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doc/Tx-cal-theory: update regarding our current FCDEV3B calibration being untrustworthy
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 13 Aug 2017 05:04:48 +0000
parents 615df2fb1ec3
children 7bcbbba94c41
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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
Mychaela Falconia <falcon@freecalypso.org>
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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
Mychaela Falconia <falcon@freecalypso.org>
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50
98
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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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
Mychaela Falconia <falcon@freecalypso.org>
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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>
parents: 66
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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>
parents: 66
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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>
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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
Mychaela Falconia <falcon@freecalypso.org>
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68
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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69 In my own experience with getting the Tx power levels calibrated on our current
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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70 FCDEV3B boards, our hardware is able to hit all of the lowest spec power levels
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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71 with good margin (the APC DAC value never goes below the 60-70 range), hence
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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72 the low targets did not need to be shifted, but the highest spec power level
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
parents: 66
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73 could not be achieved in every band. Following TI's practice with LoCosto, I
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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74 set the highest power level target to 32.2 dBm for the EGSM band and to 29.2 dBm
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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75 for PCS, but I was consistently able to get the full 30 dBm out in the DCS band,
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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76 hence I set the DCS band highest power level target to the official number of
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
parents: 66
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77 30 dBm. It needs to be emphasized, however, that these results are quite
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
parents: 66
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78 specific to our current flock of FCDEV3B hardware, and your results will most
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
parents: 66
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79 likely be different if you are working with a different hw design such as a
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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80 non-FreeCalypso derivative of Openmoko's GTA02 design.
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
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81
103
16aab86384a7 doc/Tx-cal-theory: update regarding our current FCDEV3B calibration
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82 Update: the above calibration observations for the FCDEV3B should be held
16aab86384a7 doc/Tx-cal-theory: update regarding our current FCDEV3B calibration
Mychaela Falconia <falcon@freecalypso.org>
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83 doubly in doubt because the calibration status of my CMU200 itself is in
16aab86384a7 doc/Tx-cal-theory: update regarding our current FCDEV3B calibration
Mychaela Falconia <falcon@freecalypso.org>
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84 question, i.e., it is not currently known if the measurements it reports are
16aab86384a7 doc/Tx-cal-theory: update regarding our current FCDEV3B calibration
Mychaela Falconia <falcon@freecalypso.org>
parents: 98
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85 really trustworthy. After the above paragraph was written, I replaced the
16aab86384a7 doc/Tx-cal-theory: update regarding our current FCDEV3B calibration
Mychaela Falconia <falcon@freecalypso.org>
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86 Rx/Tx module inside my CMU (the original one was good for Rx, but the Tx side
16aab86384a7 doc/Tx-cal-theory: update regarding our current FCDEV3B calibration
Mychaela Falconia <falcon@freecalypso.org>
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87 was dead), and after this internal module replacement the power measurements
16aab86384a7 doc/Tx-cal-theory: update regarding our current FCDEV3B calibration
Mychaela Falconia <falcon@freecalypso.org>
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88 reported by the instrument are 0.5 dB less than what it reported before. Which
16aab86384a7 doc/Tx-cal-theory: update regarding our current FCDEV3B calibration
Mychaela Falconia <falcon@freecalypso.org>
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89 raises the questions: are the new measurements low by 0.5 dB relative to the
16aab86384a7 doc/Tx-cal-theory: update regarding our current FCDEV3B calibration
Mychaela Falconia <falcon@freecalypso.org>
parents: 98
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90 real truth, or were the old ones high by 0.5 dB relative to the real truth? Or
16aab86384a7 doc/Tx-cal-theory: update regarding our current FCDEV3B calibration
Mychaela Falconia <falcon@freecalypso.org>
parents: 98
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91 is the real truth something else altogether? Until we raise a LOT more money
16aab86384a7 doc/Tx-cal-theory: update regarding our current FCDEV3B calibration
Mychaela Falconia <falcon@freecalypso.org>
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92 to get this CMU200 properly calibrated, we are limited to guesswork and the
16aab86384a7 doc/Tx-cal-theory: update regarding our current FCDEV3B calibration
Mychaela Falconia <falcon@freecalypso.org>
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93 true transmitter behaviour of our FCDEV3B hardware remains unknown.
