Hello Stefan,

Yes, the charm of the GPS-sync'd sampling is that it allows truly coherent transmission - even after the program was shortly stopped, PC rebooted, generator re-fuelled, or whatever strange things may happen. The sine wave generator will (optionally) not calculate the argument based on the number of soundcard samples, but derived from the GPS-derived time of day. Which, with the GPS 18x LVC mentioned before, seems to have a resolution (not neccessarily a precision) in the sub-microsecond range (192 kHz SR -> 5.x microsecond samples, but internally interpolated to a higher sample rate near the slope of the sync pulse).

At 192 kHz sampling rate, I get a standard deviation in the measured pulse position of only 30 to 60 nanoseconds (per one-second-interval) measured over one minute, which almost sounds too good to be true. Thus i'm a bit sceptic, this needs more testing.

Some preliminary info about the GPS-based calibration is already available here:
http://www.qsl.net/dl4yhf/speclab/frqcalib.htm#gps_pps_output

Note: The NMEA decoder for the soundcard is not implemented in the SL version on my website. It will be implemented when the weather(!) permits ;-)

73,  Wolf DL4YHF
(off for a walk now..)