# HG changeset patch # User Mychaela Falconia # Date 1495941694 0 # Node ID 131abadbd74de0cdcf4354725f0696a7c92f73a9 # Parent a98873b713c3ba2fe7214d02f5a45603a112d829 doc/Rx-cal-theory written diff -r a98873b713c3 -r 131abadbd74d doc/Rx-cal-theory --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/doc/Rx-cal-theory Sun May 28 03:21:34 2017 +0000 @@ -0,0 +1,92 @@ +The incoming radio signal from the antenna passes through a chain of amplifiers +before it reaches the Calypso DSP, and the dBm power measurement reported by the +latter is significantly greater than the actual power level at the antenna +connector. The mobile station firmware needs to be able to determine the actual +input level from the DSP's power measurement (not only for reporting purposes, +but also to select the proper AGC setting), hence it needs to know the +difference in dB between the DSP's power measurement and the input signal level +in dBm. For this purpose the firmware maintains a constant called GMagic, +defined as: + +GMagic = PM - IL - AGC + +where PM is the DSP's power measurement in notional dBm, IL is the input level +in actual dBm, and AGC is the AGC programmable gain setting in dB. GMagic is +the sum of all amplifier gains and insertion losses along the Rx signal path, +and is on the order of about 100 dB, although the fw reckons it in half-dB +units. + +Some Calypso device manufacters have calibrated the GMagic constant for each +band on a per-unit basis, and the unit-to-unit difference is on the order of +1-2 half-dB units, i.e., rarely more than 1 dB. The difference from one board +design to another tends to be a little greater, on the order of 10 half-dB +units, and is probably due to the difference in insertion losses at various +points in the Rx signal path. (Ideally these insertion losses should be zero, +but the world is not ideal.) + +GMagic calibration is done by connecting a signal generator putting out a +precisely controlled signal to the DUT's antenna connector or RF test port, +making a power measurement with the Calypso DSP and computing GMagic per the +simple equation above. + +Our FreeCalypso version of the GMagic calibration procedure is closely based on +the Sara version, described on pages 28-32 of the Sara document +rf_calibration.pdf (see TI-docs). The LoCosto calibration document does not +include a GMagic calibration procedure, but LoCosto is a very different beast, +with both the baseband processor and the RF transceiver in the same chip, and +using that mysterious DRP - perhaps LoCosto doesn't need GMagic calibration, +perhaps it is absorbed into DRP calibration instead. + +Per the available Sara RF calibration document, we set the signal generator's +output level to -74.5 dBm, and set the AGC in the Rita RF chip to 34 dB for the +GMagic calibration step. However, we use the ARFCNs from the LoCosto document: +37 for the EGSM band, 698 for DCS, 661 for PCS and 189 for GSM850. The old Sara +document covers only 900 and 1800 MHz bands (I'm guessing Sara probably did not +support 1900 MHz, let alone 850), the LoCosto document is quadband throughout, +and the LoCosto ARFCNs are closer to the true centre of each band. + +RSSI channel compensation +========================= + +The GMagic constant is calibrated at the centre frequency for each downlink +band, but the true value of this "magic gain" varies slightly with the frequency +channel. The drawing on page 34 of the Sara document suggests that this +frequency channel-dependent variation is caused by frequency-dependent variation +in the insertion loss of the Rx SAW filters. This frequency channel-dependent +variation in the "magic gain" is customarily calibrated on a per-unit basis +(even on LoCosto, apparently), and the available documents suggest that this +calibration is done in order to make RSSI reporting more accurate. + +Once again our FreeCalypso version of this RSSI channel calibration procedure +is based on the Sara document, rather than LoCosto, except for the ARFCNs. The +LoCosto document gives ARFCN ranges for Rx channel calibration for all 4 bands, +and these ranges fully agree with what Openmoko's factory programmed in their +/gsm/rf/rx/calchan.* files, thus we have adopted the ARFCN ranges and test +ARFCNs from LoCosto for all 4 bands. + +Note that the LoCosto document indicates the use of a different signal generator +level (-68.5 dBm instead of -74.5), and tells the operator to set the AGC gain +to 0 dB. The latter must be a LoCosto-ism, as it is definitely not correct for +our Calypso+Iota+Rita chipset. If we set the signal generator level to +-68.5 dBm, we would need to get the Rita AGC to 28 dB, but the Mother of +FreeCalypso decided to go with the safer choice of using the Sara settings of +-74.5 dBm generator level, 34 dB AGC. + +FreeCalypso Rx calibration tools +================================ + +The present suite of RF calibration tools includes two programs for Rx path +calibration: + +fc-rfcal-gmagic + + This program is a study-only tool. It measures the GMagic constant for + the user-specified band (either at that band's default ARFCN or at a + user-specified ARFCN) and prints it out, but does not write anything + into the Calypso device's FFS. + +fc-rfcal-rxband + + This program is intended for production use. It performs both the + GMagic and the RSSI channel compensation calibration steps, uploads the + results to the DUT and saves them in FFS. diff -r a98873b713c3 -r 131abadbd74d doc/TI-docs --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/doc/TI-docs Sun May 28 03:21:34 2017 +0000 @@ -0,0 +1,26 @@ +We don't have a TI calibration document that exactly matches the +Calypso+Iota+Rita chipset we are working with, but we do have two different +documents from other points in TI history: + +ftp://ftp.freecalypso.org/pub/GSM/Calypso/rf_calibration.pdf + +https://www.freecalypso.org/LoCosto-docs/Production%20test%20and%20calibration/i_sample_rf_test_and_calibration_13_03_04_01991%20-%20v026.pdf + +The rf_calibration.pdf document we found back in 2011 dates from well before +our Calypso+Iota+Rita chipset and describes TI's much earlier Sara RF, whereas +the LoCosto document naturally deals with TI's LoCosto/I-Sample platform. + +To the best of our understanding of TI history, Sara RF is two generations +before our Rita RF (as far as we know, the evolutionary line went from Sara to +Clara to Rita), whereas LoCosto (combining the baseband processor and RF +in the same chip) was a later development after the Calypso, hence our chipset +and the corresponding TCS211 firmware fall right in between the two calibration +docs we have. + +The details of the calibration procedures that are correct for our +Calypso+Iota+Rita chipset and closely match those that have been performed by +Openmoko's factory have been reconstructed by Mychaela Falconia after very +careful scrutiny of both Sara and LoCosto calibration documents, combined with +detective analysis of calibration artifact files found in Openmoko-made GTA02 +units; neither the Sara document nor the LoCosto one is particularly instructive +by itself, and a *lot* of intelligent filling of the blanks was required.