Hi
Two different meanings of the term sawtooth.
The first plot shows a very normal phase rollover process. That (when
incorrectly
unwound) can create errors. In some cases those sorts of errors are inevitable
and
it is always wise to see if data “noise” occurs right at a wrap around point.
The other meaning of sawtooth in the case of GPS is rooted in the same process.
The GPS time solution and local time have a pattern very much like the first
plot.
As long as it is running as shown in the first plot, it averages out (provided
there is
averaging). The problem comes in when the plot does *not* look like the first
graph.
If the TCXO comes to a flat spot on it’s compensation curve, it can “hang out”
to one
side of the error band. Instead of a sawtooth you get a curve. That process of
staying
to one side of the error band does not average out and you get a bump in the
time
record.
If you dig into the archives, there are lots of plots showing typical data.
Bob
> On May 4, 2018, at 5:18 AM, Gabs Ricalde wrote:
>
> On Thu, Apr 26, 2018 at 8:36 PM, Bob kb8tq wrote:
>> Hi
>>
>> What you are looking at appear to be sawtooth jumps. Simply put, the module
>> is
>> looking at the closest edge on the TCXO to do it’s timing. When the device
>> drifts,
>> it can “slip” to another cycle. If you watch the PPS out, there are also
>> artifacts that
>> result from this process operating in a non-ideal fashion. Lots of details
>> on that in
>> the archives.
>>
>
> Hi Bob,
>
> The raw data has a sawtooth pattern due to millisecond clock jumps
> [1], but TimeLab unwraps it and they happen more frequently than the
> frequency jumps.
> I redid the the experiment using the same receiver (pink trace) and
> another LEA-6T (green trace), there were no frequency jumps. SBAS was
> enabled in the old measurements (blue trace). I'm not sure if that
> explains it, but the manual does not recommend SBAS for timing.
>
>
> [1] "GNSS Receiver Clocks" http://www.insidegnss.com/node/2512
> ___
> time-nuts mailing list -- time-nuts@febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
