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annotate Quadband-ideas @ 28:3799892b1a79

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Quadband-ideas article rewritten for the new situation since Tango discovery
author Mychaela Falconia <falcon@freecalypso.org>
date Sat, 25 Jan 2020 22:41:11 +0000
parents 00216b7cfc4d
children 6d7486db31cb
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1 I, Mother Mychaela, hold the belief that any newly designed FreeCalypso hardware
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2 made in 2020 or later needs to be fully quadband, supporting all 4 possible GSM
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3 frequency bands of 850, 900, 1800 and 1900 MHz, nothing less. Furthermore, we
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4 do not need to invent or innovate anything in order to produce a quadband
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5 Calypso phone or modem: TI already had a quadband reference design back in the
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6 early 2000s, called Leonardo or Leonardo+ (the exact proper designation is
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7 unclear), we just need to resurrect it, adding some updates of our own like we
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8 always do.
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9
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10 Our current FCDEV3B modem board is triband rather than quadband because of
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11 historical circumstances: while I had *always* desired a quadband solution
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12 since the very beginning of FreeCalypso (since I started gathering TI Calypso
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13 docs from Chinese sites in 2011), back in 2015 we did not have the necessary
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14 know-how to confidently resurrect TI's lost quadband reference design by doing
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15 a new PCB layout on the basis of the available surviving documentation, which
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16 consists of just schematics and a floorplan drawing - instead going with
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17 Openmoko's PCB layout and their triband RFFE was the only viable option at that
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18 time. We have now successfully produced both 900 MHz and 850 MHz versions of
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19 our FCDEV3B (different SAW filter parts populated on the same PCB footprint)
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20 and both work flawlessly as verified with our CMU200 instrument, so we do
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21 already support all 4 GSM frequency bands in a way - but only 3 at a time with
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22 our current hardware.
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24 A major breakthrough happened in December of 2019: we found a certain extremely
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25 obscure historical commercial Calypso modem module that is almost a verbatim
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26 clone of the core section of TI's Leonardo+ quadband reference design, and the
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27 knowledge gained from examination of this obscure historically-made modem module
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28 (both physical PCB reverse eng and evaluation of its RF performance with our
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29 CMU200 instrument) has given us the necessary confidence boost with this
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30 particular way of implementing a quadband GSM MS.
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32 Epcos M034F
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33 ===========
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35 TI's Leonardo+ and E-Sample boards used a magic component made by Epcos (the
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36 canonical SAW filter manufacturer during that era) that was called M034 or M034F
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37 at the time of TI's designs in question. Epcos later gave it a completely
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38 different name when they released it into volume production, thus the name that
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39 is needed in order to buy this part and that is physically marked on the
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40 component package bears to resemblance to M034 - but we will not be able to
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41 disclose this other name until we physically produce our first FreeCalypso
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42 hardware product with this quadband FEM in it, meaning until and unless someone
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43 pays for it.
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45 The component in question is an integrated quadband FEM (front end module),
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46 integrating the antenna switch and SAW filters in one component package, with a
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47 special twist. The special twist is that even though there are 4 separate Rx
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48 band SAW filters inside that M034 "chip" module, corresponding to its advertised
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49 quadband capability, only 3 Rx signal path differential pairs come out of it,
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50 neatly corresponding to the 3 LNA inputs on TI's Rita transceiver. This twist
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51 is important because even though the Rita transceiver itself is fully quadband
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52 internally, it has only 3 LNA inputs, with GSM850 and EGSM bands sharing the
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53 same LNA input while each of DCS and PCS get their own. Thus this M034 FEM is
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54 special in that it facilitates building a quadband GSM MS using transceivers
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55 like TI Rita or Silabs Si4200 with only 3 LNA inputs, and the name M034 itself
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56 reflects this specialness: the digit 3 in the name refers to the 3 differential
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57 Rx signal paths, while the digit 4 refers to the 4 SAW filters inside which
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58 provide full quadband capability.
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60 We do have an M034F.pdf datasheet for this magic component (came along with
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61 Calypso and Leonardo docs), and the block diagram on page 6 shows the magic
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62 quite clearly: there is a baseband-controlled switch selecting between EGSM Rx
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63 and GSM850 Rx (in addition to the two usual Tx switches), this switch directs
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64 the low band Rx path toward one of two different SAW filters, and the outputs
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65 of those two filters are then joined. The high band Rx path always goes to both
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66 DCS and PCS band SAW filters, and each of those high band Rx SAW filters gets
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67 its own output going to its own dedicated Rita LNA input. Note the lack of a
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68 baseband-controlled switch between DCS and PCS in the high band Rx path!
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70 For a long time prior to the 2019-12 breakthrough I was concerned about this
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71 lack of a baseband-controlled switch between DCS and PCS in the high band Rx
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72 path: this switch is present in every triband RFFE design I am familiar with,
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73 including the one we got from Openmoko, and I was concerned that feeding the
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74 high band Rx path to both DCS and PCS SAW filters without a switch would
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75 introduce a 3 dB penalty into these high band Rx paths. But when I got my
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76 hands on the newly discovered Tango modem modules, looked at the GMagic numbers
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77 written into their FFS as part of the manufacturer's production calibration and
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78 then independently verified them with our CMU200, all fears were dispelled: the
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79 Rx performance of this M034 FEM in all 4 bands is exactly the same as our
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80 current Openmoko-based triband RFFE, with GMagic around 200 half-dB units.
