[time-nuts] Ã¢ NEO-M8N vs. NEO-M8T
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
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)
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.
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:
220.127.116.11 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:
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