> Where is the sawtooth coming from? Hal,
The 1 PPS "sawtooth" effect is found on many OEM GPS precise timing receiver boards. The pulse output of a GPS board is usually derived from the internal VCXO so the timing resolution of the 1 PPS edge is thus tied to the frequency of the oscillator. For example, a 25 MHz oscillator in a GPS engine allows the processor to pick, each second, which one of 25 million edges it wants to be the official 1 PPS edge for that second. Now 25 MHz frequency is 40 ns period so that gives a 1 PPS granularity of +/- 20 ns in this example. See: http://www.leapsecond.com/pages/m12/sawtooth.htm If that makes sense, then what's sawtooth correction? Based on satellite ranging and oscillator time or frequency drift calculations for the previous second, the GPS engine can calculate where the 1 PPS pulse ideally should occur (to within a few ns). So even though the hardware 1 PPS is slightly limited by the granularity of the clock, the firmware in some GPS timing receivers, can report the difference (in ns) between where it wanted the 1 PPS to be in software vs. where the 1 PPS actually occurred in hardware. This +/- value is called the "sawtooth correction", or sometimes the "negative sawtooth correction". In short, the sawtooth correction describes, to the best guess of the receiver, how early or late the hardware 1 PPS was. /tvb _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
