[time-nuts] LEA-6T TCXO measurements

Bob kb8tq kb8tq at n1k.org
Fri May 4 08:56:16 EDT 2018


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 <gsricalde at gmail.com> wrote:
> 
> On Thu, Apr 26, 2018 at 8:36 PM, Bob kb8tq <kb8tq at n1k.org> 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
> <phase diff.png><freq2.png>_______________________________________________
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