FreeCalypso > hg > freecalypso-docs
annotate Quadband-ideas @ 21:69ee60206c53
FC-modem-family article written
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Wed, 23 Oct 2019 00:11:37 +0000 |
parents | 3fa4006696d0 |
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rev | line source |
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Quadband-ideas article written
Mychaela Falconia <falcon@freecalypso.org>
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1 Our current Openmoko-based Calypso+RF modem core is very very good, but it has |
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2 one shortcoming compared to TI's Leonardo+ reference design: it is triband |
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3 rather than quadband. This triband restriction stems from OM's use of discrete |
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4 antenna switch and SAW filter components as opposed to an integrated FEM (front |
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5 end module) like on Leonardo+. In addition to the band restriction, our current |
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6 triband RF design suffers from one other very unpleasant problem: we have no |
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7 datasheet for the antenna switch component which we have to use. We know from |
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8 Openmoko's BOM data that the manufacturer is Darfon and that the part number for |
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9 this antenna switch component is ASM4532T0P06-1, we are able to buy this part |
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Mychaela Falconia <falcon@freecalypso.org>
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10 from our Chinese grey market suppliers, we build our boards with these parts and |
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11 our boards do work perfectly fine when we get a good batch, but we have to do |
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12 this entire process blindly, without any datasheet or other documentation for |
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13 this mystery part. |
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14 |
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15 This article outlines some ideas for how we may be able to move from this RFFE |
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16 to a different one, replacing our current mystery antenna switch with something |
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17 less mysterious and better documented, and improving our radio capability from |
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Mychaela Falconia <falcon@freecalypso.org>
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18 triband to quadband at the same time. |
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Mychaela Falconia <falcon@freecalypso.org>
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19 |
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Mychaela Falconia <falcon@freecalypso.org>
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20 Epcos M034F |
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21 =========== |
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22 |
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23 TI's Leonardo+ and E-Sample boards used a magic component made by Epcos (the |
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Mychaela Falconia <falcon@freecalypso.org>
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24 canonical SAW filter manufacturer during that era) called M034 or M034F (the |
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25 exact proper designation is unclear). It was an integrated quadband FEM, |
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26 integrating the antenna switch and SAW filters in one component package, with a |
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27 special twist. The special twist is that even though there are 4 separate Rx |
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28 band SAW filters inside that M034 "chip" module, corresponding to its advertised |
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29 quadband capability, only 3 Rx signal path differential pairs come out of it, |
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30 neatly corresponding to the 3 LNA inputs on TI's Rita transceiver. This twist |
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31 is important because even though the Rita transceiver itself is fully quadband |
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32 internally, it has only 3 LNA inputs, with GSM850 and EGSM bands sharing the |
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33 same LNA input while each of DCS and PCS get their own. |
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34 |
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35 We do have an M034F.pdf datasheet for this magic component (came along with |
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36 Calypso and Leonardo docs), and the block diagram on page 6 shows the magic |
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37 quite clearly: there is a baseband-controlled switch selecting between EGSM Rx |
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38 and GSM850 Rx (in addition to the two usual Tx switches), this switch directs |
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Mychaela Falconia <falcon@freecalypso.org>
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39 the low band Rx path toward one of two different SAW filters, and the outputs |
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40 of those two filters are then joined. The high band Rx path always goes to both |
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41 DCS and PCS band SAW filters (I assume it is a 50/50 split of the total incoming |
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42 energy, with each path suffering by 3 dB as a result), and each of those high |
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43 band Rx SAW filters gets its own output going to its own dedicated Rita LNA |
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44 input. |
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45 |
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46 I (Mother Mychaela) would absolutely love to play with an M034-based quadband |
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47 Calypso+Iota+Rita board in my lab with the trusty CMU200 instrument, and to see |
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48 how well it actually performs, especially in comparison with our current |
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49 OM-based triband version. However, in all of my years of searching I have never |
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Mychaela Falconia <falcon@freecalypso.org>
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50 found a physical Leonardo board (any version), nor have we ever found any |
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51 Leonardo PCB layout files which would allow us to build a modern recreation - |
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52 thus the magic of M034 remains elusive. |
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53 |
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54 Unless a miracle happens and we are able to obtain either a physical Leonardo+ |
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55 board or a PADS PCB file for one, there is no quick or low-effort way to "just |
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56 try" this M034 RFFE. Instead if we wish to build a FreeCalypso board with this |
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57 RFFE, it would have to be "the full 9 yards": a full-blown PCB design and layout |
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58 effort. There is no way to just "drop" the M034 into our existing PCB design |
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59 in the place of our current triband RFFE, it would have to be either a very |
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60 disruptive RF section layout change or an entirely new PCB layout, making this |
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61 idea very open-ended - an open-ended venture with quite uncertain outcome, but |
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62 with a high dollar cost attached to it. Given the massive effort required and |
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63 PCB layout labor costs, I currently have no active plans to pursue this idea |
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64 beyond hypothetical. |
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65 |
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66 Commissioning a new custom RF FEM |
