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doc/Tx-cal-theory written
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 28 May 2017 06:42:29 +0000
parents
children 615df2fb1ec3
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66
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
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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51 In the present FreeCalypso RF calibration tools suite the planned
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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52 fc-rfcal-txband program that will perform the entire Tx band calibration
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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53 procedure has not been written yet. Instead we have the following two building
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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54 blocks:
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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55
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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56 fc-rfcal-txbasis performs the preliminary "basis" step of Tx power level
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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57 calibration: it sets the APC DAC to each of the values given on the command line
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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58 in turn, and reports the resulting output power level in dBm. The temporary
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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59 APC DAC values do get written into the Tx levels table in L1 (there is no other
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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60 way to set the APC DAC value for the active transmitter through Test Mode), but
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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61 the MISC_ENABLE command to save Tx calibration tables in FFS is NOT issued,
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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62 thus the corruption of the Tx levels table inflicted by this program will get
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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63 blown away on the next rfpw 7 (Test Mode band selection) command.
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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64
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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65 fc-rfcal-txcheck is a completely non-invasive program (does not make any writes
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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66 to any L1 RF tables or to FFS) that steps through all power levels for the
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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67 specified band (5-19 for EGSM or GSM850, or 0-15 for DCS or PCS), performs a
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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68 power measurement at each level, and displays the measured result. An
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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69 equivalent operation will be the finishing sanity check step in the full Tx
8e675142cca7 doc/Tx-cal-theory written
Mychaela Falconia <falcon@freecalypso.org>
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70 power level calibration program.