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doc/Tx-cal-theory: update for the new conservative rf3166 profile set
author Mychaela Falconia <falcon@freecalypso.org>
date Wed, 14 Feb 2018 03:46:34 +0000
parents 7bcbbba94c41
children 76faa4f52a4d
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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
120
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parents: 111
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28 set of APC DAC values. Looking at the Vout/LSB slope in each segment of this
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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29 constructed piecewise linear function, one can see that the relation stays
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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30 mostly linear throughout most of the RF PA's valid control range, and at least
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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31 in the case of GSM MS devices made by Openmoko, the factory-calibrated power
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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32 levels all fall into this "good" range in which the PA's response to the APC DAC
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
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33 control value is mostly linear.
66
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34
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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35 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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36 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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37 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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38 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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39 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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40 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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41 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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42 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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43
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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44 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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45 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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46 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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47 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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48 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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49 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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50 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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51 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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52 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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53 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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54 but an artifact remains in the levels.900 table in the unused slot 0.
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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55
98
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Mychaela Falconia <falcon@freecalypso.org>
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56 Target power level numbers
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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57 ==========================
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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58
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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59 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>
parents: 66
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60 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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61 5 dBm for the EGSM and GSM850 bands or from 30 dBm down to 0 dBm for DCS and
120
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Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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62 PCS, in 2 dB steps in each case. However, the evidence from TI's calibration
98
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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63 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>
parents: 66
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64 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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65 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>
parents: 66
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66 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>
parents: 66
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67 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>
parents: 66
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68 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>
parents: 66
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69 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>
parents: 66
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70 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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71 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>
parents: 66
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72 29.2 dBm for DCS and PCS, while all other levels are set to the spec numbers.
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73
111
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
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74 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>
parents: 103
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75 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>
parents: 103
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76 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>
parents: 103
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77 the next one down is 30.5/27.5 dBm instead of 05.05-spec-given 31/28 dBm, and
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
parents: 103
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78 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>
parents: 103
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79 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>
parents: 103
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80 level at 7 dBm instead of 5 dBm, and the lowest DCS and PCS power levels at
7bcbbba94c41 doc/Tx-cal-theory: update with properly calibrated CMU200 and cabling setup
Mychaela Falconia <falcon@freecalypso.org>
parents: 103
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81 3 dBm instead of 0 dBm.
98
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82
120
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Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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83 When it comes to the highest power levels, my (Mychaela's) first thought was
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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84 that OM's calibration had them reduced to 31.8/28.8 dBm for no good reason other
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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85 than OM/FIC mindlessly using some TI-provided calibration software that had
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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86 those settings by default, as a leftover from some much older hw platform that
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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87 was poorer in this department than OM's GTA0x hardware. As a matter of fact,
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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88 it *is* possible to make an Openmoko-made GTA02 device put out the full 33 dBm
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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89 in the EGSM band and the full 30 dBm in the DCS and PCS bands by setting the
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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90 APC DAC to higher values than those used by OM's factory calibration. However,
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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91 a closer look reveals that those higher-than-OM's APC DAC values which produce
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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92 the full 33/30 dBm output put the PA outside of its linear range, i.e., the
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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93 relation between the APC DAC control value and the resulting RF Vout no longer
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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94 has the same linearity as is maintained at the lower power levels. In our own
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parents: 111
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95 FreeCalypso calibration, we used to set the higher 32.2/29.2 or even the full
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
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parents: 111
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96 33/30 dBm targets for the highest power levels prior to the discovery of the
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Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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97 linearity issue, but our current rf3166 profile set (see below) uses the same
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
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parents: 111
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98 conversative upper-end targets as OM's original: 31.8/28.8 dBm for the highest
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parents: 111
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99 power level and 30.5/27.5 dBm for the next one down.
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parents: 111
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100
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
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parents: 111
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101 When it comes to the lowest power levels, the empirical observation is that the
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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102 official spec bottom power levels of 5 dBm for the low bands and 0 dBm for the
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Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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103 high bands are perfectly attainable both on Openmoko-made units and on our own
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Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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104 FCDEV3B, with APC DAC values that are only a tiny bit lower than the lowest
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Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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105 values in OM's calibration. We currently use 70 as the lowest basis point in
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Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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106 the EGSM and GSM850 bands and 60 as the lowest basis point for DCS and PCS,
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parents: 111
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107 following the LoCosto document; we are consistently able to get 5 dBm for the
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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108 low bands or 0 dBm for the high bands within our basis range, and examination
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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109 of the piecewise Vout/LSB slopes shows that the PA appears to be happily linear
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Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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110 in this range. Therefore, we use the official spec numbers for the lowest
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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111 power level targets in our current rf3166 profile set, not the raised target
32bf19fb8a2a doc/Tx-cal-theory: update for the new conservative rf3166 profile set
Mychaela Falconia <falcon@freecalypso.org>
parents: 111
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112 numbers from TI's older platforms.
