FreeCalypso > hg > fc-rfcal-tools

annotate doc/VCXO-notes @ 21:a3f48378658d

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autocal: beginning of fc-rfcal-vcxo
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 22 May 2017 20:51:55 +0000
parents bd62be88259d
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1 How to calibrate the VCXO on your FreeCalypso development board
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2 ===============================================================
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3
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4 The process of calibrating the VCXO on a Calypso+Iota+Rita GSM MS consists of
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5 the following fundamental parts:
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6
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7 * The antenna needs to be disconnected and the FreeCalypso device's RF output
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8 (SMA on the FCDEV3B) needs to be connected to an RF test station such as an
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9 R&S CMU200.
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10
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11 * The DUT is commanded to transmit semi-continuously as if it were transmitting
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12 on TCH: Tx in one timeslot out of 8, but with the DUT running its own notion
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13 of the TDMA frame not synchronized to anything, and keep transmitting
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14 endlessly in 1/8 out of every 4.615 ms.
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15
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16 * The RF test station connected to the DUT is used in the RF analyzer mode to
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17 measure the frequency offset of the DUT's signal, relative to the ideal uplink
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18 frequency corresponding to the selected ARFCN.
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19
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20 * The above frequency offset measurement is performed with the AFC DAC on the
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21 Calypso device set to different values, the results of the initial
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22 measurements are used to guide some additional measurements, some computations
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23 are made from these results, and the computed values are written into the
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24 FreeCalypso device's FFS.
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25
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26 This procedure is meant to be automated by way of a program that talks both to
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27 the FreeCalypso DUT and to the RF test station and orchestrates all of the
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28 measurement and computation steps, but until this program gets written (we
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29 weren't able to get a hold of TI's original, hence we have to develop our own),
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30 use the following instructions to perform the VCXO calibration procedure
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31 manually. You still need a CMU200 or equivalent, though - it is not possible
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32 to do any kind of calibration on a Calypso device by itself, without connecting
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33 it to some appropriate RF test equipment.
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34
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35 Reference documentation
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36 =======================
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37
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38 We have the following two TI documents which describe some of the RF calibration
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39 procedures including the one for the VCXO:
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40
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41 ftp://ftp.freecalypso.org/pub/GSM/Calypso/rf_calibration.pdf
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42
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43 https://www.freecalypso.org/LoCosto-docs/Production%20test%20and%20calibration/i_sample_rf_test_and_calibration_13_03_04_01991%20-%20v026.pdf
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44
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45 Unfortunately neither of them corresponds to the exact evolutionary time point
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46 of interest to us: the first one corresponds to some chipset much earlier than
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47 the one we are working with, and to firmware versions much earlier than ours,
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48 whereas the second one is for TI's later LoCosto chipset.
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49
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50 Commanding the DUT to transmit semi-continuously
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51 ================================================
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52
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53 There is only one VCXO calibration that is subsequently used for all bands in
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54 normal MS operation. Both of the calibration instruction documents above
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55 instruct the operator to run the Tx in GSM900 mode on ARFCN 40, hence we shall
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56 do likewise until and unless we find some good reason to do differently.
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57
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58 Issue the following commands through fc-tmsh to start the semi-continuous Tx:
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59
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60 tms 1 # enter RF Test Mode
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61 rfpw 7 6 0 # select GSM 900+1800 band pair, GSM900 band within the pair
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62 rfpw 2 40 # set ARFCN to 40
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63 rfpw 8 0 # disable AFC algorithm, i.e., control the AFC DAC manually
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64 txpw 1 12 # Tx power level
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65 rfe 3 # start Rx & Tx without network sync
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66
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67 WARNING: Before issuing the above commands, ensure that the antenna is
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68 disconnected and that the RF output will be going into your test equipment,
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69 not on the air! Do not EVER issue these commands with a real antenna connected,
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70 unless your intent is to operate a rogue transmitter or jammer.
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71
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72 At this point your CMU200 or equivalent should detect the uplink signal
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73 generated by the DUT (on the CMU200 one needs to set TSC to 5, dunno about
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74 other test equipment), and you should see some frequency offset.
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75
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76 The actual calibration procedure
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77 ================================
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78
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79 1. Set the AFC DAC to -2048:
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80
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81 rfpw 9 -2048
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82
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83 and measure the frequency offset. Note it down.
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84
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85 2. Set the AFC DAC to +2048:
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86
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87 rfpw 9 2048
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88
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89 and again measure the frequency offset. Note it down.
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90
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91 Now you need to create an ASCII text file with your frequency offset
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92 measurements. Each line represents one measurement and consists of two fields:
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93 the first field is the DAC value and the second field is the measured frequency
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94 offset in Hz. On my FCDEV3B S/N 001 the first two measurements were:
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95
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96 -2048 -30008
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97 +2048 +21394
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98
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99 Next you need to apply a linear model to the VCXO frequency offset as a function
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100 of the DAC input: if x is the DAC value and F is the resulting frequency offset,
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101 then the linear model is F = ax + b, where a and b need to be determined from
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102 two measured points (x1, F1) and (x2, F2). Then once you have a and b, find the
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103 x value that should produce F = 0. The fc-vcxo-linear utility will do this math
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104 for you: run it with the name of your text file with measurements as its only
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105 argument.
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106
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107 With my measurements, the DAC_CENTER value computed by fc-vcxo-linear is 343.
