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doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 13 Feb 2018 03:18:38 +0000
parents 16aab86384a7
children 32bf19fb8a2a
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8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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1 The Tx power levels put out by a GSM mobile station are subject to strict
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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2 specifications, and on the Calypso+Iota+Rita chipset they are produced by
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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3 writing different values into the APC DAC register. But what value should be
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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4 written into the APC DAC in order to produce a given Tx power level in dBm?
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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5 The answer varies from one produced device unit to the next, and even more from
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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6 one board design to the next, hence these APC values need to be calibrated.
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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7
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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8 The general procedure for calibrating Tx power levels is described in both Sara
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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9 and LoCosto calibration documents (see TI-docs), but the LoCosto document adds
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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10 one crucial detail which the Sara one fails to describe. The instructions given
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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11 in the Sara document appear to be perfectly intuitive at first: go through all
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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12 power levels in turn (PL numbers 5-19 for EGSM and GSM850 or 0-15 for DCS and
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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13 PCS), and for each power level command the DUT to transmit at that level,
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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14 measure the actual Tx output power with the CMU200 or other RF test equipment,
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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15 and adjust the APC DAC value up or down to make the output power match the spec.
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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16
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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17 The problem is with the very last part: the output power you get with the
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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18 compiled-in APC DAC value is one number, the spec-defined target power is a
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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19 different number, exactly how do you adjust the APC DAC value to hit the target
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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20 power level?
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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21
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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22 The LoCosto document gives the answer which was not at all obvious to me at
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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23 first. I draw the reader's attention to section A.3.1 on page 43 of that
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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24 document. The trick is as follows: the relation between the APC DAC value and
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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25 the resulting Tx output power (or rather the equivalent in terms of voltage) is
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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26 modeled as a piecewise linear function, and this piecewise linear function is
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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27 constructed from a preliminary set of output power measurements with a fixed
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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28 set of APC DAC values.
8e675142cca7 doc/Tx-cal-theory written
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29
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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30 We also have a strong piece of evidence that the Tx power level calibration
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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31 procedure performed by Openmoko at their factory was based on the same principle
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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32 as the LoCosto document version. If you look at the /gsm/rf/tx/levels.900 table
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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33 in Om-calibrated GTA02 units and compare it against the firmware's compiled-in
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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34 version, you will see that the APC DAC values for power levels 5-19 have been
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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35 replaced with calibrated ones as expected, but there is also this oddity: the
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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36 APC DAC value for power level 0 (not a valid number for this band) has been
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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37 replaced with 600. Always 600, a perfect round number, on every unit.
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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38
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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39 Having understood the calibration method described in the LoCosto document, the
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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40 mystery of this fixed number 600 becomes clear: the calibration program
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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41 establishes the basis for the calibration by temporarily overwriting the APC DAC
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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42 value in the power level 0 slot with a series of fixed values, 600 being the
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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43 last of them. It constructs the piecewise linear model for the APC from power
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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44 measurements made with these fixed DAC values, and uses this model to compute
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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45 the DAC value for each target power level. The latter DAC values are then
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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46 written into Tx levels table slots 5-19 for EGSM and GSM850, or 0-15 for DCS
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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47 and PCS. In the case of DCS and PCS no visible artifacts remain from the
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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48 calibration basis run (because slot 0 is a valid power level for these bands),
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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49 but an artifact remains in the levels.900 table in the unused slot 0.
8e675142cca7 doc/Tx-cal-theory written
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50
98
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
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51 Target power level numbers
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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52 ==========================
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
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53
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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54 Ideally the power level numbers to be used as the targets for the calibration
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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55 process would be the levels given in the GSM 05.05 spec: from 33 dBm down to
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
parents: 66
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56 5 dBm for the EGSM and GSM850 bands or from 30 dBm down to 0 dBm for DCS and
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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57 PCS, in 2 dBm steps in each case. However, the evidence from TI's calibration
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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58 documents (both Sara and LoCosto) is that some devices are not able to produce
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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59 the very highest and sometimes the very lowest power levels given in the spec,
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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60 and getting a successful calibration requires setting the calibration targets
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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61 to slightly shifted numbers, although still within the tolerances allowed by
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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62 the GSM 05.05 spec. The table in TI's Sara calibration document (page 26) has
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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63 the highest power level reduced by 1.2 relative to the spec, the next highest
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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64 by 0.5, whereas the lowest 4 (EGSM) or 6 (DCS) levels are raised and compacted
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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65 together. The table in the LoCosto document (page 44) has the highest power
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
parents: 66
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66 level for each band reduced by 0.8, making it 32.2 dBm for EGSM and GSM850 and
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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67 29.2 dBm for DCS and PCS, while all other levels are set to the spec numbers.
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
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68
111
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
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69 The calibration that was performed by Openmoko's factory (FIC?) on their GTA02
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
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70 units is similar to what the old Sara document calls for: the highest power
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
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71 level is set to 31.8 dBm for EGSM or GSM850 and to 28.8 dBm for DCS and PCS,
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
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72 the next one down is 30.5/27.5 dBm instead of 05.05-spec-given 31/28 dBm, and
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Mychaela Falconia <falcon@freecalypso.org>
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73 for the lowest 4 (EGSM or GSM850) or 6 (DCS and PCS) power levels the decrement
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
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74 between levels is 1.5 instead of 2, putting the lowest EGSM or GSM850 power
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
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75 level at 7 dBm instead of 5 dBm, and the lowest DCS and PCS power levels at
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Mychaela Falconia <falcon@freecalypso.org>
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76 3 dBm instead of 0 dBm.
