 
NEO-M8T and ZED-F9T raw, and NEO-M8T and ZED-F9T sawtooth corrected PPS. 
  The M8F is definitely better than the raw M8T and even the raw F9T, 
but the corrected M8T and F9T are both much better than the M8F.  And 
the M8F does *not* have sawtooth correction available.

So, it's a neat implementation and has some applications (basically as a 
modest performance 30.72 MHz GPSDO), but TANSTAAFL.

John
----

On 2/26/21 5:37 PM, ed breya wrote:
> John, if you look back at the recent GPSDO discussion initiated by 
> "dandober" in hpaligent keysight group, you'll see a post by Leo Bodnar, 
> linking a Ublox model here
> 
> https://www.u-blox.com/sites/default/files/products/documents/LEA-M8F_DataSheet_%28UBX-14001772%29.pdf 
> 
> 
> I looked it up, and I recall the PPS timing spec is +/- 5 nSec. After I 
> studied it some, I replied back
> 
> "Thanks Leo, that's a great example of what I've been picturing. This is 
> from the data sheet, regarding the PPS output:
> 
> "After an initial phase of acquisition the time-pulse becomes 
> essentially jitter-free, generated coherently from the built-in 
> reference oscillator and guaranteeing an exact number of reference 
> frequency cycles between each time-pulse."
> 
> It looks like this is an implementation of what we've talked about here 
> before a few times, for saw-tooth error reduction. BTW I didn't call it 
> "1PPS" above, because it can be programmed for different rates - I don't 
> recall what they are.
> 
> BTW, did dandober ever sign up here?
> 
> Ed
> 
> 
> 
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