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108 However, the linear model is not perfect, thus when you write this computed
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109 value into the DAC with the rfpw 9 command, the resulting frequency offset on
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110 the CMU200 screen may be quite far from 0.
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111
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112 TI's instructions in the LoCosto document direct the calibration operator to do
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113 two more measurements at DAC_CENTER-100 and DAC_CENTER+100, where DAC_CENTER is
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114 the value we just computed by applying the linear model to the first two
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115 measurements. However, in my case the frequency offset at DAC=343 (DAC_CENTER)
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116 was so negative that at DAC=443 (DAC_CENTER+100) it was still negative - and I
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117 assume that TI's intent was to capture a close range around the zero crossing.
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118
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119 Therefore, when I get to writing the automated calibration program, I intend to
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120 change this part of the algorithm as follows: instead of adding or subtracting
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121 100 right now, first do an rfpw 9 with the DAC_CENTER value as computed from
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122 the linear model, make a frequency offset measurement, and see if it is negative
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123 or positive. Then step the DAC value in the appropriate direction by some
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124 reasonable increment (e.g., 100) until the frequency offset changes sign. Then
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125 take the two DAC values closest to the output frequency offset sign change.
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126
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127 After doing the above, my measurement notes file became:
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128
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129 -2048 -30008
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130 +2048 +21394
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131 443 -669
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132 543 634
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133
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134 This file needs to contain all four measurements, with the first two being at
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135 the extreme DAC values and with the second two hugging the empirically located
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136 zero crossing, when you feed it to the next step:
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137
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138 fc-vcxo-param myvcxo.meas
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139
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140 The fc-vcxo-param utility will compute the final math steps to produce the
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141 actual calibration values which will need to be uploaded to the FreeCalypso
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142 device and stored in its FFS. With my measurements above, I got the following
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143 output:
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144
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145 rf_table afcparams
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146
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147 3434 # Psi_sta_inv
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148 15 # Psi_st
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149 1000341 # Psi_st_32
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150 4293 # Psi_st_inv
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151
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152 3954 # DAC_INIT * 8
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153 -5860 # DAC_MIN * 8
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154 11351 # DAC_MAX * 8
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155 2560 # snr_thr
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156
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157 # DAC_INIT: rfpw 10 494
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158
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159 The output from fc-vcxo-param is in the rf_table format which our implementation
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160 of the rftw command takes as input, and the latter is the fc-tmsh command which
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161 you will need to issue in order to send this table to the FreeCalypso firmware
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162 in the DUT.
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163
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164 Explanation of the numbers:
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165
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166 * The Psi constants are computed from the slope of the VCXO, and are
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167 subsequently used for the steering: when the DSP reports a particular
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168 frequency offset (in the form of an angle in radians), by how much should the
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169 DAC value be adjusted? The slope I use for computing these Psi constants is
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170 the one from the first two measurements at the extreme DAC values, as the
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171 LoCosto document seems to indicate.
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172
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173 * DAC_INIT is the DAC value at which the resulting frequency offset should be 0;
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174 it is computed per the linear model from the second pair of measurements.
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175
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176 * DAC_MIN and DAC_MAX are the DAC values which should produce frequency offsets
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177 of -15 and +15 ppm, respectively, according to the LoCosto document. I
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178 compute them per the linear model from the first pair of measurements (the
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179 extreme DAC ones), as that is what the LoCosto document says.
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180
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181 * The SNR threshold is a constant that never needs to change.
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182
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183 The 3 dac_* values in the afcparams structure are stored in the times 8 form.
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184 Examination of the afcparams values read out of several Openmoko-made GTA02
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185 units shows that the low 3 bits aren't necessarily zeros, indicating that TI's
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186 calibration program probably multiplied by 8 before converting from floating
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187 point to integer; I do likewise in fc-vcxo-param.
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188
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189 Examination of the same afcparams values read out of Openmoko-made units also
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190 shows that the center, min and max DAC values do vary quite a bit from one unit
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191 to the next, whereas the Psi constants change very little. The Psi constants
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192 which my program computed from my manual measurements on FCDEV3B S/N 001 are in
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193 the same range as those read out of Openmoko-made units, which is definitely a
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194 reassuring sign.
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195
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196 Writing your VCXO calibration into FFS
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197 ======================================
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198
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199 Save the fc-vcxo-param output in a file, e.g.:
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200
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201 fc-vcxo-param myvcxo.meas myvcxo.param
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202
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203 Upload the generated afcparams table to your FreeCalypso device:
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204
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205 rftw 9 myvcxo.param
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206
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207 There is one more variable in the firmware, outside of the afcparams structure,
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208 which also holds the DAC_INIT value. Set it with an rfpw 10 command as
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209 instructed in the last comment line emitted by fc-vcxo-param; in my case it was:
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210
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211 rfpw 10 494
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212
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213 Now save all of these values in the non-volatile flash file system:
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214
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215 me 102
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216 me 103
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217
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218 Cleaning up
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219 ===========
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220
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221 To shut off the transmitter you started earlier, issue this command:
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222
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223 rfe 0
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224
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225 Now power off your FreeCalypso device, disconnect the RF test setup, connect the
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226 antenna back, insert a SIM, do a fresh boot and see if you can connect to a real
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227 live GSM network with your VCXO calibration!