Hi,
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