FreeCalypso > hg > fc-rfcal-tools
annotate doc/Rx-cal-theory @ 131:8e26cb57c7f8
doc/Test-system-interface: additions for Tx ramp testing
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Mon, 20 Dec 2021 03:29:23 +0000 |
parents | e7b9a9903f14 |
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1 The incoming radio signal from the antenna passes through a chain of amplifiers |
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2 before it reaches the Calypso DSP, and the dBm power measurement reported by the |
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3 latter is significantly greater than the actual power level at the antenna |
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4 connector. The mobile station firmware needs to be able to determine the actual |
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5 input level from the DSP's power measurement (not only for reporting purposes, |
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6 but also to select the proper AGC setting), hence it needs to know the |
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7 difference in dB between the DSP's power measurement and the input signal level |
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8 in dBm. For this purpose the firmware maintains a constant called GMagic, |
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9 defined as: |
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10 |
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11 GMagic = PM - IL - AGC |
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12 |
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13 where PM is the DSP's power measurement in notional dBm, IL is the input level |
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14 in actual dBm, and AGC is the AGC programmable gain setting in dB. GMagic is |
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15 the sum of all amplifier gains and insertion losses along the Rx signal path, |
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16 and is on the order of about 100 dB, although the fw reckons it in half-dB |
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17 units. |
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18 |
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19 Some Calypso device manufacters have calibrated the GMagic constant for each |
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20 band on a per-unit basis, and the unit-to-unit difference is on the order of |
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21 1-2 half-dB units, i.e., rarely more than 1 dB. The difference from one board |
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22 design to another tends to be a little greater, on the order of 10 half-dB |
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23 units, and is probably due to the difference in insertion losses at various |
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24 points in the Rx signal path. (Ideally these insertion losses should be zero, |
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25 but the world is not ideal.) |
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26 |
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27 GMagic calibration is done by connecting a signal generator putting out a |
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28 precisely controlled signal to the DUT's antenna connector or RF test port, |
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29 making a power measurement with the Calypso DSP and computing GMagic per the |
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30 simple equation above. |
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31 |
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32 Our FreeCalypso version of the GMagic calibration procedure is closely based on |
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33 the Sara version, described on pages 28-32 of the Sara document |
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34 rf_calibration.pdf (see TI-docs). The LoCosto calibration document does not |
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35 include a GMagic calibration procedure, but LoCosto is a very different beast, |
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36 with both the baseband processor and the RF transceiver in the same chip, and |
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37 using that mysterious DRP - perhaps LoCosto doesn't need GMagic calibration, |
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38 perhaps it is absorbed into DRP calibration instead. |
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39 |
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40 Per the available Sara RF calibration document, we set the signal generator's |
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41 output level to -74.5 dBm, and set the AGC in the Rita RF chip to 34 dB for the |
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42 GMagic calibration step. However, we use the ARFCNs from the LoCosto document: |
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43 37 for the EGSM band, 698 for DCS, 661 for PCS and 189 for GSM850. The old Sara |
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44 document covers only 900 and 1800 MHz bands (I'm guessing Sara probably did not |
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45 support 1900 MHz, let alone 850), the LoCosto document is quadband throughout, |
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46 and the LoCosto ARFCNs are closer to the true centre of each band. |
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47 |
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48 RSSI channel compensation |
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49 ========================= |
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50 |
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51 The GMagic constant is calibrated at the centre frequency for each downlink |
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52 band, but the true value of this "magic gain" varies slightly with the frequency |
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53 channel. The drawing on page 34 of the Sara document suggests that this |
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54 frequency channel-dependent variation is caused by frequency-dependent variation |
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55 in the insertion loss of the Rx SAW filters. This frequency channel-dependent |
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56 variation in the "magic gain" is customarily calibrated on a per-unit basis |
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57 (even on LoCosto, apparently), and the available documents suggest that this |
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58 calibration is done in order to make RSSI reporting more accurate. |
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59 |
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60 Once again our FreeCalypso version of this RSSI channel calibration procedure |
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61 is based on the Sara document, rather than LoCosto, except for the ARFCNs. The |
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62 LoCosto document gives ARFCN ranges for Rx channel calibration for all 4 bands, |
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63 and these ranges fully agree with what Openmoko's factory programmed in their |
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64 /gsm/rf/rx/calchan.* files, thus we have adopted the ARFCN ranges and test |
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65 ARFCNs from LoCosto for all 4 bands. |
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66 |
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67 Note that the LoCosto document indicates the use of a different signal generator |
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68 level (-68.5 dBm instead of -74.5), and tells the operator to set the AGC gain |
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69 to 0 dB. The latter must be a LoCosto-ism, as it is definitely not correct for |
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70 our Calypso+Iota+Rita chipset. If we set the signal generator level to |
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71 -68.5 dBm, we would need to set the Rita AGC to 28 dB, but the Mother of |
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72 FreeCalypso decided to go with the safer choice of using the Sara settings of |
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73 -74.5 dBm generator level, 34 dB AGC. |
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74 |
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75 FreeCalypso Rx calibration tools |
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76 ================================ |
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77 |
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78 The present suite of RF calibration tools includes two programs for Rx path |
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79 calibration: |
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80 |
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81 fc-rfcal-gmagic |
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82 |
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83 This program is a study-only tool. It measures the GMagic constant for |
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84 the user-specified band (either at that band's default ARFCN or at a |
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85 user-specified ARFCN) and prints it out, but does not write anything |
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86 into the Calypso device's FFS. |
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87 |
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88 fc-rfcal-rxband |
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89 |
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90 This program is intended for production use. It performs both the |
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91 GMagic and the RSSI channel compensation calibration steps, uploads the |
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92 results to the DUT and saves them in FFS. |