# HG changeset patch # User Mychaela Falconia # Date 1518584098 0 # Node ID aa2533be2ec1ef75d0d385fa4a8ff029c744d02f # Parent 76faa4f52a4d0643e46fd1d9f85880c54ca92d95 doc/Tx-cal-theory: channel calibration documented diff -r 76faa4f52a4d -r aa2533be2ec1 doc/Tx-cal-theory --- a/doc/Tx-cal-theory Wed Feb 14 03:55:03 2018 +0000 +++ b/doc/Tx-cal-theory Wed Feb 14 04:54:58 2018 +0000 @@ -185,10 +185,48 @@ calibrating, just run fc-rfcal-txband with the band number and the name of your profile. +Channel correction calibration +============================== + +The APC DAC values given in the levels tables apply directly only to the +channels in the centre of each band. The transfer function from APC DAC control +values to power output levels in dBm depends on the frequency, and the APC DAC +values need to be adjusted for different subbands within each band in order to +produce the correct output power levels for all channels. TI's RF driver code +and calibration data structures provide for a Tx channel calibration, and all +historical commercial manufacturers of Calypso GSM devices that are known to us +have performed this Tx channel calibration on a per-unit basis on their +production lines. + +Our fc-rfcal-txband calibration tool implements Tx channel correction +calibration as of 2018-02, and we do it a little better than the way it was done +by the big folks Back In The Day. TI's RF data structures allow for up to 4 +channel calibration tables, to be used for different power levels, but +Openmoko's factory only did one channel calibration set (calibrated at the +highest power level for each band), using only one table out of the possible 4, +and this one table (calibrated at the highest power level) is then used for all +power levels. As far as we can tell, other Calypso GSM device manufacturers +have done likewise. + +Our Tx calibration implementation makes use of all 4 available channel +correction table slots. We divide the 15 power levels of EGSM and GSM850 bands +or the 16 power levels of DCS and PCS into 4 groups (5-7, 8-11, 12-15, 16-19 +for EGSM and GSM850; 0-3, 4-7, 8-11, 12-15 for DCS and PCS), perform channel +calibration at the highest power level in each group, and set the resulting +channel calibration table to be used for all 3 or 4 power levels in that group. +Our procedure produces Tx behaviour that is identical to Openmoko's for the +highest 3 or 4 power levels, but the lower power levels run with channel +calibration that was made closer to them, as opposed to the very highest power +level. The price of our approach is more time spent in RF calibration for each +unit on the production line, but our approach is perfectly suited for low volume +production and aftermarket recalibration services. + Checking existing calibration ============================= fc-rfcal-txcheck is a completely non-invasive program (does not make any writes to any L1 RF tables or to FFS) that steps through all power levels for the specified band (5-19 for EGSM or GSM850, or 0-15 for DCS or PCS), performs a -power measurement at each level, and displays the measured result. +power measurement at each level, and displays the measured result. By default +the test is performed at the centre frequency of the specified band, but the +ARFCN to be used can also be specified on the command line.