DC calibration

Goal: lower middle "hill" in Spectrum mode.

• Turn off osmo-trx.
• Set analyser to Spectrum, span 1kHz.
• Run host/build/utils/uhd_cal_tx_dc_offset_lms script from UHD (source) and note .csv file it writes:

uhd_cal_tx_dc_offset_lms --freq-start 940e6 --freq-stop 950e6

• Take a look at .csv file, "real" and "imag" columns. Values should differ between rows at no more than 10 or so. Calculate averages for these columns. Write them down as I (real) and Q (imag). Yes, it's not a typo.
• Edit umtrx_scripts/service/lms_tune.sh (source) to write I to send_lms_cmd 0x42 and Q to … 0x43. In hex!
• Run lms_tune.sh 1 and check "hill" in analyzer, so that it becomes lower. Play with these I and Q values a little.
• Write values to EEPROM. Some docs are at umtrx.org/hardware/calibration, but for single channel only. Commands to write values For the first channel of two channels:

cd /usr/local/share/uhd/utils
./usrp_burn_mb_eeprom --key tx1-vga1-dc-i --val <I goes here, in decimal>
./usrp_burn_mb_eeprom --key tx1-vga1-dc-q --val <Q goes here, in decimal>

• After the above changes uhd_usrp_probe should start showing values (because they now differ from default values).
• Run lms_tune.sh (commenting out 0x42 and 0x43 lines) to check that values from EEPROM are now getting used.
• Do the check for second channel. TBD.