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81
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82 Now that we have physical proof that our desired quadband RFFE based on this
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83 Epcos M034 FEM and other aspects of TI's Leonardo design that go with it really
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84 works exactly as we would like and has been used successfully by a historical
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85 commercial modem module manufacturer (albeit an obscure non-mainstream one),
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86 the way forward for FreeCalypso is clear: instead of continuing with Openmoko's
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87 PCB layout and triband RFFE for our future hardware products, we can confidently
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88 go with a new PCB layout based on Leonardo/Tango, using Epcos M034 as our FEM
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89 and achieving the full quadband capability we have always wanted.
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90
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91 The matching networks ("RF black magic") that would need to be placed between
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92 M034 Rx outputs and Rita LNA inputs were also a big area of concern for a long
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93 time - I did not feel confident with blindly going with the matching networks
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94 depicted on Leonardo schematics (or the slightly different E-Sample version)
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95 with no ability to actually understand them - but the new Tango find has once
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96 again saved the day. Tango uses exactly the same matching networks as depicted
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97 on Leonardo+ schematics (not the slightly different E-Sample version), or more
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98 precisely, it uses exactly the same topology (PCB layout), but with slightly
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99 different component values populated. The safest approach for FreeCalypso is
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100 once again clear: copy Leonardo+ matching network topology and Tango component
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101 values.
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102
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103 Firmware compatibility
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104 ======================
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105
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106 Our current FreeCalypso firmwares drive TSPACT RFFE control signals as follows
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107 on FC hw family targets (CONFIG_TARGET_FCFAM):
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108
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109 TSPACT1 = Rx PCS band
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110 TSPACT2 = Tx high bands
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111 TSPACT4 = Tx low bands
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112 TSPACT5 = Rx GSM850 band
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113
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114 The driving of TSPACT1, TSPACT2 and TSPACT4 matches the way these signals have
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115 been assigned by Openmoko and thus the way they function on our current OM-based
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116 triband RFFE, whereas TSPACT5 is a new signal which is not wired anywhere on
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117 our current FCDEV3B. If and when we actually produce a new FreeCalypso hw
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118 product with the newly-confident M034 FEM, we will need to wire its control
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119 signals (going through logic-inverting PNP transistors as usual) to TSPACT2,
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120 TSPACT4 and TSPACT5 as mapped above, leaving TSPACT1 unconnected as there is no
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121 switch between DCS and PCS in the high band Rx path. This new control signal
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122 wiring is an original FreeCalypso invention, different from TI's original
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123 Leonardo and E-Sample TSPACT signal assignments, but in my maternal opinion
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124 having the same fcmodem firmware build run on both legacy FCDEV3B and future FC
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125 quadband modems would be more valuable than paying tribute to the historical
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126 Leonardo.
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127
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128 Other ideas
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129 ===========
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130
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131 In a previous version of this article I entertained the idea of getting an
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132 entirely new M034-like FEM custom-designed and made specifically for us, asking
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133 someone with the necessary capabilities to produce a new FEM for us that would
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134 be very much like the historical M034F, but with just two key differences:
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135
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136 1) A rearranged pinout so that the putative new FEM could be just "plopped"
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137 into our current OM-based PCB layout in the place of OM's triband RFFE;
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138
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139 2) Better-understood Rx output impedance specifications so that simpler and
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140 more intuitive matching networks like OM's could be retained as well.
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141
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142 That desire was driven by the mystery of Epcos M034 and the lack of empirical
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143 test data for it, stemming from the lack of an already-existing historically-
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144 made physical specimen that could be examined and tested: I simply could not
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145 stomach the idea of expending a monumental effort on a new PCB layout followed
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146 by thousands of dollars of cost to physically produce the new experimental board
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147 batch while doing it as a completely blind stab-in-the-dark with the unknown
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148 Epcos FEM. But the discovery of Tango modems in 2019-12 has drastically changed
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149 the situation, and all mysteries and unknowns surrounding that M034 FEM have now
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150 been cleared.
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151
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152 In the new reality that exists from 2020 onward that idea of commissioning the
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153 design of a new FEM to replace Epcos M034 no longer makes any sense and can be
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154 taken off the table: while it is true that using the existing M034 FEM will
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155 require doing an entirely new PCB layout (no more reuse of OM's version), it is
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156 now just a matter of doing the layout labor, no more risks or uncertainties,
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157 and the cost of this PCB layout job will certainly be much less than having a
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158 new FEM custom-made for us.
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159
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160 Of course there are also myriad other, completely different ways of implementing
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161 a quadband GSM MS without using an M034-style FEM. The approach used by TI in
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162 the days of Calypso, using FEMs that combine the antenna switch and SAW filters
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163 in one component while the PA is separate, fell out of fashion shortly
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164 afterward, replaced with a competing approach where the PA and the antenna
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165 switch are combined into one component while Rx SAW filters are external - the
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166 latter approach is featured on TI's I-Sample (LoCosto) board, and it is
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167 quadband - LoCosto has 4 separate LNA inputs, not 3 like Rita. One could even
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168 implement the same approach while keeping the Calypso+Iota baseband, perhaps
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169 replacing TI's Rita transceiver with Silabs Si4210 (Aero II) that has 4 LNA
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170 inputs like LoCosto. But such ideas are far outside the scope of FreeCalypso,
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171 so if anyone feels like pursuing them, feel free to do so on your own, not
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172 involving FC - my mission in life is NOT to invent or innovate anything new,
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173 instead I am all about resurrecting and bringing back to availability those
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174 perfectly good and perfectly working technical solutions which already existed
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175 once before, but which have been wrongfully discontinued and thus taken away
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176 from us.
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177
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178 In the present situation, the goal of producing a quadband GSM MS while reusing
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179 as much of the already existing FreeCalypso IP and know-how as possible would
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180 be accomplished most efficiently by using the newly-confident M034 FEM and doing
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181 a new PCB layout with it, hence that is the approach I am currently pursuing.