[time-nuts] â NEO-M8N vs. NEO-M8T

Chris Caudle chris at chriscaudle.org
Mon May 21 09:23:27 EDT 2018


On Sun, May 20, 2018 9:23 pm, Gary E. Miller wrote:
> I do not see the keyword 'sawtooth' in the u-blox 8 doc.  Can I buy a
> clue?

Sawtooth is what it looks like when you plot the quantization error of the
PPS output, the documentation will just refer to it as quantization error.

Referencing this doc (not sure it is the exact match for your model)
https://www.u-blox.com/sites/default/files/products/documents/u-blox8-M8_ReceiverDescrProtSpec_%28UBX-13003221%29_Public.pdf

18.1 Introduction
u-blox receivers include a time pulse function providing clock pulses with
configurable duration and frequency.
The time pulse function can be configured using the UBX-CFG-TP5 message.
The UBX-TIM-TP
 message provides time information for the next pulse, time source and the
quantization error of the output pin

The UBX-TIM-TP message is described in:
32.21.8.1 Time Pulse Timedata
byte offset 8, name: qErr unit: ps
Quantization error of time pulse (not supported for the FTS product variant).

I believe that means  you ready byte offset 8 of that message, and it 
tells you how many picoseconds the next PPS output is expected to be early
or late compared to the nominally correct location of the PPS pulse. 
Inject that offset into the math for your software implemented PLL and you
can cancel out that noise caused by the GPS clock used to generate the PPS
being asynchronous to the GPS satellite clocks.

This document has some nice graphs of sawtooth shaped quantization errors,
just search for sawtooth:
https://ivscc.gsfc.nasa.gov/meetings/tow2011/Hambly.Sem.pdf

-- 
Chris Caudle





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