16aab86384a7 doc/Tx-cal-theory: update regarding our current FCDEV3B calibration
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94
98
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95 Profiles for Tx levels calibration
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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96 ==================================
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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97
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98 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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99 power levels for each band requires a preconfigured txlevels calibration profile
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100 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>
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101 individual units of a board design for which the correct profiles have already
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
parents: 66
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102 been crafted, you simply use those given profiles (fcom1 for our current flock
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103 of FCDEV3B boards), but if you are doing Tx power level calibration on a new
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Mychaela Falconia <falcon@freecalypso.org>
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104 board design for the first time, you first need to characterize the Tx output
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
parents: 66
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105 level behaviour of your new board design and craft the appropriate set of
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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106 profiles.
66
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107
98
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108 The profiles for txlevels calibration reside in /opt/freecalypso/rfcal/txlevels;
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
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109 each per-band profile is sought in a file named profile_name-band_number, where
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parents: 66
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110 profile_name is the profile name argument given to fc-rfcal-txband and
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111 band_number is one of 850, 900, 1800 or 1900. For example, the profile set for
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Mychaela Falconia <falcon@freecalypso.org>
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112 our current FCDEV3B hardware is named fcom1 ("FreeCalypso hardware, Openmoko-
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113 based, version 1"), this hw platform is 900/1800/1900 MHz triband, and the
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114 individual profile config files are fcom1-900, fcom1-1800 and fcom1-1900.
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115
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116 Each profile provides two key pieces of data: the list of fixed APC DAC values
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
Mychaela Falconia <falcon@freecalypso.org>
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117 to be used as the basis set for constructing the piecewise linear model, and
615df2fb1ec3 doc/Tx-cal-theory: update with both theory and practice
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parents: 66
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118 the list of power levels in dBm that will be the targets for the calibration.
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119
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120 Crafting the txlevels calibration profiles for a new board design
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121 =================================================================
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122
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123 If you are working with a new (or new-to-you) board design with no previous Tx
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124 levels calibration experience, you need to begin by characterizing its Tx
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125 output level behaviour so you can determine what APC values are appropriate for
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126 the basis set, whether or not you need to shift the highest and/or lowest
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127 target levels from their nominal spec values, and if some target shifting is
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128 needed, which levels need to be shifted and by how much.
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129
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130 The tool you will need for this investigation is fc-rfcal-txbasis. It requires
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131 fc-cmu200d and rvinterf to be running, talking to the CMU200 and to the DUT,
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132 respectively, and it takes the band number (850/900/1800/1900) and a set of
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133 APC DAC values on the command line; no pre-existing txlevels profile is needed.
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134 It commands the DUT to transmit at each of the specified APC DAC values in turn
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135 and reports the power output levels measured by the CMU200.
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136
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137 VERY IMPORTANT: You need to be very sure that your CMU200 or other measuring
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138 instrument is itself in good calibration standing, i.e., that the measurements
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139 it reports are trustworthy, and that the insertion loss in your cabling setup
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140 (all the way from the actual DUT's antenna connector or RF test port to your
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141 CMU200) really matches the numbers you have put in your cable configuration file
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142 (see Cable-config-howto). If you fail to ensure these prerequisites, your
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143 fc-rfcal-txbasis observations will be meaningless, as the cable insertion losses
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144 are typically of the same order of magnitude as the transmitter differences you
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145 are trying to determine.
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146
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147 You will need to run fc-rfcal-txbasis with a guesstimated set of APC DAC values,
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148 to be revised iteratively, and get a feel for what your DUT is putting out.
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149 Are you able to hit the lowest spec power level consistently while the APC DAC
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150 value remains well above zero? Are you able to hit the highest spec power level
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151 as well, or does the measured power output fall short of this target no matter
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152 how high you crank the APC DAC? These observations will tell you whether or not
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153 you need to shift the highest and/or lowest target power levels in your new
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154 txlevels calibration profile.
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155
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156 Regular calibration runs
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157 ========================
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158
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159 Once you have a good set of profiles for the type of Calypso GSM device you are
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160 calibrating, just run fc-rfcal-txband with the band number and the name of your
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161 profile.
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162
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163 Checking existing calibration
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164 =============================
66
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165
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166 fc-rfcal-txcheck is a completely non-invasive program (does not make any writes
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167 to any L1 RF tables or to FFS) that steps through all power levels for the
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168 specified band (5-19 for EGSM or GSM850, or 0-15 for DCS or PCS), performs a
98
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169 power measurement at each level, and displays the measured result.