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67 ================================= |
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68 |
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69 Here is a wild thought: what if instead of twisting over backwards trying to |
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70 hammer an existing RF FEM like M034F into our not-quite-fitting PCB design, we |
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71 were to get an entirely new FEM made specially for us, made exactly the way we |
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72 need it? If we were to venture that way, I would ask for a FEM very similar |
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73 conceptually to M034F, but with a few changes: |
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74 |
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75 1) Instead of diplexing between DCS and PCS SAW filter inputs with a 50/50 |
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76 energy split, implement another switch (just like the GSM850 Rx switch) for |
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77 PCS Rx, exactly the same way how it is done in classic triband designs like |
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78 our current OM-based one. This change should eliminate the extra 3 dB |
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79 penalty which I assume (for lack of experimental data) must happen with the |
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80 existing M034 FEM. Or as an alternative to making this change, if someone |
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81 who is more knowledgeable than me in this area can explain to me why it isn't |
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82 necessary, I would accept that option as well. |
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83 |
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84 2) I would ask for a rearranged pinout: the existing M034F pinout does not fit |
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85 at all into our OM-based PCB layout, but it would fit much better with some |
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86 rearrangement. |
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87 |
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88 3) The hypothetical M034-like FEM would fit into our OM-based PCB layout a lot |
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89 better if it were made a little smaller than the 8.2x5.5 mm size of M034F. |
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90 Considering that the original M034F was created some 15-16 y ago, I assume |
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91 that it should be possible to make a smaller version in 2020 or 2021 or |
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92 whenever. |
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93 |
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94 Timeline sequentiality |
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95 ====================== |
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96 |
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97 All of the above ideas will be considered on a less hypothetical level after we |
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98 get our already-committed FCM40 product built. FCM40 will be a modem module in |
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99 the same 56.5x36 mm form factor as Huawei GTM900 (with a mostly-compatible FPC |
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100 interface with only a few changes), featuring the same OM-based triband modem |
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101 core as FCDEV3B V2. The reason for this sequencing is that our current FCDEV3B |
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102 suffers from a couple of issues which FCM40 is expected to solve: |
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103 |
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104 1) FCDEV3B has a very tight PCB layout: not only do we have the tightly laid out |
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105 core from GTA02, but also the whole board is quite small for the implemented |
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106 peripheral complexity, imposing further constraints from all sides. This |
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107 tight and complex layout makes a poor choice of starting point for bold |
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108 experiments like RFFE changes. |
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109 |
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110 2) FCDEV3B is locked into Altium. Layout data migration from Altium to FOSS |
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111 appears to be much less feasible than migration from PADS to FOSS, thus |
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112 freeing our PCB layout from the clutches of proprietary software will most |
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113 likely require giving up (or rather setting aside) all of FCDEV3B new layout |
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114 and going back to the GTA02 starting point, which is in PADS format rather |
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115 than Altium. Redoing all of FCDEV3B anew does not sound appealing at all, |
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116 but the much simpler FCM40 board offers a perfect opportunity for a fresh |
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117 start. |
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118 |
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119 FCM40 will have exactly the same OM-based triband RFFE as our current FCDEV3B, |
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120 but it will be a much simpler board, and if we can get it done in FOSS instead |
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121 of continuing the Altium track, then we would have a very solid reference and a |
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122 good starting point for potential RFFE change experiments. |
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123 |
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124 Firmware compatibility |
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125 ====================== |
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126 |
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127 Our current FreeCalypso firmwares drive TSPACT RFFE control signals as follows |
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128 on FC hw family targets (CONFIG_TARGET_FCFAM): |
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129 |
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130 TSPACT1 = Rx PCS band |
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131 TSPACT2 = Tx high bands |
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132 TSPACT4 = Tx low bands |
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133 TSPACT5 = Rx GSM850 band |
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134 |
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135 The driving of TSPACT1, TSPACT2 and TSPACT4 matches the way these signals have |
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136 been assigned by Openmoko and thus the way they function on our current OM-based |
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137 triband RFFE, whereas TSPACT5 is a new signal which is not wired anywhere on |
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138 our current FCDEV3B. This signal driving arrangement is expected to be |
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139 compatible with all 3 RFFE hw possibilities under consideration: |
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140 |
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141 * On our current OM-based triband RFFE it works as is. |
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142 |
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143 * If we use Epcos M034 or a semi-clone thereof that has the two Tx switches and |
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144 a GSM850 Rx switch but no PCS Rx switch, then we will need to connect TSPACT2, |
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145 TSPACT4 and TSPACT5 per the table above, and leave TSPACT1 unconnected. |
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146 |
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147 * If we get a new M034-like FEM custom-made with a full set of all 4 switches, |
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148 then all 4 TSPACT signals will need to be connected per the table above. |