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comparison doc/Tx-cal-theory @ 66:8e675142cca7

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doc/Tx-cal-theory written
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 28 May 2017 06:42:29 +0000
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children 615df2fb1ec3
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65:e7b9a9903f14 66:8e675142cca7
1 The Tx power levels put out by a GSM mobile station are subject to strict
2 specifications, and on the Calypso+Iota+Rita chipset they are produced by
3 writing different values into the APC DAC register. But what value should be
4 written into the APC DAC in order to produce a given Tx power level in dBm?
5 The answer varies from one produced device unit to the next, and even more from
6 one board design to the next, hence these APC values need to be calibrated.
7
8 The general procedure for calibrating Tx power levels is described in both Sara
9 and LoCosto calibration documents (see TI-docs), but the LoCosto document adds
10 one crucial detail which the Sara one fails to describe. The instructions given
11 in the Sara document appear to be perfectly intuitive at first: go through all
12 power levels in turn (PL numbers 5-19 for EGSM and GSM850 or 0-15 for DCS and
13 PCS), and for each power level command the DUT to transmit at that level,
14 measure the actual Tx output power with the CMU200 or other RF test equipment,
15 and adjust the APC DAC value up or down to make the output power match the spec.
16
17 The problem is with the very last part: the output power you get with the
18 compiled-in APC DAC value is one number, the spec-defined target power is a
19 different number, exactly how do you adjust the APC DAC value to hit the target
20 power level?
21
22 The LoCosto document gives the answer which was not at all obvious to me at
23 first. I draw the reader's attention to section A.3.1 on page 43 of that
24 document. The trick is as follows: the relation between the APC DAC value and
25 the resulting Tx output power (or rather the equivalent in terms of voltage) is
26 modeled as a piecewise linear function, and this piecewise linear function is
27 constructed from a preliminary set of output power measurements with a fixed
28 set of APC DAC values.
29
30 We also have a strong piece of evidence that the Tx power level calibration
31 procedure performed by Openmoko at their factory was based on the same principle
32 as the LoCosto document version. If you look at the /gsm/rf/tx/levels.900 table
33 in Om-calibrated GTA02 units and compare it against the firmware's compiled-in
34 version, you will see that the APC DAC values for power levels 5-19 have been
35 replaced with calibrated ones as expected, but there is also this oddity: the
36 APC DAC value for power level 0 (not a valid number for this band) has been
37 replaced with 600. Always 600, a perfect round number, on every unit.
38
39 Having understood the calibration method described in the LoCosto document, the
40 mystery of this fixed number 600 becomes clear: the calibration program
41 establishes the basis for the calibration by temporarily overwriting the APC DAC
42 value in the power level 0 slot with a series of fixed values, 600 being the
43 last of them. It constructs the piecewise linear model for the APC from power
44 measurements made with these fixed DAC values, and uses this model to compute
45 the DAC value for each target power level. The latter DAC values are then
46 written into Tx levels table slots 5-19 for EGSM and GSM850, or 0-15 for DCS
47 and PCS. In the case of DCS and PCS no visible artifacts remain from the
48 calibration basis run (because slot 0 is a valid power level for these bands),
49 but an artifact remains in the levels.900 table in the unused slot 0.
50
51 In the present FreeCalypso RF calibration tools suite the planned
52 fc-rfcal-txband program that will perform the entire Tx band calibration
53 procedure has not been written yet. Instead we have the following two building
54 blocks:
55
56 fc-rfcal-txbasis performs the preliminary "basis" step of Tx power level
57 calibration: it sets the APC DAC to each of the values given on the command line
58 in turn, and reports the resulting output power level in dBm. The temporary
59 APC DAC values do get written into the Tx levels table in L1 (there is no other
60 way to set the APC DAC value for the active transmitter through Test Mode), but
61 the MISC_ENABLE command to save Tx calibration tables in FFS is NOT issued,
62 thus the corruption of the Tx levels table inflicted by this program will get
63 blown away on the next rfpw 7 (Test Mode band selection) command.
64
65 fc-rfcal-txcheck is a completely non-invasive program (does not make any writes
66 to any L1 RF tables or to FFS) that steps through all power levels for the
67 specified band (5-19 for EGSM or GSM850, or 0-15 for DCS or PCS), performs a
68 power measurement at each level, and displays the measured result. An
69 equivalent operation will be the finishing sanity check step in the full Tx
70 power level calibration program.