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comparison doc/Rx-cal-theory @ 63:131abadbd74d

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doc/Rx-cal-theory written
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 28 May 2017 03:21:34 +0000
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children e7b9a9903f14
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62:a98873b713c3 63:131abadbd74d
1 The incoming radio signal from the antenna passes through a chain of amplifiers
2 before it reaches the Calypso DSP, and the dBm power measurement reported by the
3 latter is significantly greater than the actual power level at the antenna
4 connector. The mobile station firmware needs to be able to determine the actual
5 input level from the DSP's power measurement (not only for reporting purposes,
6 but also to select the proper AGC setting), hence it needs to know the
7 difference in dB between the DSP's power measurement and the input signal level
8 in dBm. For this purpose the firmware maintains a constant called GMagic,
9 defined as:
10
11 GMagic = PM - IL - AGC
12
13 where PM is the DSP's power measurement in notional dBm, IL is the input level
14 in actual dBm, and AGC is the AGC programmable gain setting in dB. GMagic is
15 the sum of all amplifier gains and insertion losses along the Rx signal path,
16 and is on the order of about 100 dB, although the fw reckons it in half-dB
17 units.
18
19 Some Calypso device manufacters have calibrated the GMagic constant for each
20 band on a per-unit basis, and the unit-to-unit difference is on the order of
21 1-2 half-dB units, i.e., rarely more than 1 dB. The difference from one board
22 design to another tends to be a little greater, on the order of 10 half-dB
23 units, and is probably due to the difference in insertion losses at various
24 points in the Rx signal path. (Ideally these insertion losses should be zero,
25 but the world is not ideal.)
26
27 GMagic calibration is done by connecting a signal generator putting out a
28 precisely controlled signal to the DUT's antenna connector or RF test port,
29 making a power measurement with the Calypso DSP and computing GMagic per the
30 simple equation above.
31
32 Our FreeCalypso version of the GMagic calibration procedure is closely based on
33 the Sara version, described on pages 28-32 of the Sara document
34 rf_calibration.pdf (see TI-docs). The LoCosto calibration document does not
35 include a GMagic calibration procedure, but LoCosto is a very different beast,
36 with both the baseband processor and the RF transceiver in the same chip, and
37 using that mysterious DRP - perhaps LoCosto doesn't need GMagic calibration,
38 perhaps it is absorbed into DRP calibration instead.
39
40 Per the available Sara RF calibration document, we set the signal generator's
41 output level to -74.5 dBm, and set the AGC in the Rita RF chip to 34 dB for the
42 GMagic calibration step. However, we use the ARFCNs from the LoCosto document:
43 37 for the EGSM band, 698 for DCS, 661 for PCS and 189 for GSM850. The old Sara
44 document covers only 900 and 1800 MHz bands (I'm guessing Sara probably did not
45 support 1900 MHz, let alone 850), the LoCosto document is quadband throughout,
46 and the LoCosto ARFCNs are closer to the true centre of each band.
47
48 RSSI channel compensation
49 =========================
50
51 The GMagic constant is calibrated at the centre frequency for each downlink
52 band, but the true value of this "magic gain" varies slightly with the frequency
53 channel. The drawing on page 34 of the Sara document suggests that this
54 frequency channel-dependent variation is caused by frequency-dependent variation
55 in the insertion loss of the Rx SAW filters. This frequency channel-dependent
56 variation in the "magic gain" is customarily calibrated on a per-unit basis
57 (even on LoCosto, apparently), and the available documents suggest that this
58 calibration is done in order to make RSSI reporting more accurate.
59
60 Once again our FreeCalypso version of this RSSI channel calibration procedure
61 is based on the Sara document, rather than LoCosto, except for the ARFCNs. The
62 LoCosto document gives ARFCN ranges for Rx channel calibration for all 4 bands,
63 and these ranges fully agree with what Openmoko's factory programmed in their
64 /gsm/rf/rx/calchan.* files, thus we have adopted the ARFCN ranges and test
65 ARFCNs from LoCosto for all 4 bands.
66
67 Note that the LoCosto document indicates the use of a different signal generator
68 level (-68.5 dBm instead of -74.5), and tells the operator to set the AGC gain
69 to 0 dB. The latter must be a LoCosto-ism, as it is definitely not correct for
70 our Calypso+Iota+Rita chipset. If we set the signal generator level to
71 -68.5 dBm, we would need to get the Rita AGC to 28 dB, but the Mother of
72 FreeCalypso decided to go with the safer choice of using the Sara settings of
73 -74.5 dBm generator level, 34 dB AGC.
74
75 FreeCalypso Rx calibration tools
76 ================================
77
78 The present suite of RF calibration tools includes two programs for Rx path
79 calibration:
80
81 fc-rfcal-gmagic
82
83 This program is a study-only tool. It measures the GMagic constant for
84 the user-specified band (either at that band's default ARFCN or at a
85 user-specified ARFCN) and prints it out, but does not write anything
86 into the Calypso device's FFS.
87
88 fc-rfcal-rxband
89
90 This program is intended for production use. It performs both the
91 GMagic and the RSSI channel compensation calibration steps, uploads the
92 results to the DUT and saves them in FFS.