98
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77
111
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78 It appears that OM/FIC were using some TI-provided calibration software without
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
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79 taking the time and effort to tune its settings for their own hardware: when we
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
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80 recalibrate these OM-made devices in our own lab, we are able to hit all of the
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
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81 highest and lowest power levels given in the GSM 05.05 spec. On our own FCDEV3B
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
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82 hardware we are likewise able to get the full 30 dBm in the DCS and PCS bands,
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
parents: 103
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83 but we are not able to get the full 33 dBm in the low band. Following TI's
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
parents: 103
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84 practice with LoCosto, I set the highest power level target to 32.2 dBm for the
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
parents: 103
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85 EGSM and GSM850 bands; for DCS and PCS the highest power level target is 30 dBm
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
parents: 103
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86 per the spec. On the low end the power level targets are the official ones
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
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87 from the GSM 05.05 spec.
103
16aab86384a7 doc/Tx-cal-theory: update regarding our current FCDEV3B calibration
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88
98
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89 Profiles for Tx levels calibration
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90 ==================================
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91
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92 Our fc-rfcal-txband program that performs the actual per-unit calibration of Tx
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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93 power levels for each band requires a preconfigured txlevels calibration profile
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94 for each band as one of its inputs; if you are performing calibration on
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
parents: 66
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95 individual units of a board design for which the correct profiles have already
111
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Mychaela Falconia <falcon@freecalypso.org>
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96 been crafted, you simply use those given profiles (see the txlevels directory
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Mychaela Falconia <falcon@freecalypso.org>
parents: 103
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97 in the source tree for our current offerings), but if you are doing Tx power
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Mychaela Falconia <falcon@freecalypso.org>
parents: 103
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98 level calibration on a new board design for the first time, you first need to
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
parents: 103
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99 characterize the Tx output level behaviour of your new board design and craft
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100 the appropriate set of profiles.
66
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101
98
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102 The profiles for txlevels calibration reside in /opt/freecalypso/rfcal/txlevels;
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103 each per-band profile is sought in a file named profile_name-band_number, where
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104 profile_name is the profile name argument given to fc-rfcal-txband and
111
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105 band_number is one of 850, 900, 1800 or 1900. For example, if you are using
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parents: 103
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106 the rf3166-ideal profile set (appropriate for recalibrating Openmoko GTA02
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107 devices), the individual profile config files are rf3166-ideal-900,
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108 rf3166-ideal-1800 and rf3166-ideal-1900 for the tri900 band configuration.
98
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109
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110 Each profile provides two key pieces of data: the list of fixed APC DAC values
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111 to be used as the basis set for constructing the piecewise linear model, and
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112 the list of power levels in dBm that will be the targets for the calibration.
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113
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114 Crafting the txlevels calibration profiles for a new board design
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115 =================================================================
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116
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117 If you are working with a new (or new-to-you) board design with no previous Tx
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118 levels calibration experience, you need to begin by characterizing its Tx
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119 output level behaviour so you can determine what APC values are appropriate for
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120 the basis set, whether or not you need to shift the highest and/or lowest
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121 target levels from their nominal spec values, and if some target shifting is
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122 needed, which levels need to be shifted and by how much.
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123
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124 The tool you will need for this investigation is fc-rfcal-txbasis. It requires
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125 fc-cmu200d and rvinterf to be running, talking to the CMU200 and to the DUT,
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126 respectively, and it takes the band number (850/900/1800/1900) and a set of
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127 APC DAC values on the command line; no pre-existing txlevels profile is needed.
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128 It commands the DUT to transmit at each of the specified APC DAC values in turn
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129 and reports the power output levels measured by the CMU200.
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130
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131 VERY IMPORTANT: You need to be very sure that your CMU200 or other measuring
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132 instrument is itself in good calibration standing, i.e., that the measurements
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133 it reports are trustworthy, and that the insertion loss in your cabling setup
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134 (all the way from the actual DUT's antenna connector or RF test port to your
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135 CMU200) really matches the numbers you have put in your cable configuration file
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136 (see Cable-config-howto). If you fail to ensure these prerequisites, your
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137 fc-rfcal-txbasis observations will be meaningless, as the cable insertion losses
111
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138 and instrument errors are typically of the same order of magnitude as the
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139 transmitter differences you are trying to determine.
98
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140
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141 You will need to run fc-rfcal-txbasis with a guesstimated set of APC DAC values,
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142 to be revised iteratively, and get a feel for what your DUT is putting out.
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143 Are you able to hit the lowest spec power level consistently while the APC DAC
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144 value remains well above zero? Are you able to hit the highest spec power level
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145 as well, or does the measured power output fall short of this target no matter
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146 how high you crank the APC DAC? These observations will tell you whether or not
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147 you need to shift the highest and/or lowest target power levels in your new
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148 txlevels calibration profile.
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149
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150 Regular calibration runs
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151 ========================
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152
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153 Once you have a good set of profiles for the type of Calypso GSM device you are
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154 calibrating, just run fc-rfcal-txband with the band number and the name of your
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155 profile.
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156
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157 Checking existing calibration
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158 =============================
66
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159
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160 fc-rfcal-txcheck is a completely non-invasive program (does not make any writes
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161 to any L1 RF tables or to FFS) that steps through all power levels for the
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162 specified band (5-19 for EGSM or GSM850, or 0-15 for DCS or PCS), performs a
98
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163 power measurement at each level, and displays the measured result.