103
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parents: 98
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113
98
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114 Profiles for Tx levels calibration
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115 ==================================
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116
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117 Our fc-rfcal-txband program that performs the actual per-unit calibration of Tx
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118 power levels for each band requires a preconfigured txlevels calibration profile
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119 for each band as one of its inputs; if you are performing calibration on
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120 individual units of a board design for which the correct profiles have already
120
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121 been crafted, you simply use those given profiles (rf3166 for Openmoko and
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122 OM-based FreeCalypso hardware), but if you are doing Tx power level calibration
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123 on a new board design for the first time, you first need to characterize the Tx
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124 output level behaviour of your new board design and craft the appropriate set
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125 of profiles.
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126
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127 The profiles for txlevels calibration reside in /opt/freecalypso/rfcal/txlevels;
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128 each per-band profile is sought in a file named profile_name-band_number, where
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129 profile_name is the profile name argument given to fc-rfcal-txband and
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130 band_number is one of 850, 900, 1800 or 1900. For example, if you are using
120
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131 the rf3166 profile set (appropriate for recalibrating Openmoko GTA02 devices
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132 and for factory production of new FreeCalypso devices with the same RF PA), the
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133 individual profile config files are rf3166-900, rf3166-1800 and rf3166-1900 for
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134 the tri900 band configuration.
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135
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136 Each profile provides two key pieces of data: the list of fixed APC DAC values
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137 to be used as the basis set for constructing the piecewise linear model, and
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138 the list of power levels in dBm that will be the targets for the calibration.
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139
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140 Crafting the txlevels calibration profiles for a new board design
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141 =================================================================
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142
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143 If you are working with a new (or new-to-you) board design with no previous Tx
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144 levels calibration experience, you need to begin by characterizing its Tx
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145 output level behaviour so you can determine what APC values are appropriate for
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146 the basis set, whether or not you need to shift the highest and/or lowest
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147 target levels from their nominal spec values, and if some target shifting is
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148 needed, which levels need to be shifted and by how much.
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149
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150 The tool you will need for this investigation is fc-rfcal-txbasis. It requires
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151 fc-cmu200d and rvinterf to be running, talking to the CMU200 and to the DUT,
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152 respectively, and it takes the band number (850/900/1800/1900) and a set of
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153 APC DAC values on the command line; no pre-existing txlevels profile is needed.
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154 It commands the DUT to transmit at each of the specified APC DAC values in turn
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155 and reports the power output levels measured by the CMU200.
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156
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157 VERY IMPORTANT: You need to be very sure that your CMU200 or other measuring
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158 instrument is itself in good calibration standing, i.e., that the measurements
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159 it reports are trustworthy, and that the insertion loss in your cabling setup
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160 (all the way from the actual DUT's antenna connector or RF test port to your
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161 CMU200) really matches the numbers you have put in your cable configuration file
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162 (see Cable-config-howto). If you fail to ensure these prerequisites, your
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163 fc-rfcal-txbasis observations will be meaningless, as the cable insertion losses
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164 and instrument errors are typically of the same order of magnitude as the
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165 transmitter differences you are trying to determine.
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166
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167 You will need to run fc-rfcal-txbasis with a guesstimated set of APC DAC values,
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168 to be revised iteratively, and get a feel for what your DUT is putting out.
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169 Are you able to hit the lowest spec power level consistently while the APC DAC
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170 value remains well above zero? Are you able to hit the highest spec power level
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171 as well, or does the measured power output fall short of this target no matter
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172 how high you crank the APC DAC? These observations will tell you whether or not
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173 you need to shift the highest and/or lowest target power levels in your new
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174 txlevels calibration profile.
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175
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176 Regular calibration runs
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177 ========================
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178
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179 Once you have a good set of profiles for the type of Calypso GSM device you are
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180 calibrating, just run fc-rfcal-txband with the band number and the name of your
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181 profile.
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182
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183 Checking existing calibration
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184 =============================
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185
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186 fc-rfcal-txcheck is a completely non-invasive program (does not make any writes
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187 to any L1 RF tables or to FFS) that steps through all power levels for the
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188 specified band (5-19 for EGSM or GSM850, or 0-15 for DCS or PCS), performs a
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189 power measurement at each level, and displays the measured result.