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Andreas Seltenreich andreas+osmocom at gate450.dyndns.orgHi, I just found this discussion on the topic after sending some patches to the list (pending moderation)... Benedikt Heinz writes: > Another option might be storing the value in a non-used eeprom > location and adding get/set functions to librtlsdr. I modified my copy of librtlsdr to use the last four bytes of the eeprom as the default XTAL frequency instead of the compiled-in 28800000Hz, if it is within ±1kHz. That way applications don't have to deal with the calibration aspect at all. Also, rtl_eeprom got an -x parameter to store a value there. git://gate450.dyndns.org/git/rtl-sdr > Is it possible to write arbitrary values to unused eeprom locations > without fucking up the realtek eeprom handling? Well, my two sticks do still enumerate nicely with the change. I wonder if eeproms other than 2kbit ones are in use though, as that would make "last four bytes" slightly ambiguous... > (Of course the ppm value still changes with temperature, but having a > base value is still closer to the truth than 0ppm I'd say. > I measured the offset directly after grabbing samples for 20 minutes > at room-temperature and jitter is <1ppm.) After warmup, plotting the output of kalibrate-rtl[1] with my specimens indicates an accuracy of about 0.1ppm as long as the room temperature doesn't deviate more than 1K. Alan Corey writes: > While you're at it it would be nice to use maybe at least a 16 bit > value if possible. I had mine all worked out to about 5 or 6 places > then found I could only store what might be an 8 bit integer. SDR > Sharp would only store mine as 102 instead of about 102.4567. Since > the correction is done in software there shouldn't be a limit on the > size (data type) of the number. Hmm, my solution would yield a resolution of about 0.035ppm. A better resolution is probably not meaningful when calibrating a plain XO. regards, andreas Footnotes: [1] https://github.com/steve-m/kalibrate-rtl